Refactor AppURL usage (#30885)

Fix #30883
Fix #29591

---------

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

modules/httplib/url.go

@@ -4,6 +4,8 @@
 package httplib
 
 import (
+	"context"
+	"net/http"
 	"net/url"
 	"strings"
 
@@ -11,6 +13,10 @@ import (
 	"code.gitea.io/gitea/modules/util"
 )
 
+type RequestContextKeyStruct struct{}
+
+var RequestContextKey = RequestContextKeyStruct{}
+
 func urlIsRelative(s string, u *url.URL) bool {
 	// Unfortunately browsers consider a redirect Location with preceding "//", "\\", "/\" and "\/" as meaning redirect to "http(s)://REST_OF_PATH"
 	// Therefore we should ignore these redirect locations to prevent open redirects
@@ -26,7 +32,56 @@ func IsRelativeURL(s string) bool {
 	return err == nil && urlIsRelative(s, u)
 }
 
-func IsCurrentGiteaSiteURL(s string) bool {
+func guessRequestScheme(req *http.Request, def string) string {
+	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto
+	if s := req.Header.Get("X-Forwarded-Proto"); s != "" {
+		return s
+	}
+	if s := req.Header.Get("X-Forwarded-Protocol"); s != "" {
+		return s
+	}
+	if s := req.Header.Get("X-Url-Scheme"); s != "" {
+		return s
+	}
+	if s := req.Header.Get("Front-End-Https"); s != "" {
+		return util.Iif(s == "on", "https", "http")
+	}
+	if s := req.Header.Get("X-Forwarded-Ssl"); s != "" {
+		return util.Iif(s == "on", "https", "http")
+	}
+	return def
+}
+
+func guessForwardedHost(req *http.Request) string {
+	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Host
+	return req.Header.Get("X-Forwarded-Host")
+}
+
+// GuessCurrentAppURL tries to guess the current full URL by http headers. It always has a '/' suffix, exactly the same as setting.AppURL
+func GuessCurrentAppURL(ctx context.Context) string {
+	req, ok := ctx.Value(RequestContextKey).(*http.Request)
+	if !ok {
+		return setting.AppURL
+	}
+	if host := guessForwardedHost(req); host != "" {
+		// if it is behind a reverse proxy, use "https" as default scheme in case the site admin forgets to set the correct forwarded-protocol headers
+		return guessRequestScheme(req, "https") + "://" + host + setting.AppSubURL + "/"
+	} else if req.Host != "" {
+		// if it is not behind a reverse proxy, use the scheme from config options, meanwhile use "https" as much as possible
+		defaultScheme := util.Iif(setting.Protocol == "http", "http", "https")
+		return guessRequestScheme(req, defaultScheme) + "://" + req.Host + setting.AppSubURL + "/"
+	}
+	return setting.AppURL
+}
+
+func MakeAbsoluteURL(ctx context.Context, s string) string {
+	if IsRelativeURL(s) {
+		return GuessCurrentAppURL(ctx) + strings.TrimPrefix(s, "/")
+	}
+	return s
+}
+
+func IsCurrentGiteaSiteURL(ctx context.Context, s string) bool {
 	u, err := url.Parse(s)
 	if err != nil {
 		return false
@@ -45,5 +100,6 @@ func IsCurrentGiteaSiteURL(s string) bool {
 	if u.Path == "" {
 		u.Path = "/"
 	}
-	return strings.HasPrefix(strings.ToLower(u.String()), strings.ToLower(setting.AppURL))
+	urlLower := strings.ToLower(u.String())
+	return strings.HasPrefix(urlLower, strings.ToLower(setting.AppURL)) || strings.HasPrefix(urlLower, strings.ToLower(GuessCurrentAppURL(ctx)))
 }