path: root/pingora-proxy/src/proxy_cache.rs

// Copyright 2024 Cloudflare, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use super::*;
use http::StatusCode;
use pingora_cache::key::CacheHashKey;
use pingora_cache::lock::LockStatus;
use pingora_cache::max_file_size::ERR_RESPONSE_TOO_LARGE;
use pingora_cache::{HitStatus, RespCacheable::*};
use pingora_core::protocols::http::conditional_filter::to_304;
use pingora_core::protocols::http::v1::common::header_value_content_length;
use pingora_core::ErrorType;

impl<SV> HttpProxy<SV> {
    // return bool: server_session can be reused, and error if any
    pub(crate) async fn proxy_cache(
        self: &Arc<Self>,
        session: &mut Session,
        ctx: &mut SV::CTX,
    ) -> Option<(bool, Option<Box<Error>>)>
    // None: continue to proxy, Some: return
    where
        SV: ProxyHttp + Send + Sync + 'static,
        SV::CTX: Send + Sync,
    {
        // Cache logic request phase
        if let Err(e) = self.inner.request_cache_filter(session, ctx) {
            // TODO: handle this error
            warn!(
                "Fail to request_cache_filter: {e}, {}",
                self.inner.request_summary(session, ctx)
            );
        }

        // cache key logic, should this be part of request_cache_filter?
        if session.cache.enabled() {
            match self.inner.cache_key_callback(session, ctx) {
                Ok(key) => {
                    session.cache.set_cache_key(key);
                }
                Err(e) => {
                    // TODO: handle this error
                    session.cache.disable(NoCacheReason::StorageError);
                    warn!(
                        "Fail to cache_key_callback: {e}, {}",
                        self.inner.request_summary(session, ctx)
                    );
                }
            }
        }

        // cache purge logic: PURGE short-circuits rest of request
        if self.inner.is_purge(session, ctx) {
            return self.proxy_purge(session, ctx).await;
        }

        // bypass cache lookup if we predict to be uncacheable
        if session.cache.enabled() && !session.cache.cacheable_prediction() {
            session.cache.bypass();
        }

        if !session.cache.enabled() {
            return None;
        }

        // cache lookup logic
        loop {
            // for cache lock, TODO: cap the max number of loops
            match session.cache.cache_lookup().await {
                Ok(res) => {
                    if let Some((mut meta, handler)) = res {
                        // Vary logic
                        // Because this branch can be called multiple times in a loop, and we only
                        // need to update the vary once, check if variance is already set to
                        // prevent unnecessary vary lookups.
                        let cache_key = session.cache.cache_key();
                        if let Some(variance) = cache_key.variance_bin() {
                            // We've looked up a secondary slot.
                            // Adhoc double check that the variance found is the variance we want.
                            if Some(variance) != meta.variance() {
                                warn!("Cache variance mismatch, {variance:?}, {cache_key:?}");
                                session.cache.disable(NoCacheReason::InternalError);
                                break None;
                            }
                        } else {
                            // Basic cache key; either variance is off, or this is the primary slot.
                            let req_header = session.req_header();
                            let variance = self.inner.cache_vary_filter(&meta, ctx, req_header);
                            if let Some(variance) = variance {
                                // Variance is on. This is the primary slot.
                                if !session.cache.cache_vary_lookup(variance, &meta) {
                                    // This wasn't the desired variant. Updated cache key variance, cause another
                                    // lookup to get the desired variant, which would be in a secondary slot.
                                    continue;
                                }
                            } // else: vary is not in use
                        }

                        // Either no variance, or the current handler targets the correct variant.

                        // hit
                        // TODO: maybe round and/or cache now()
                        let hit_status = if meta.is_fresh(std::time::SystemTime::now()) {
                            // check if we should force expire
                            // (this is a soft purge which tries to revalidate,
                            // vs. hard purge which forces miss)
                            // TODO: allow hard purge
                            match self.inner.cache_hit_filter(session, &meta, ctx).await {
                                Err(e) => {
                                    error!(
                                        "Failed to filter cache hit: {e}, {}",
                                        self.inner.request_summary(session, ctx)
                                    );
                                    // this return value will cause us to fetch from upstream
                                    HitStatus::FailedHitFilter
                                }
                                Ok(expired) => {
                                    // force expired asset should not be serve as stale
                                    // because force expire is usually to remove data
                                    if expired {
                                        meta.disable_serve_stale();
                                        HitStatus::ForceExpired
                                    } else {
                                        HitStatus::Fresh
                                    }
                                }
                            }
                        } else {
                            HitStatus::Expired
                        };
                        // init cache for hit / stale
                        session.cache.cache_found(meta, handler, hit_status);

