fix(turnstile): route CAPTCHA verification through the configured proxy (#38412)

Fixes #38217

## Problem

Turnstile CAPTCHA verification uses `http.DefaultClient`, so the request
to `challenges.cloudflare.com` bypasses Gitea's configured HTTP proxy —
unlike other outbound HTTP clients such as the update checker
(`modules/updatechecker/update_checker.go`) and migrations. In
deployments where egress is only permitted through the configured proxy,
verification fails.

## Fix

Build the client with `proxy.Proxy()` as the transport proxy, mirroring
the update checker:

```go
func httpClient() *http.Client {
	return &http.Client{
		Transport: &http.Transport{
			Proxy: proxy.Proxy(),
		},
	}
}
```

The client is built per call (rather than a package-level var) because
`proxy.Proxy()` reads `setting.Proxy` when invoked; building it at
request time ensures it reflects the loaded settings. When no proxy is
configured, behavior is unchanged (`proxy.Proxy()` returns a no-op /
`http.ProxyFromEnvironment`).

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/turnstile/turnstile.go

@@ -12,9 +12,20 @@ import (
 	"strings"
 
 	"gitea.dev/modules/json"
+	"gitea.dev/modules/proxy"
 	"gitea.dev/modules/setting"
+	"gitea.dev/modules/util"
 )
 
+// httpClient returns an HTTP client that honors Gitea's proxy configuration.
+var httpClient = util.OnceValue[*http.Client]{
+	Func: func() *http.Client {
+		transport := http.DefaultTransport.(*http.Transport).Clone()
+		transport.Proxy = proxy.Proxy()
+		return &http.Client{Transport: transport}
+	},
+}
+
 // Response is the structure of JSON returned from API
 type Response struct {
 	Success     bool        `json:"success"`
@@ -40,7 +51,7 @@ func Verify(ctx context.Context, response string) (bool, error) {
 	}
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 
-	resp, err := http.DefaultClient.Do(req)
+	resp, err := httpClient.Value().Do(req)
 	if err != nil {
 		return false, fmt.Errorf("Failed to send CAPTCHA response: %w", err)
 	}

modules/turnstile/turnstile_test.go

@@ -0,0 +1,38 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package turnstile
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+
+	"gitea.dev/modules/setting"
+	"gitea.dev/modules/test"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestHTTPClientHonorsProxy(t *testing.T) {
+	proxyURL, err := url.Parse("http://proxy.example.com:3128")
+	require.NoError(t, err)
+
+	defer test.MockVariableValue(&setting.Proxy.Enabled, true)()
+	defer test.MockVariableValue(&setting.Proxy.ProxyURL, proxyURL.String())()
+	defer test.MockVariableValue(&setting.Proxy.ProxyURLFixed, proxyURL)()
+	defer test.MockVariableValue(&setting.Proxy.ProxyHosts, []string{"**"})()
+	httpClient.Reset()
+	transport, ok := httpClient.Value().Transport.(*http.Transport)
+	require.True(t, ok)
+	require.NotNil(t, transport.Proxy)
+
+	// The Turnstile verification request must be routed through the configured proxy.
+	req := httptest.NewRequest(http.MethodPost, "https://any.example.com", nil)
+	got, err := transport.Proxy(req)
+	require.NoError(t, err)
+	require.NotNil(t, got)
+	assert.Equal(t, proxyURL.String(), got.String())
+}

modules/util/sync.go

@@ -0,0 +1,51 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+type onceValueResult[T any] struct {
+	value T
+	panic any
+}
+
+// OnceValue is similar to Golang's "sync.OnceValue", but can be reset.
+type OnceValue[T any] struct {
+	Func func() T
+	mu   sync.Mutex
+	res  atomic.Pointer[onceValueResult[T]]
+}
+
+func (o *OnceValue[T]) Value() T {
+	res := o.res.Load()
+	if res == nil {
+		o.mu.Lock()
+		defer o.mu.Unlock()
+		res = o.res.Load()
+		if res == nil {
+			res = &onceValueResult[T]{}
+			defer func() {
+				res.panic = recover()
+				o.res.Store(res)
+				if res.panic != nil {
+					panic(res.panic)
+				}
+			}()
+			res.value = o.Func()
+		}
+	}
+	if res.panic != nil {
+		panic(res.panic)
+	}
+	return res.value
+}
+
+func (o *OnceValue[T]) Reset() {
+	o.mu.Lock()
+	defer o.mu.Unlock()
+	o.res.Store(nil)
+}

modules/util/sync_test.go

@@ -0,0 +1,49 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestOnceValue(t *testing.T) {
+	t.Run("RepeatCall", func(t *testing.T) {
+		callCount := 0
+		o := OnceValue[int]{Func: func() int {
+			callCount++
+			return 42
+		}}
+		assert.Equal(t, 42, o.Value())
+		assert.Equal(t, 42, o.Value())
+		assert.Equal(t, 1, callCount)
+		o.Reset()
+		assert.Equal(t, 42, o.Value())
+		assert.Equal(t, 2, callCount)
+		assert.Equal(t, 42, o.Value())
+		assert.Equal(t, 2, callCount)
+	})
+
+	t.Run("Panic", func(t *testing.T) {
+		callCount := 0
+		doPanic := true
+		o := OnceValue[int]{Func: func() int {
+			callCount++
+			if doPanic {
+				panic("some error")
+			}
+			return 42
+		}}
+		assert.PanicsWithValue(t, "some error", func() { o.Value() })
+		assert.PanicsWithValue(t, "some error", func() { o.Value() })
+		assert.Equal(t, 1, callCount)
+		doPanic = false
+		o.Reset()
+		assert.Equal(t, 42, o.Value())
+		assert.Equal(t, 2, callCount)
+		assert.Equal(t, 42, o.Value())
+		assert.Equal(t, 2, callCount)
+	})
+}