tpl/partials: Fix recently introduced deadlock in partials cache

The change in lock logic for `partialCached` in  0927cf739f was naive as it didn't consider cached partials calling other cached partials.

This changeset may look on the large side for this particular issue, but it pulls in part of a working branch, introducing `context.Context` in the template execution.

Note that the context is only partially implemented in this PR, but the upcoming use cases will, as one example, include having access to the top "dot" (e.g. `Page`) all the way down into partials and shortcodes etc.

The earlier benchmarks rerun against master:

```bash
name              old time/op    new time/op    delta
IncludeCached-10    13.6ms ± 2%    13.8ms ± 1%    ~     (p=0.343 n=4+4)

name              old alloc/op   new alloc/op   delta
IncludeCached-10    5.30MB ± 0%    5.35MB ± 0%  +0.96%  (p=0.029 n=4+4)

name              old allocs/op  new allocs/op  delta
IncludeCached-10     74.7k ± 0%     75.3k ± 0%  +0.77%  (p=0.029 n=4+4)
```

Fixes #9519

tpl/internal/go_templates/texttemplate/hugo_template.go

@@ -1,4 +1,4 @@
-// Copyright 2019 The Hugo Authors. All rights reserved.
+// Copyright 2022 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.
@@ -14,6 +14,7 @@
 package template
 
 import (
+	"context"
 	"io"
 	"reflect"
 
@@ -39,14 +40,15 @@ type Preparer interface {
 
 // ExecHelper allows some custom eval hooks.
 type ExecHelper interface {
-	GetFunc(tmpl Preparer, name string) (reflect.Value, bool)
-	GetMethod(tmpl Preparer, receiver reflect.Value, name string) (method reflect.Value, firstArg reflect.Value)
-	GetMapValue(tmpl Preparer, receiver, key reflect.Value) (reflect.Value, bool)
+	Init(ctx context.Context, tmpl Preparer)
+	GetFunc(ctx context.Context, tmpl Preparer, name string) (reflect.Value, reflect.Value, bool)
+	GetMethod(ctx context.Context, tmpl Preparer, receiver reflect.Value, name string) (method reflect.Value, firstArg reflect.Value)
+	GetMapValue(ctx context.Context, tmpl Preparer, receiver, key reflect.Value) (reflect.Value, bool)
 }
 
 // Executer executes a given template.
 type Executer interface {
-	Execute(p Preparer, wr io.Writer, data interface{}) error
+	ExecuteWithContext(ctx context.Context, p Preparer, wr io.Writer, data interface{}) error
 }
 
 type executer struct {
@@ -57,6 +59,48 @@ func NewExecuter(helper ExecHelper) Executer {
 	return &executer{helper: helper}
 }
 
+type (
+	dataContextKeyType    string
+	hasLockContextKeyType string
+)
+
+const (
+	// The data object passed to Execute or ExecuteWithContext gets stored with this key if not already set.
+	DataContextKey = dataContextKeyType("data")
+	// Used in partialCached to signal to nested templates that a lock is already taken.
+	HasLockContextKey = hasLockContextKeyType("hasLock")
+)
+
+// Note: The context is currently not fully implemeted in Hugo. This is a work in progress.
+func (t *executer) ExecuteWithContext(ctx context.Context, p Preparer, wr io.Writer, data interface{}) error {
+	tmpl, err := p.Prepare()
+	if err != nil {
+		return err
+	}
+
+	if v := ctx.Value(DataContextKey); v == nil {
+		ctx = context.WithValue(ctx, DataContextKey, data)
+	}
+
+	value, ok := data.(reflect.Value)
+	if !ok {
+		value = reflect.ValueOf(data)
+	}
+
+	state := &state{
+		ctx:    ctx,
+		helper: t.helper,
+		prep:   p,
+		tmpl:   tmpl,
+		wr:     wr,
+		vars:   []variable{{"$", value}},
+	}
+
+	t.helper.Init(ctx, p)
+
+	return tmpl.executeWithState(state, value)
+}
+
 func (t *executer) Execute(p Preparer, wr io.Writer, data interface{}) error {
 	tmpl, err := p.Prepare()
 	if err != nil {
@@ -77,7 +121,6 @@ func (t *executer) Execute(p Preparer, wr io.Writer, data interface{}) error {
 	}
 