                        if !hit_status.is_fresh() {
                            // expired or force expired asset
                            if session.cache.is_cache_locked() {
                                // first if this is the sub request for the background cache update
                                if let Some(write_lock) = session
                                    .subrequest_ctx
                                    .as_mut()
                                    .and_then(|ctx| ctx.write_lock.take())
                                {
                                    // Put the write lock in the request
                                    session.cache.set_write_lock(write_lock);
                                    session.cache.tag_as_subrequest();
                                    // and then let it go to upstream
                                    break None;
                                }
                                let will_serve_stale = session.cache.can_serve_stale_updating()
                                    && self.inner.should_serve_stale(session, ctx, None);
                                if !will_serve_stale {
                                    let lock_status = session.cache.cache_lock_wait().await;
                                    if self.handle_lock_status(session, ctx, lock_status) {
                                        continue;
                                    } else {
                                        break None;
                                    }
                                }
                                // else continue to serve stale
                                session.cache.set_stale_updating();
                            } else if session.cache.is_cache_lock_writer() {
                                // stale while revalidate logic for the writer
                                let will_serve_stale = session.cache.can_serve_stale_updating()
                                    && self.inner.should_serve_stale(session, ctx, None);
                                if will_serve_stale {
                                    // create a background thread to do the actual update
                                    let subrequest =
                                        Box::new(crate::subrequest::create_dummy_session(session));
                                    let new_app = self.clone(); // Clone the Arc
                                    let sub_req_ctx = Box::new(SubReqCtx {
                                        write_lock: Some(session.cache.take_write_lock()),
                                    });
                                    tokio::spawn(async move {
                                        new_app.process_subrequest(subrequest, sub_req_ctx).await;
                                    });
                                    // continue to serve stale for this request
                                    session.cache.set_stale_updating();
                                } else {
                                    // return to fetch from upstream
                                    break None;
                                }
                            } else {
                                // return to fetch from upstream
                                break None;
                            }
                        }
                        let (reuse, err) = self.proxy_cache_hit(session, ctx).await;
                        if let Some(e) = err.as_ref() {
                            error!(
                                "Fail to serve cache: {e}, {}",
                                self.inner.request_summary(session, ctx)
                            );
                        }
                        // responses is served from cache, exit
                        break Some((reuse, err));
                    } else {
                        // cache miss
                        if session.cache.is_cache_locked() {
                            // Another request is filling the cache; try waiting til that's done and retry.
                            let lock_status = session.cache.cache_lock_wait().await;
                            if self.handle_lock_status(session, ctx, lock_status) {
                                continue;
                            } else {
                                break None;
                            }
                        } else {
                            self.inner.cache_miss(session, ctx);
                            break None;
                        }
                    }
                }
                Err(e) => {
                    // Allow cache miss to fill cache even if cache lookup errors
                    // this is mostly to support backward incompatible metadata update
                    // TODO: check error types
                    // session.cache.disable();
                    self.inner.cache_miss(session, ctx);
                    warn!(
                        "Fail to cache lookup: {e}, {}",
                        self.inner.request_summary(session, ctx)
                    );
                    break None;
                }
            }
        }
    }

    // return bool: server_session can be reused, and error if any
    pub(crate) async fn proxy_cache_hit(
        &self,
        session: &mut Session,
        ctx: &mut SV::CTX,
    ) -> (bool, Option<Box<Error>>)
    where
        SV: ProxyHttp + Send + Sync,
        SV::CTX: Send + Sync,
    {
        use range_filter::*;

        let seekable = session.cache.hit_handler().can_seek();
        let mut header = cache_hit_header(&session.cache);

        let req = session.req_header();

        let not_modified = match self.inner.cache_not_modified_filter(session, &header, ctx) {
            Ok(not_modified) => not_modified,
            Err(e) => {
                // fail open if cache_not_modified_filter errors,
                // just return the whole original response
                warn!(
                    "Failed to run cache not modified filter: {e}, {}",
                    self.inner.request_summary(session, ctx)
                );
                false
            }
        };
        if not_modified {
            to_304(&mut header);
        }
        let header_only = not_modified || req.method == http::method::Method::HEAD;

        // process range header if the cache storage supports seek
        let range_type = if seekable && !session.ignore_downstream_range {
            range_header_filter(req, &mut header)
        } else {
            RangeType::None
        };

        // return a 416 with an empty body for simplicity
        let header_only = header_only || matches!(range_type, RangeType::Invalid);

        // TODO: use ProxyUseCache to replace the logic below
        match self.inner.response_filter(session, &mut header, ctx).await {
            Ok(_) => {
                if let Err(e) = session
                    .as_mut()
                    .write_response_header(header)
                    .await
                    .map_err(|e| e.into_down())
                {
                    // downstream connection is bad already
                    return (false, Some(e));
                }
            }
            Err(e) => {
                // TODO: more logging and error handling
                session.as_mut().respond_error(500).await;
                // we have not write anything dirty to downstream, it is still reusable
                return (true, Some(e));
            }
        }
        debug!("finished sending cached header to downstream");

        if !header_only {
            if let RangeType::Single(r) = range_type {
                if let Err(e) = session.cache.hit_handler().seek(r.start, Some(r.end)) {
                    return (false, Some(e));
                }
            }
            loop {
                match session.cache.hit_handler().read_body().await {
                    Ok(mut body) => {
                        let end = body.is_none();
                        match self
                            .inner
                            .response_body_filter(session, &mut body, end, ctx)
                        {
                            Ok(Some(duration)) => {
                                trace!("delaying response for {duration:?}");
                                time::sleep(duration).await;
                            }
                            Ok(None) => { /* continue */ }
                            Err(e) => {
                                // body is being sent, don't treat downstream as reusable
                                return (false, Some(e));
                            }
                        }

