Only invoke a given cached partial once

Note that this is backed by a LRU cache (which we soon shall see more usage of), so if you're a heavy user of cached partials it may be evicted and
refreshed if needed. But in most cases every partial is only invoked once.

This commit also adds a timeout (the global `timeout` config option) to make infinite recursion in partials
easier to reason about.

```
name              old time/op    new time/op    delta
IncludeCached-10    8.92ms ± 0%    8.48ms ± 1%   -4.87%  (p=0.016 n=4+5)

name              old alloc/op   new alloc/op   delta
IncludeCached-10    6.65MB ± 0%    5.17MB ± 0%  -22.32%  (p=0.002 n=6+6)

name              old allocs/op  new allocs/op  delta
IncludeCached-10      117k ± 0%       71k ± 0%  -39.44%  (p=0.002 n=6+6)
```

Closes #4086
Updates #9588

3 files changed, 128 insertions, 1 deletions

diff --git a/identity/identity.go b/identity/identity.go
index 9236f0876..033409b80 100644
--- a/identity/identity.go
+++ b/identity/identity.go
@@ -1,3 +1,16 @@
+// Copyright 2023 The Hugo Authors. All rights reserved.
+//
+// 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.
+
 package identity
 
 import (
@@ -107,7 +120,7 @@ func (id KeyValueIdentity) Name() string {
 	return id.Key
 }
 
-// Provider provides the hashable Identity.
+// Provider provides the comparable Identity.
 type Provider interface {
 	// GetIdentity is for internal use.
 	GetIdentity() Identity
diff --git a/identity/identityhash.go b/identity/identityhash.go
new file mode 100644
index 000000000..ef7b5afa7
--- /dev/null
+++ b/identity/identityhash.go
@@ -0,0 +1,69 @@
+// Copyright 2023 The Hugo Authors. All rights reserved.
+//
+// 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.
+
+package identity
+
+import (
+	"strconv"
+
+	"github.com/mitchellh/hashstructure"
+)
+
+// HashString returns a hash from the given elements.
+// It will panic if the hash cannot be calculated.
+// Note that this hash should be used primarily for identity, not for change detection as
+// it in the more complex values (e.g. Page) will not hash the full content.
+func HashString(vs ...any) string {
+	hash := HashUint64(vs...)
+	return strconv.FormatUint(hash, 10)
+}
+
+// HashUint64 returns a hash from the given elements.
+// It will panic if the hash cannot be calculated.
+// Note that this hash should be used primarily for identity, not for change detection as
+// it in the more complex values (e.g. Page) will not hash the full content.
+func HashUint64(vs ...any) uint64 {
+	var o any
+	if len(vs) == 1 {
+		o = toHashable(vs[0])
+	} else {
+		elements := make([]any, len(vs))
+		for i, e := range vs {
+			elements[i] = toHashable(e)
+		}
+		o = elements
+	}
+
+	hash, err := hashstructure.Hash(o, nil)
+	if err != nil {
+		panic(err)
+	}
+	return hash
+}
+
+type keyer interface {
+	Key() string
+}
+
+// For structs, hashstructure.Hash only works on the exported fields,
+// so rewrite the input slice for known identity types.
+func toHashable(v any) any {
+	switch t := v.(type) {
+	case Provider:
+		return t.GetIdentity()
+	case keyer:
+		return t.Key()
+	default:
+		return v
+	}
+}
diff --git a/identity/identityhash_test.go b/identity/identityhash_test.go
new file mode 100644
index 000000000..378c0160d
--- /dev/null
+++ b/identity/identityhash_test.go
@@ -0,0 +1,45 @@
+// Copyright 2023 The Hugo Authors. All rights reserved.
+//
+// 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.
+
+package identity
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestHashString(t *testing.T) {
+	c := qt.New(t)
+
+	c.Assert(HashString("a", "b"), qt.Equals, "2712570657419664240")
+	c.Assert(HashString("ab"), qt.Equals, "590647783936702392")
+
+	var vals []any = []any{"a", "b", tstKeyer{"c"}}
+
+	c.Assert(HashString(vals...), qt.Equals, "12599484872364427450")
+	c.Assert(vals[2], qt.Equals, tstKeyer{"c"})
+
+}
+
+type tstKeyer struct {
+	key string
+}
+
+func (t tstKeyer) Key() string {
+	return t.key
+}
+
+func (t tstKeyer) String() string {
+	return "key: " + t.key
+}