 	return tmpl.executeWithState(state, value)
-
 }
 
 // Prepare returns a template ready for execution.
@@ -101,8 +144,9 @@ func (t *Template) executeWithState(state *state, value reflect.Value) (err erro
 // can execute in parallel.
 type state struct {
 	tmpl   *Template
-	prep   Preparer   // Added for Hugo.
-	helper ExecHelper // Added for Hugo.
+	ctx    context.Context // Added for Hugo. The orignal data context.
+	prep   Preparer        // Added for Hugo.
+	helper ExecHelper      // Added for Hugo.
 	wr     io.Writer
 	node   parse.Node // current node, for errors
 	vars   []variable // push-down stack of variable values.
@@ -114,10 +158,11 @@ func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd
 	name := node.Ident
 
 	var function reflect.Value
+	// Added for Hugo.
+	var first reflect.Value
 	var ok bool
 	if s.helper != nil {
-		// Added for Hugo.
-		function, ok = s.helper.GetFunc(s.prep, name)
+		function, first, ok = s.helper.GetFunc(s.ctx, s.prep, name)
 	}
 
 	if !ok {
@@ -127,6 +172,9 @@ func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd
 	if !ok {
 		s.errorf("%q is not a defined function", name)
 	}
+	if first != zero {
+		return s.evalCall(dot, function, cmd, name, args, final, first)
+	}
 	return s.evalCall(dot, function, cmd, name, args, final)
 }
 
@@ -159,7 +207,7 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
 	var first reflect.Value
 	var method reflect.Value
 	if s.helper != nil {
-		method, first = s.helper.GetMethod(s.prep, ptr, fieldName)
+		method, first = s.helper.GetMethod(s.ctx, s.prep, ptr, fieldName)
 	} else {
 		method = ptr.MethodByName(fieldName)
 	}
@@ -198,7 +246,7 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
 			var result reflect.Value
 			if s.helper != nil {
 				// Added for Hugo.
-				result, _ = s.helper.GetMapValue(s.prep, receiver, nameVal)
+				result, _ = s.helper.GetMapValue(s.ctx, s.prep, receiver, nameVal)
 			} else {
 				result = receiver.MapIndex(nameVal)
 			}

tpl/internal/go_templates/texttemplate/hugo_template_test.go

@@ -1,4 +1,4 @@
-// Copyright 2019 The Hugo Authors. All rights reserved.
+// Copyright 2022 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.
@@ -15,6 +15,7 @@ package template
 
 import (
 	"bytes"
+	"context"
 	"reflect"
 	"strings"
 	"testing"
@@ -35,24 +36,26 @@ func (t TestStruct) Hello2(arg1, arg2 string) string {
 	return arg1 + " " + arg2
 }
 
-type execHelper struct {
+type execHelper struct{}
+
+func (e *execHelper) Init(ctx context.Context, tmpl Preparer) {
 }
 
-func (e *execHelper) GetFunc(tmpl Preparer, name string) (reflect.Value, bool) {
+func (e *execHelper) GetFunc(ctx context.Context, tmpl Preparer, name string) (reflect.Value, reflect.Value, bool) {
 	if name == "print" {
-		return zero, false
+		return zero, zero, false
 	}
 	return reflect.ValueOf(func(s string) string {
 		return "hello " + s
-	}), true
+	}), zero, true
 }
 
-func (e *execHelper) GetMapValue(tmpl Preparer, m, key reflect.Value) (reflect.Value, bool) {
+func (e *execHelper) GetMapValue(ctx context.Context, tmpl Preparer, m, key reflect.Value) (reflect.Value, bool) {
 	key = reflect.ValueOf(strings.ToLower(key.String()))
 	return m.MapIndex(key), true
 }
 
-func (e *execHelper) GetMethod(tmpl Preparer, receiver reflect.Value, name string) (method reflect.Value, firstArg reflect.Value) {
+func (e *execHelper) GetMethod(ctx context.Context, tmpl Preparer, receiver reflect.Value, name string) (method reflect.Value, firstArg reflect.Value) {
 	if name != "Hello1" {
 		return zero, zero
 	}
@@ -78,12 +81,11 @@ Method: {{ .Hello1 "v1" }}
 	var b bytes.Buffer
 	data := TestStruct{S: "sv", M: map[string]string{"a": "av"}}
 
-	c.Assert(ex.Execute(templ, &b, data), qt.IsNil)
+	c.Assert(ex.ExecuteWithContext(context.Background(), templ, &b, data), qt.IsNil)
 	got := b.String()
 
 	c.Assert(got, qt.Contains, "foo")
 	c.Assert(got, qt.Contains, "hello hugo")
 	c.Assert(got, qt.Contains, "Map: av")
 	c.Assert(got, qt.Contains, "Method: v2 v1")
-
 }