                        if let Some(b) = body {
                            // write to downstream
                            if let Err(e) = session
                                .as_mut()
                                .write_response_body(b, false)
                                .await
                                .map_err(|e| e.into_down())
                            {
                                return (false, Some(e));
                            }
                        } else {
                            break;
                        }
                    }
                    Err(e) => return (false, Some(e)),
                }
            }
        }

        if let Err(e) = session.cache.finish_hit_handler().await {
            warn!("Error during finish_hit_handler: {}", e);
        }

        match session.as_mut().finish_body().await {
            Ok(_) => {
                debug!("finished sending cached body to downstream");
                (true, None)
            }
            Err(e) => (false, Some(e)),
        }
    }

    /* Downstream revalidation, only needed when cache is on because otherwise origin
     * will handle it */
    pub(crate) fn downstream_response_conditional_filter(
        &self,
        use_cache: &mut ServeFromCache,
        session: &Session,
        resp: &mut ResponseHeader,
        ctx: &mut SV::CTX,
    ) where
        SV: ProxyHttp,
    {
        // TODO: range
        let req = session.req_header();

        let not_modified = match self.inner.cache_not_modified_filter(session, resp, ctx) {
            Ok(not_modified) => not_modified,
            Err(e) => {
                // fail open if cache_not_modified_filter errors,
                // just return the whole original response
                warn!(
                    "Failed to run cache not modified filter: {e}, {}",
                    self.inner.request_summary(session, ctx)
                );
                false
            }
        };

        if not_modified {
            to_304(resp);
        }
        let header_only = not_modified || req.method == http::method::Method::HEAD;
        if header_only {
            if use_cache.is_on() {
                // tell cache to stop after yielding header
                use_cache.enable_header_only();
            } else {
                // headers only during cache miss, upstream should continue send
                // body to cache, `session` will ignore body automatically because
                // of the signature of `header` (304)
                // TODO: we should drop body before/within this filter so that body
                // filter only runs on data downstream sees
            }
        }
    }

    // TODO: cache upstream header filter to add/remove headers

    pub(crate) async fn cache_http_task(
        &self,
        session: &mut Session,
        task: &HttpTask,
        ctx: &mut SV::CTX,
        serve_from_cache: &mut ServeFromCache,
    ) -> Result<()>
    where
        SV: ProxyHttp + Send + Sync,
        SV::CTX: Send + Sync,
    {
        if !session.cache.enabled() && !session.cache.bypassing() {
            return Ok(());
        }

        match task {
            HttpTask::Header(header, end_stream) => {
                // decide if cacheable and create cache meta
                // for now, skip 1xxs (should not affect response cache decisions)
                // However 101 is an exception because it is the final response header
                if header.status.is_informational()
                    && header.status != StatusCode::SWITCHING_PROTOCOLS
                {
                    return Ok(());
                }
                match self.inner.response_cache_filter(session, header, ctx)? {
                    Cacheable(meta) => {
                        let mut fill_cache = true;
                        if session.cache.bypassing() {
                            // The cache might have been bypassed because the response exceeded the
                            // maximum cacheable asset size. If that looks like the case (there
                            // is a maximum file size configured and we don't know the content
                            // length up front), attempting to re-enable the cache now would cause
                            // the request to fail when the chunked response exceeds the maximum
                            // file size again.
                            if session.cache.max_file_size_bytes().is_some()
                                && !header.headers.contains_key(header::CONTENT_LENGTH)
                            {
                                session.cache.disable(NoCacheReason::ResponseTooLarge);
                                return Ok(());
                            }

                            session.cache.response_became_cacheable();

                            if header.status == StatusCode::OK {
                                self.inner.cache_miss(session, ctx);
                            } else {
                                // we've allowed caching on the next request,
                                // but do not cache _this_ request if bypassed and not 200
                                // (We didn't run upstream request cache filters to strip range or condition headers,
                                // so this could be an uncacheable response e.g. 206 or 304.
                                // Exclude all non-200 for simplicity, may expand allowable codes in the future.)
                                fill_cache = false;
                                session.cache.disable(NoCacheReason::Deferred);
                            }
                        }

