admin: support ETag on config endpoints (#4579)

* admin: support ETags

* support etags

Co-authored-by: Matt Holt <[email protected]>

3 files changed, 105 insertions, 7 deletions

diff --git a/admin.go b/admin.go
index 214b734d8..e37caf83a 100644
--- a/admin.go
+++ b/admin.go
@@ -21,10 +21,13 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/base64"
+	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"expvar"
 	"fmt"
+	"hash"
+	"hash/fnv"
 	"io"
 	"net"
 	"net/http"
@@ -894,16 +897,30 @@ func (h adminHandler) originAllowed(origin *url.URL) bool {
 	return false
 }
 
+// etagHasher returns a the hasher we used on the config to both
+// produce and verify ETags.
+func etagHasher() hash.Hash32 { return fnv.New32a() }
+
 func handleConfig(w http.ResponseWriter, r *http.Request) error {
 	switch r.Method {
 	case http.MethodGet:
 		w.Header().Set("Content-Type", "application/json")
-
-		err := readConfig(r.URL.Path, w)
+		// Set the ETag as a trailer header.
+		// The alternative is to write the config to a buffer, and
+		// then hash that.
+		w.Header().Set("Trailer", "ETag")
+
+		hash := etagHasher()
+		configWriter := io.MultiWriter(w, hash)
+		err := readConfig(r.URL.Path, configWriter)
 		if err != nil {
 			return APIError{HTTPStatus: http.StatusBadRequest, Err: err}
 		}
 
+		// we could consider setting up a sync.Pool for the summed
+		// hashes to reduce GC pressure.
+		w.Header().Set("ETag", r.URL.Path+" "+hex.EncodeToString(hash.Sum(nil)))
+
 		return nil
 
 	case http.MethodPost,
@@ -937,7 +954,7 @@ func handleConfig(w http.ResponseWriter, r *http.Request) error {
 
 		forceReload := r.Header.Get("Cache-Control") == "must-revalidate"
 
-		err := changeConfig(r.Method, r.URL.Path, body, forceReload)
+		err := changeConfig(r.Method, r.URL.Path, body, r.Header.Get("If-Match"), forceReload)
 		if err != nil && !errors.Is(err, errSameConfig) {
 			return err
 		}
diff --git a/admin_test.go b/admin_test.go
index 608a32c67..32f20c627 100644
--- a/admin_test.go
+++ b/admin_test.go
@@ -15,7 +15,9 @@
 package caddy
 
 import (
+	"encoding/hex"
 	"encoding/json"
+	"net/http"
 	"reflect"
 	"sync"
 	"testing"
@@ -139,10 +141,57 @@ func TestLoadConcurrent(t *testing.T) {
 			wg.Done()
 		}()
 	}
-
 	wg.Wait()
 }
 
