Start using template context function (#26254)

Before:

* `{{.locale.Tr ...}}`
* `{{$.locale.Tr ...}}`
* `{{$.root.locale.Tr ...}}`
* `{{template "sub" .}}`
* `{{template "sub" (dict "locale" $.locale)}}`
* `{{template "sub" (dict "root" $)}}`
* .....

With context function: only need to `{{ctx.Locale.Tr ...}}`

The "ctx" could be considered as a super-global variable for all
templates including sub-templates.


To avoid potential risks (any bug in the template context function
package), this PR only starts using "ctx" in "head.tmpl" and
"footer.tmpl" and it has a "DataRaceCheck". If there is anything wrong,
the code can be fixed or reverted easily.

3 files changed, 63 insertions, 5 deletions

diff --git a/modules/context/context.go b/modules/context/context.go
index de0518a1d2..b75ba9ab67 100644
--- a/modules/context/context.go
+++ b/modules/context/context.go
@@ -5,6 +5,7 @@
 package context
 
 import (
+	"context"
 	"html"
 	"html/template"
 	"io"
@@ -31,14 +32,16 @@ import (
 
 // Render represents a template render
 type Render interface {
-	TemplateLookup(tmpl string) (templates.TemplateExecutor, error)
-	HTML(w io.Writer, status int, name string, data any) error
+	TemplateLookup(tmpl string, templateCtx context.Context) (templates.TemplateExecutor, error)
+	HTML(w io.Writer, status int, name string, data any, templateCtx context.Context) error
 }
 
 // Context represents context of a request.
 type Context struct {
 	*Base
 
+	TemplateContext TemplateContext
+
 	Render   Render
 	PageData map[string]any // data used by JavaScript modules in one page, it's `window.config.pageData`
 
@@ -60,6 +63,8 @@ type Context struct {
 	Package *Package
 }
 
+type TemplateContext map[string]any
+
 func init() {
 	web.RegisterResponseStatusProvider[*Context](func(req *http.Request) web_types.ResponseStatusProvider {
 		return req.Context().Value(WebContextKey).(*Context)
@@ -133,8 +138,12 @@ func Contexter() func(next http.Handler) http.Handler {
 			}
 			defer baseCleanUp()
 
+			// TODO: "install.go" also shares the same logic, which should be refactored to a general function
+			ctx.TemplateContext = NewTemplateContext(ctx)
+			ctx.TemplateContext["Locale"] = ctx.Locale
+
 			ctx.Data.MergeFrom(middleware.CommonTemplateContextData())
-			ctx.Data["Context"] = &ctx
+			ctx.Data["Context"] = ctx // TODO: use "ctx" in template and remove this
 			ctx.Data["CurrentURL"] = setting.AppSubURL + req.URL.RequestURI()
 			ctx.Data["Link"] = ctx.Link
 			ctx.Data["locale"] = ctx.Locale
diff --git a/modules/context/context_response.go b/modules/context/context_response.go
index 9dc6d1fc0e..5729865561 100644
--- a/modules/context/context_response.go
+++ b/modules/context/context_response.go
@@ -75,7 +75,7 @@ func (ctx *Context) HTML(status int, name base.TplName) {
 		return strconv.FormatInt(time.Since(tmplStartTime).Nanoseconds()/1e6, 10) + "ms"
 	}
 
-	err := ctx.Render.HTML(ctx.Resp, status, string(name), ctx.Data)
+	err := ctx.Render.HTML(ctx.Resp, status, string(name), ctx.Data, ctx.TemplateContext)
 	if err == nil {
 		return
 	}
@@ -93,7 +93,7 @@ func (ctx *Context) HTML(status int, name base.TplName) {
 // RenderToString renders the template content to a string
 func (ctx *Context) RenderToString(name base.TplName, data map[string]any) (string, error) {
 	var buf strings.Builder
-	err := ctx.Render.HTML(&buf, http.StatusOK, string(name), data)
+	err := ctx.Render.HTML(&buf, http.StatusOK, string(name), data, ctx.TemplateContext)
 	return buf.String(), err
 }
 
diff --git a/modules/context/context_template.go b/modules/context/context_template.go
new file mode 100644
index 0000000000..ba90fc170a
--- /dev/null
+++ b/modules/context/context_template.go
@@ -0,0 +1,49 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package context
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"code.gitea.io/gitea/modules/log"
+)
+
+var _ context.Context = TemplateContext(nil)
+
+func NewTemplateContext(ctx context.Context) TemplateContext {
+	return TemplateContext{"_ctx": ctx}
+}
+
+func (c TemplateContext) parentContext() context.Context {
+	return c["_ctx"].(context.Context)
+}
+
+func (c TemplateContext) Deadline() (deadline time.Time, ok bool) {
+	return c.parentContext().Deadline()
+}
+
+func (c TemplateContext) Done() <-chan struct{} {
+	return c.parentContext().Done()
+}
+
+func (c TemplateContext) Err() error {
+	return c.parentContext().Err()
+}
+
+func (c TemplateContext) Value(key any) any {
+	return c.parentContext().Value(key)
+}
+
+// DataRaceCheck checks whether the template context function "ctx()" returns the consistent context
+// as the current template's rendering context (request context), to help to find data race issues as early as possible.
+// When the code is proven to be correct and stable, this function should be removed.
+func (c TemplateContext) DataRaceCheck(dataCtx context.Context) (string, error) {
+	if c.parentContext() != dataCtx {
+		log.Error("TemplateContext.DataRaceCheck: parent context mismatch\n%s", log.Stack(2))
+		return "", errors.New("parent context mismatch")
+	}
+	return "", nil
+}