                        // If the Content-Length is known, and a maximum asset size has been configured
                        // on the cache, validate that the response does not exceed the maximum asset size.
                        if session.cache.enabled() {
                            if let Some(max_file_size) = session.cache.max_file_size_bytes() {
                                let content_length_hdr = header.headers.get(header::CONTENT_LENGTH);
                                if let Some(content_length) =
                                    header_value_content_length(content_length_hdr)
                                {
                                    if content_length > max_file_size {
                                        fill_cache = false;
                                        session.cache.response_became_uncacheable(
                                            NoCacheReason::ResponseTooLarge,
                                        );
                                        session.cache.disable(NoCacheReason::ResponseTooLarge);
                                    }
                                }
                                // if the content-length header is not specified, the miss handler
                                // will count the response size on the fly, aborting the request
                                // mid-transfer if the max file size is exceeded
                            }
                        }
                        if fill_cache {
                            let req_header = session.req_header();
                            // Update the variance in the meta via the same callback,
                            // cache_vary_filter(), used in cache lookup for consistency.
                            // Future cache lookups need a matching variance in the meta
                            // with the cache key to pick up the correct variance
                            let variance = self.inner.cache_vary_filter(&meta, ctx, req_header);
                            session.cache.set_cache_meta(meta);
                            session.cache.update_variance(variance);
                            // this sends the meta and header
                            session.cache.set_miss_handler().await?;
                            if session.cache.miss_body_reader().is_some() {
                                serve_from_cache.enable_miss();
                            }
                            if *end_stream {
                                session
                                    .cache
                                    .miss_handler()
                                    .unwrap() // safe, it is set above
                                    .write_body(Bytes::new(), true)
                                    .await?;
                                session.cache.finish_miss_handler().await?;
                            }
                        }
                    }
                    Uncacheable(reason) => {
                        if !session.cache.bypassing() {
                            // mark as uncacheable, so we bypass cache next time
                            session.cache.response_became_uncacheable(reason);
                        }
                        session.cache.disable(reason);
                    }
                }
            }
            HttpTask::Body(data, end_stream) => match data {
                Some(d) => {
                    if session.cache.enabled() {
                        // this will panic if more data is sent after we see end_stream
                        // but should be impossible in real world
                        let miss_handler = session.cache.miss_handler().unwrap();
                        // TODO: do this async
                        let res = miss_handler.write_body(d.clone(), *end_stream).await;
                        if let Err(err) = res {
                            if err.etype == ERR_RESPONSE_TOO_LARGE {
                                debug!("chunked response exceeded max cache size, remembering that it is uncacheable");
                                session
                                    .cache
                                    .response_became_uncacheable(NoCacheReason::ResponseTooLarge);
                            }

                            return Err(err);
                        }
                        if *end_stream {
                            session.cache.finish_miss_handler().await?;
                        }
                    }
                }
                None => {
                    if session.cache.enabled() && *end_stream {
                        session.cache.finish_miss_handler().await?;
                    }
                }
            },
            HttpTask::Trailer(_) => {} // h1 trailer is not supported yet
            HttpTask::Done => {
                if session.cache.enabled() {
                    session.cache.finish_miss_handler().await?;
                }
            }
            HttpTask::Failed(_) => {
                // TODO: handle this failure: delete the temp files?
            }
        }
        Ok(())
    }

    // Decide if local cache can be used according to upstream http header
    // 1. when upstream returns 304, the local cache is refreshed and served fresh
    // 2. when upstream returns certain HTTP error status, the local cache is served stale
    // Return true if local cache should be used, false otherwise
    pub(crate) async fn revalidate_or_stale(
        &self,
        session: &mut Session,
        task: &mut HttpTask,
        ctx: &mut SV::CTX,
    ) -> bool
    where
        SV: ProxyHttp + Send + Sync,
        SV::CTX: Send + Sync,
    {
        if !session.cache.enabled() {
            return false;
        }

        match task {
            HttpTask::Header(resp, _eos) => {
                if resp.status == StatusCode::NOT_MODIFIED {
                    if session.cache.maybe_cache_meta().is_some() {
                        // run upstream response filters on upstream 304 first
                        self.inner.upstream_response_filter(session, resp, ctx);
                        // 304 doesn't contain all the headers, merge 304 into cached 200 header
                        // in order for response_cache_filter to run correctly
                        let merged_header = session.cache.revalidate_merge_header(resp);
                        match self
                            .inner
                            .response_cache_filter(session, &merged_header, ctx)
                        {
                            Ok(Cacheable(mut meta)) => {
                                // For simplicity, ignore changes to variance over 304 for now.
                                // Note this means upstream can only update variance via 2xx
                                // (expired response).
                                //
                                // TODO: if we choose to respect changing Vary / variance over 304,
                                // then there are a few cases to consider. See `update_variance` in
                                // the `pingora-cache` module.
                                let old_meta = session.cache.maybe_cache_meta().unwrap(); // safe, checked above
                                if let Some(old_variance) = old_meta.variance() {
                                    meta.set_variance(old_variance);
                                }
                                if let Err(e) = session.cache.revalidate_cache_meta(meta).await {
                                    // Fail open: we can continue use the revalidated response even
                                    // if the meta failed to write to storage
                                    warn!("revalidate_cache_meta failed {e:?}");
                                }
                            }
                            Ok(Uncacheable(reason)) => {
                                // This response was once cacheable, and upstream tells us it has not changed
                                // but now we decided it is uncacheable!
                                // RFC 9111: still allowed to reuse stored response this time because
                                // it was "successfully validated"
                                // https://www.rfc-editor.org/rfc/rfc9111#constructing.responses.from.caches
                                // Serve the response, but do not update cache