+type fooModule struct {
+	IntField int
+	StrField string
+}
+
+func (fooModule) CaddyModule() ModuleInfo {
+	return ModuleInfo{
+		ID:  "foo",
+		New: func() Module { return new(fooModule) },
+	}
+}
+func (fooModule) Start() error { return nil }
+func (fooModule) Stop() error  { return nil }
+
+func TestETags(t *testing.T) {
+	RegisterModule(fooModule{})
+
+	if err := Load([]byte(`{"apps": {"foo": {"strField": "abc", "intField": 0}}}`), true); err != nil {
+		t.Fatalf("loading: %s", err)
+	}
+
+	const key = "/" + rawConfigKey + "/apps/foo"
+
+	// try update the config with the wrong etag
+	err := changeConfig(http.MethodPost, key, []byte(`{"strField": "abc", "intField": 1}}`), "/"+rawConfigKey+" not_an_etag", false)
+	if apiErr, ok := err.(APIError); !ok || apiErr.HTTPStatus != http.StatusPreconditionFailed {
+		t.Fatalf("expected precondition failed; got %v", err)
+	}
+
+	// get the etag
+	hash := etagHasher()
+	if err := readConfig(key, hash); err != nil {
+		t.Fatalf("reading: %s", err)
+	}
+
+	// do the same update with the correct key
+	err = changeConfig(http.MethodPost, key, []byte(`{"strField": "abc", "intField": 1}`), key+" "+hex.EncodeToString(hash.Sum(nil)), false)
+	if err != nil {
+		t.Fatalf("expected update to work; got %v", err)
+	}
+
+	// now try another update. The hash should no longer match and we should get precondition failed
+	err = changeConfig(http.MethodPost, key, []byte(`{"strField": "abc", "intField": 2}`), key+" "+hex.EncodeToString(hash.Sum(nil)), false)
+	if apiErr, ok := err.(APIError); !ok || apiErr.HTTPStatus != http.StatusPreconditionFailed {
+		t.Fatalf("expected precondition failed; got %v", err)
+	}
+}
+
 func BenchmarkLoad(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Load(testCfg, true)
diff --git a/caddy.go b/caddy.go
index d15509228..0c6dfcd00 100644
--- a/caddy.go
+++ b/caddy.go
@@ -17,6 +17,7 @@ package caddy
 import (
 	"bytes"
 	"context"
+	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -111,7 +112,7 @@ func Load(cfgJSON []byte, forceReload bool) error {
 		}
 	}()
 
-	err := changeConfig(http.MethodPost, "/"+rawConfigKey, cfgJSON, forceReload)
+	err := changeConfig(http.MethodPost, "/"+rawConfigKey, cfgJSON, "", forceReload)
 	if errors.Is(err, errSameConfig) {
 		err = nil // not really an error
 	}
@@ -125,7 +126,12 @@ func Load(cfgJSON []byte, forceReload bool) error {
 // occur unless forceReload is true. If the config is unchanged and not
 // forcefully reloaded, then errConfigUnchanged This function is safe for
 // concurrent use.
-func changeConfig(method, path string, input []byte, forceReload bool) error {
+// The ifMatchHeader can optionally be given a string of the format:
+//    "<path> <hash>"
+// where <path> is the absolute path in the config and <hash> is the expected hash of
+// the config at that path. If the hash in the ifMatchHeader doesn't match
+// the hash of the config, then an APIError with status 412 will be returned.
+func changeConfig(method, path string, input []byte, ifMatchHeader string, forceReload bool) error {
 	switch method {
 	case http.MethodGet,
 		http.MethodHead,
@@ -138,6 +144,32 @@ func changeConfig(method, path string, input []byte, forceReload bool) error {
 	currentCfgMu.Lock()
 	defer currentCfgMu.Unlock()
 
+	if ifMatchHeader != "" {
+		// read out the parts
+		parts := strings.Fields(ifMatchHeader)
+		if len(parts) != 2 {
+			return APIError{
+				HTTPStatus: http.StatusBadRequest,
+				Err:        fmt.Errorf("malformed If-Match header; expect format \"<path> <hash>\""),
+			}
+		}
+
+		// get the current hash of the config
+		// at the given path
+		hash := etagHasher()
+		err := unsyncedConfigAccess(http.MethodGet, parts[0], nil, hash)
+		if err != nil {
+			return err
+		}
+
+		if hex.EncodeToString(hash.Sum(nil)) != parts[1] {
+			return APIError{
+				HTTPStatus: http.StatusPreconditionFailed,
+				Err:        fmt.Errorf("If-Match header did not match current config hash"),
+			}
+		}
+	}
+
 	err := unsyncedConfigAccess(method, path, input, nil)
 	if err != nil {
 		return err
@@ -500,7 +532,7 @@ func finishSettingUp(ctx Context, cfg *Config) error {
 
 		runLoadedConfig := func(config []byte) error {
 			logger.Info("applying dynamically-loaded config")
-			err := changeConfig(http.MethodPost, "/"+rawConfigKey, config, false)
+			err := changeConfig(http.MethodPost, "/"+rawConfigKey, config, "", false)
 			if errors.Is(err, errSameConfig) {
 				return err
 			}