Fix missing images in editor preview due to wrong links (#31299) (#31393)

Backport #31299

Parse base path and tree path so that media links can be correctly
created with /media/.

Resolves #31294

---------

Co-authored-by: Brecht Van Lommel <brecht@blender.org>

routers/common/markup.go

@@ -7,62 +7,66 @@ package common
 import (
 	"fmt"
 	"net/http"
+	"path"
 	"strings"
 
+	"code.gitea.io/gitea/modules/httplib"
 	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/markup/markdown"
 	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/services/context"
-
-	"mvdan.cc/xurls/v2"
 )
 
 // RenderMarkup renders markup text for the /markup and /markdown endpoints
-func RenderMarkup(ctx *context.Base, repo *context.Repository, mode, text, urlPrefix, filePath string, wiki bool) {
-	var markupType string
-	relativePath := ""
+func RenderMarkup(ctx *context.Base, repo *context.Repository, mode, text, urlPathContext, filePath string, wiki bool) {
+	// urlPathContext format is "/subpath/{user}/{repo}/src/{branch, commit, tag}/{identifier/path}/{file/dir}"
+	// filePath is the path of the file to render if the end user is trying to preview a repo file (mode == "file")
+	// filePath will be used as RenderContext.RelativePath
 
-	if len(text) == 0 {
-		_, _ = ctx.Write([]byte(""))
-		return
+	// for example, when previewing file "/gitea/owner/repo/src/branch/features/feat-123/doc/CHANGE.md", then filePath is "doc/CHANGE.md"
+	// and the urlPathContext is "/gitea/owner/repo/src/branch/features/feat-123/doc"
+
+	var markupType, relativePath string
+
+	links := markup.Links{AbsolutePrefix: true}
+	if urlPathContext != "" {
+		links.Base = fmt.Sprintf("%s%s", httplib.GuessCurrentHostURL(ctx), urlPathContext)
 	}
 
 	switch mode {
 	case "markdown":
 		// Raw markdown
 		if err := markdown.RenderRaw(&markup.RenderContext{
-			Ctx: ctx,
-			Links: markup.Links{
-				AbsolutePrefix: true,
-				Base:           urlPrefix,
-			},
+			Ctx:   ctx,
+			Links: links,
 		}, strings.NewReader(text), ctx.Resp); err != nil {
 			ctx.Error(http.StatusInternalServerError, err.Error())
 		}
 		return
 	case "comment":
-		// Comment as markdown
+		// Issue & comment content
 		markupType = markdown.MarkupName
 	case "gfm":
-		// Github Flavored Markdown as document
+		// GitHub Flavored Markdown
 		markupType = markdown.MarkupName
 	case "file":
-		// File as document based on file extension
-		markupType = ""
+		markupType = "" // render the repo file content by its extension
 		relativePath = filePath
 	default:
 		ctx.Error(http.StatusUnprocessableEntity, fmt.Sprintf("Unknown mode: %s", mode))
 		return
 	}
 
-	if !strings.HasPrefix(setting.AppSubURL+"/", urlPrefix) {
-		// check if urlPrefix is already set to a URL
-		linkRegex, _ := xurls.StrictMatchingScheme("https?://")
-		m := linkRegex.FindStringIndex(urlPrefix)
-		if m == nil {
-			urlPrefix = util.URLJoin(setting.AppURL, urlPrefix)
-		}
+	fields := strings.SplitN(strings.TrimPrefix(urlPathContext, setting.AppSubURL+"/"), "/", 5)
+	if len(fields) == 5 && fields[2] == "src" && (fields[3] == "branch" || fields[3] == "commit" || fields[3] == "tag") {
+		// absolute base prefix is something like "https://host/subpath/{user}/{repo}"
+		absoluteBasePrefix := fmt.Sprintf("%s%s/%s", httplib.GuessCurrentAppURL(ctx), fields[0], fields[1])
+
+		fileDir := path.Dir(filePath)                      // it is "doc" if filePath is "doc/CHANGE.md"
+		refPath := strings.Join(fields[3:], "/")           // it is "branch/features/feat-12/doc"
+		refPath = strings.TrimSuffix(refPath, "/"+fileDir) // now we get the correct branch path: "branch/features/feat-12"
+
+		links = markup.Links{AbsolutePrefix: true, Base: absoluteBasePrefix, BranchPath: refPath, TreePath: fileDir}
 	}
 
 	meta := map[string]string{}
@@ -78,11 +82,8 @@ func RenderMarkup(ctx *context.Base, repo *context.Repository, mode, text, urlPr
 	}
 
 	if err := markup.Render(&markup.RenderContext{
-		Ctx: ctx,
-		Links: markup.Links{
-			AbsolutePrefix: true,
-			Base:           urlPrefix,
-		},
+		Ctx:          ctx,
+		Links:        links,
 		Metas:        meta,
 		IsWiki:       wiki,
 		Type:         markupType,