                                // We also want to avoid poisoning downstream's cache with an unsolicited 304
                                // if we did not receive a conditional request from downstream
                                // (downstream may have a different cacheability assessment and could cache the 304)

                                //TODO: log more
                                warn!("Uncacheable {reason:?} 304 received");
                                session.cache.response_became_uncacheable(reason);
                                session.cache.revalidate_uncacheable(merged_header, reason);
                            }
                            Err(e) => {
                                // Error during revalidation, similarly to the reasons above
                                // (avoid poisoning downstream cache with passthrough 304),
                                // allow serving the stored response without updating cache
                                warn!("Error {e:?} response_cache_filter during revalidation");
                                session.cache.revalidate_uncacheable(
                                    merged_header,
                                    NoCacheReason::InternalError,
                                );
                                // Assume the next 304 may succeed, so don't mark uncacheable
                            }
                        }
                        // always serve from cache after receiving the 304
                        true
                    } else {
                        //TODO: log more
                        warn!("304 received without cached asset, disable caching");
                        let reason = NoCacheReason::Custom("304 on miss");
                        session.cache.response_became_uncacheable(reason);
                        session.cache.disable(reason);
                        false
                    }
                } else if resp.status.is_server_error() {
                    // stale if error logic, 5xx only for now

                    // this is response header filter, response_written should always be None?
                    if !session.cache.can_serve_stale_error()
                        || session.response_written().is_some()
                    {
                        return false;
                    }

                    // create an error to encode the http status code
                    let http_status_error = Error::create(
                        ErrorType::HTTPStatus(resp.status.as_u16()),
                        ErrorSource::Upstream,
                        None,
                        None,
                    );
                    self.inner
                        .should_serve_stale(session, ctx, Some(&http_status_error))
                } else {
                    false // not 304, not stale if error status code
                }
            }
            _ => false, // not header
        }
    }

    // None: no staled asset is used, Some(_): staled asset is sent to downstream
    // bool: can the downstream connection be reused
    pub(crate) async fn handle_stale_if_error(
        &self,
        session: &mut Session,
        ctx: &mut SV::CTX,
        error: &Error,
    ) -> Option<(bool, Option<Box<Error>>)>
    where
        SV: ProxyHttp + Send + Sync,
        SV::CTX: Send + Sync,
    {
        // the caller might already checked this as an optimization
        if !session.cache.can_serve_stale_error() {
            return None;
        }

        // the error happen halfway through a regular response to downstream
        // can't resend the response
        if session.response_written().is_some() {
            return None;
        }

        // check error types
        if !self.inner.should_serve_stale(session, ctx, Some(error)) {
            return None;
        }

        // log the original error
        warn!(
            "Fail to proxy: {}, serving stale, {}",
            error,
            self.inner.request_summary(session, ctx)
        );

        Some(self.proxy_cache_hit(session, ctx).await)
    }

    // helper function to check when to continue to retry lock (true) or give up (false)
    fn handle_lock_status(
        &self,
        session: &mut Session,
        ctx: &SV::CTX,
        lock_status: LockStatus,
    ) -> bool
    where
        SV: ProxyHttp,
    {
        debug!("cache unlocked {lock_status:?}");
        match lock_status {
            // should lookup the cached asset again
            LockStatus::Done => true,
            // should compete to be a new writer
            LockStatus::TransientError => true,
            // the request is uncacheable, go ahead to fetch from the origin
            LockStatus::GiveUp => {
                // TODO: It will be nice for the writer to propagate the real reason
                session.cache.disable(NoCacheReason::CacheLockGiveUp);
                // not cacheable, just go to the origin.
                false
            }
            // treat this the same as TransientError
            LockStatus::Dangling => {
                // software bug, but request can recover from this
                warn!(
                    "Dangling cache lock, {}",
                    self.inner.request_summary(session, ctx)
                );
                true
            }
            /* We have 3 options when a lock is held too long
             * 1. release the lock and let every request complete for it again
             * 2. let every request cache miss
             * 3. let every request through while disabling cache
             * #1 could repeat the situation but protect the origin from load
             * #2 could amplify disk writes and storage for temp file
             * #3 is the simplest option for now */
            LockStatus::Timeout => {
                warn!(
                    "Cache lock timeout, {}",
                    self.inner.request_summary(session, ctx)
                );
                session.cache.disable(NoCacheReason::CacheLockTimeout);
                // not cacheable, just go to the origin.
                false
            }
            // software bug, this status should be impossible to reach
            LockStatus::Waiting => panic!("impossible LockStatus::Waiting"),
        }
    }
}

fn cache_hit_header(cache: &HttpCache) -> Box<ResponseHeader> {
    let mut header = Box::new(cache.cache_meta().response_header_copy());
    // convert cache response

