refactor: implement mcaptcha client and add comments/tests (#38561)

End users still need it, so it's better to make it maintainable with
tests and OOM-safe.

---------

Co-authored-by: silverwind <me@silverwind.io>

modules/mcaptcha/mcaptcha.go

@@ -1,26 +1,71 @@
 // Copyright 2022 The Gitea Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
+// https://github.com/go-gitea/gitea/pull/37492#issuecomment-4355482585
+// End users need it "since it is the only one open-source option in Gitea besides the image captcha"
+
 package mcaptcha
 
 import (
+	"bytes"
 	"context"
 	"fmt"
+	"io"
+	"net/http"
+	"strings"
 
+	"gitea.dev/modules/json"
 	"gitea.dev/modules/setting"
-
-	"codeberg.org/gusted/mcaptcha"
+	"gitea.dev/modules/util"
 )
 
 func Verify(ctx context.Context, token string) (bool, error) {
-	valid, err := mcaptcha.Verify(ctx, &mcaptcha.VerifyOpts{
-		InstanceURL: setting.Service.McaptchaURL,
-		Sitekey:     setting.Service.McaptchaSitekey,
-		Secret:      setting.Service.McaptchaSecret,
-		Token:       token,
-	})
-	if err != nil {
-		return false, fmt.Errorf("wasn't able to verify mCaptcha: %w", err)
+	c := &client{
+		ServerURL: setting.Service.McaptchaURL,
+		SiteKey:   setting.Service.McaptchaSitekey,
+		Secret:    setting.Service.McaptchaSecret,
+		Token:     token,
 	}
-	return valid, nil
+	return c.Verify(ctx)
+}
+
+type client struct {
+	ServerURL string
+
+	Secret  string
+	SiteKey string
+	Token   string
+}
+
+func (c *client) Verify(ctx context.Context) (bool, error) {
+	verifyURL := strings.TrimSuffix(c.ServerURL, "/") + "/api/v1/pow/siteverify"
+	reqParams := map[string]string{"secret": c.Secret, "key": c.SiteKey, "token": c.Token}
+	reqBody, _ := json.Marshal(reqParams)
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, verifyURL, bytes.NewReader(reqBody))
+	if err != nil {
+		return false, fmt.Errorf("unable to create verify request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	res, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return false, fmt.Errorf("unable to complete verify request: %w", err)
+	}
+	defer res.Body.Close()
+
+	respContent, err := io.ReadAll(io.LimitReader(res.Body, 16*1024))
+	if err != nil {
+		return false, fmt.Errorf("unable to read response body: %w", err)
+	}
+	if res.StatusCode != http.StatusOK {
+		return false, fmt.Errorf("unexpected response status %d: %q", res.StatusCode, util.TruncateRunes(util.UnsafeBytesToString(respContent), 100))
+	}
+
+	var resp struct {
+		Valid bool `json:"valid"`
+	}
+	err = json.Unmarshal(respContent, &resp)
+	if err != nil {
+		return false, fmt.Errorf("unable to decode response: %w", err)
+	}
+	return resp.Valid, nil
 }