    // these status codes / method cannot have body, so no need to add chunked encoding
    let no_body = matches!(header.status.as_u16(), 204 | 304);

    // https://www.rfc-editor.org/rfc/rfc9111#section-4:
    // When a stored response is used to satisfy a request without validation, a cache
    // MUST generate an Age header field
    if !cache.upstream_used() {
        let age = cache.cache_meta().age().as_secs();
        header.insert_header(http::header::AGE, age).unwrap();
    }

    /* Add chunked header to tell downstream to use chunked encoding
     * during the absent of content-length in h2 */
    if !no_body
        && !header.status.is_informational()
        && header.headers.get(http::header::CONTENT_LENGTH).is_none()
    {
        header
            .insert_header(http::header::TRANSFER_ENCODING, "chunked")
            .unwrap();
    }
    header
}

// https://datatracker.ietf.org/doc/html/rfc7233#section-3
pub(crate) mod range_filter {
    use super::*;
    use http::header::*;
    use std::ops::Range;

    // parse bytes into usize, ignores specific error
    fn parse_number(input: &[u8]) -> Option<usize> {
        str::from_utf8(input).ok()?.parse().ok()
    }

    fn parse_range_header(range: &[u8], content_length: usize) -> RangeType {
        use regex::Regex;

        // single byte range only for now
        // https://datatracker.ietf.org/doc/html/rfc7233#section-2.1
        // https://datatracker.ietf.org/doc/html/rfc7233#appendix-C: case-insensitive
        static RE_SINGLE_RANGE: Lazy<Regex> =
            Lazy::new(|| Regex::new(r"(?i)bytes=(?P<start>\d*)-(?P<end>\d*)").unwrap());

        // ignore invalid range header
        let Ok(range_str) = str::from_utf8(range) else {
            return RangeType::None;
        };

        let Some(captured) = RE_SINGLE_RANGE.captures(range_str) else {
            return RangeType::None;
        };
        let maybe_start = captured
            .name("start")
            .and_then(|s| s.as_str().parse::<usize>().ok());
        let end = captured
            .name("end")
            .and_then(|s| s.as_str().parse::<usize>().ok());

        if let Some(start) = maybe_start {
            if start >= content_length {
                RangeType::Invalid
            } else {
                // open-ended range should end at the last byte
                // over sized end is allow but ignored
                // range end is inclusive
                let end = std::cmp::min(end.unwrap_or(content_length - 1), content_length - 1) + 1;
                if end <= start {
                    RangeType::Invalid
                } else {
                    RangeType::new_single(start, end)
                }
            }
        } else {
            // start is empty, this changes the meaning of the value of `end`
            // Now it means to read the last `end` bytes
            if let Some(end) = end {
                if content_length >= end {
                    RangeType::new_single(content_length - end, content_length)
                } else {
                    // over sized end is allow but ignored
                    RangeType::new_single(0, content_length)
                }
            } else {
                // both empty/invalid
                RangeType::Invalid
            }
        }
    }
    #[test]
    fn test_parse_range() {
        assert_eq!(
            parse_range_header(b"bytes=0-1", 10),
            RangeType::new_single(0, 2)
        );
        assert_eq!(
            parse_range_header(b"bYTes=0-9", 10),
            RangeType::new_single(0, 10)
        );
        assert_eq!(
            parse_range_header(b"bytes=0-12", 10),
            RangeType::new_single(0, 10)
        );
        assert_eq!(
            parse_range_header(b"bytes=0-", 10),
            RangeType::new_single(0, 10)
        );
        assert_eq!(parse_range_header(b"bytes=2-1", 10), RangeType::Invalid);
        assert_eq!(parse_range_header(b"bytes=10-11", 10), RangeType::Invalid);
        assert_eq!(
            parse_range_header(b"bytes=-2", 10),
            RangeType::new_single(8, 10)
        );
        assert_eq!(
            parse_range_header(b"bytes=-12", 10),
            RangeType::new_single(0, 10)
        );
        assert_eq!(parse_range_header(b"bytes=-", 10), RangeType::Invalid);
        assert_eq!(parse_range_header(b"bytes=", 10), RangeType::None);
    }

    #[derive(Debug, Eq, PartialEq, Clone)]
    pub enum RangeType {
        None,
        Single(Range<usize>),
        // TODO: multi-range
        Invalid,
    }

    impl RangeType {
        fn new_single(start: usize, end: usize) -> Self {
            RangeType::Single(Range { start, end })
        }
    }

    // TODO: if-range

    // single range for now
    pub fn range_header_filter(req: &RequestHeader, resp: &mut ResponseHeader) -> RangeType {
        // The Range header field is evaluated after evaluating the precondition
        // header fields defined in [RFC7232], and only if the result in absence
        // of the Range header field would be a 200 (OK) response
        if resp.status != StatusCode::OK {
            return RangeType::None;
        }

        // "A server MUST ignore a Range header field received with a request method other than GET."
        if req.method != http::Method::GET && req.method != http::Method::HEAD {
            return RangeType::None;
        }

        let Some(range_header) = req.headers.get(RANGE) else {
            return RangeType::None;
        };

        // Content-Length is not required by RFC but it is what nginx does and easier to implement
        // with this header present.
        let Some(content_length_bytes) = resp.headers.get(CONTENT_LENGTH) else {
            return RangeType::None;
        };
        // bail on invalid content length
        let Some(content_length) = parse_number(content_length_bytes.as_bytes()) else {
            return RangeType::None;
        };

        // TODO: we can also check Accept-Range header from resp. Nginx gives uses the option
        // see proxy_force_ranges

        let range_type = parse_range_header(range_header.as_bytes(), content_length);

        match &range_type {
            RangeType::None => { /* nothing to do*/ }
            RangeType::Single(r) => {
                // 206 response
                resp.set_status(StatusCode::PARTIAL_CONTENT).unwrap();
                resp.insert_header(&CONTENT_LENGTH, r.end - r.start)
                    .unwrap();
                resp.insert_header(
                    &CONTENT_RANGE,
                    format!("bytes {}-{}/{content_length}", r.start, r.end - 1), // range end is inclusive
                )
                .unwrap()
            }
            RangeType::Invalid => {
                // 416 response
                resp.set_status(StatusCode::RANGE_NOT_SATISFIABLE).unwrap();
                // empty body for simplicity
                resp.insert_header(&CONTENT_LENGTH, HeaderValue::from_static("0"))
                    .unwrap();
                // TODO: remove other headers like content-encoding
                resp.remove_header(&CONTENT_TYPE);
                resp.insert_header(&CONTENT_RANGE, format!("bytes */{content_length}"))
                    .unwrap()
            }
        }

        range_type
    }

    #[test]
    fn test_range_filter() {
        fn gen_req() -> RequestHeader {
            RequestHeader::build(http::Method::GET, b"/", Some(1)).unwrap()
        }
        fn gen_resp() -> ResponseHeader {
            let mut resp = ResponseHeader::build(200, Some(1)).unwrap();
            resp.append_header("Content-Length", "10").unwrap();
            resp
        }

        // no range
        let req = gen_req();
        let mut resp = gen_resp();
        assert_eq!(RangeType::None, range_header_filter(&req, &mut resp));
        assert_eq!(resp.status.as_u16(), 200);

        // regular range
        let mut req = gen_req();
        req.insert_header("Range", "bytes=0-1").unwrap();
        let mut resp = gen_resp();
        assert_eq!(
            RangeType::new_single(0, 2),
            range_header_filter(&req, &mut resp)
        );
        assert_eq!(resp.status.as_u16(), 206);
        assert_eq!(resp.headers.get("content-length").unwrap().as_bytes(), b"2");
        assert_eq!(
            resp.headers.get("content-range").unwrap().as_bytes(),
            b"bytes 0-1/10"
        );

        // bad range
        let mut req = gen_req();
        req.insert_header("Range", "bytes=1-0").unwrap();
        let mut resp = gen_resp();
        assert_eq!(RangeType::Invalid, range_header_filter(&req, &mut resp));
        assert_eq!(resp.status.as_u16(), 416);
        assert_eq!(resp.headers.get("content-length").unwrap().as_bytes(), b"0");
        assert_eq!(
            resp.headers.get("content-range").unwrap().as_bytes(),
            b"bytes */10"
        );
    }

    pub struct RangeBodyFilter {
        range: RangeType,
        current: usize,
    }

    impl RangeBodyFilter {
        pub fn new() -> Self {
            RangeBodyFilter {
                range: RangeType::None,
                current: 0,
            }
        }

        pub fn set(&mut self, range: RangeType) {
            self.range = range;
        }

        pub fn filter_body(&mut self, data: Option<Bytes>) -> Option<Bytes> {
            match &self.range {
                RangeType::None => data,
                RangeType::Invalid => None,
                RangeType::Single(r) => {
                    let current = self.current;
                    self.current += data.as_ref().map_or(0, |d| d.len());
                    data.and_then(|d| Self::filter_range_data(r.start, r.end, current, d))
                }
            }
        }

        fn filter_range_data(
            start: usize,
            end: usize,
            current: usize,
            data: Bytes,
        ) -> Option<Bytes> {
            if current + data.len() < start || current >= end {
                // if the current data is out side the desired range, just drop the data
                None
            } else if current >= start && current + data.len() <= end {
                // all data is within the slice
                Some(data)
            } else {
                // data:  current........current+data.len()
                // range: start...........end
                let slice_start = start.saturating_sub(current);
                let slice_end = std::cmp::min(data.len(), end - current);
                Some(data.slice(slice_start..slice_end))
            }
        }
    }

    #[test]
    fn test_range_body_filter() {
        let mut body_filter = RangeBodyFilter::new();
        assert_eq!(body_filter.filter_body(Some("123".into())).unwrap(), "123");

        let mut body_filter = RangeBodyFilter::new();
        body_filter.set(RangeType::Invalid);
        assert!(body_filter.filter_body(Some("123".into())).is_none());

        let mut body_filter = RangeBodyFilter::new();
        body_filter.set(RangeType::new_single(0, 1));
        assert_eq!(body_filter.filter_body(Some("012".into())).unwrap(), "0");
        assert!(body_filter.filter_body(Some("345".into())).is_none());

        let mut body_filter = RangeBodyFilter::new();
        body_filter.set(RangeType::new_single(4, 6));
        assert!(body_filter.filter_body(Some("012".into())).is_none());
        assert_eq!(body_filter.filter_body(Some("345".into())).unwrap(), "45");
        assert!(body_filter.filter_body(Some("678".into())).is_none());

        let mut body_filter = RangeBodyFilter::new();
        body_filter.set(RangeType::new_single(1, 7));
        assert_eq!(body_filter.filter_body(Some("012".into())).unwrap(), "12");
        assert_eq!(body_filter.filter_body(Some("345".into())).unwrap(), "345");
        assert_eq!(body_filter.filter_body(Some("678".into())).unwrap(), "6");
    }
}

// a state machine for proxy logic to tell when to use cache in the case of
// miss/revalidation/error.
#[derive(Debug)]
pub(crate) enum ServeFromCache {
    Off,             // not using cache
    CacheHeader,     // should serve cache header
    CacheHeaderOnly, // should serve cache header
    CacheBody,       // should serve cache body
    CacheHeaderMiss, // should serve cache header but upstream response should be admitted to cache
    CacheBodyMiss,   // should serve cache body but upstream response should be admitted to cache
    Done,            // should serve cache body
}

impl ServeFromCache {
    pub fn new() -> Self {
        Self::Off
    }

    pub fn is_on(&self) -> bool {
        !matches!(self, Self::Off)
    }

    pub fn is_miss(&self) -> bool {
        matches!(self, Self::CacheHeaderMiss | Self::CacheBodyMiss)
    }

    pub fn is_miss_header(&self) -> bool {
        matches!(self, Self::CacheHeaderMiss)
    }

    pub fn is_miss_body(&self) -> bool {
        matches!(self, Self::CacheBodyMiss)
    }

    pub fn should_discard_upstream(&self) -> bool {
        self.is_on() && !self.is_miss()
    }

    pub fn should_send_to_downstream(&self) -> bool {
        !self.is_on()
    }

    pub fn enable(&mut self) {
        *self = Self::CacheHeader;
    }

    pub fn enable_miss(&mut self) {
        if !self.is_on() {
            *self = Self::CacheHeaderMiss;
        }
    }

    pub fn enable_header_only(&mut self) {
        match self {
            Self::CacheBody => *self = Self::Done, // TODO: make sure no body is read yet
            _ => *self = Self::CacheHeaderOnly,
        }
    }

    // This function is (best effort) cancel-safe to be used in select
    pub async fn next_http_task(&mut self, cache: &mut HttpCache) -> Result<HttpTask> {
        if !cache.enabled() {
            // Cache is disabled due to internal error
            // TODO: if nothing is sent to eyeball yet, figure out a way to recovery by
            // fetching from upstream
            return Error::e_explain(InternalError, "Cache disabled");
        }
        match self {
            Self::Off => panic!("ProxyUseCache not enabled"),
            Self::CacheHeader => {
                *self = Self::CacheBody;
                Ok(HttpTask::Header(cache_hit_header(cache), false)) // false for now
            }
            Self::CacheHeaderMiss => {
                *self = Self::CacheBodyMiss;
                Ok(HttpTask::Header(cache_hit_header(cache), false)) // false for now
            }
            Self::CacheHeaderOnly => {
                *self = Self::Done;
                Ok(HttpTask::Header(cache_hit_header(cache), true))
            }
            Self::CacheBody => {
                if let Some(b) = cache.hit_handler().read_body().await? {
                    Ok(HttpTask::Body(Some(b), false)) // false for now
                } else {
                    *self = Self::Done;
                    Ok(HttpTask::Done)
                }
            }
            Self::CacheBodyMiss => {
                // safety: called of enable_miss() call it only if the async_body_reader exist
                if let Some(b) = cache.miss_body_reader().unwrap().read_body().await? {
                    Ok(HttpTask::Body(Some(b), false)) // false for now
                } else {
                    *self = Self::Done;
                    Ok(HttpTask::Done)
                }
            }
            Self::Done => Ok(HttpTask::Done),
        }
    }
}