Fix relative links in postprocessed images (#16334)

If a pre-post-processed file contains relative img tags these need to be updated
and joined correctly with the prefix. Finally, the node attributes need to be updated.

Fix #16308

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>

modules/markup/html.go

@@ -364,7 +364,7 @@ func visitNode(ctx *RenderContext, procs []processor, node *html.Node, visitText
 		}
 	case html.ElementNode:
 		if node.Data == "img" {
-			for _, attr := range node.Attr {
+			for i, attr := range node.Attr {
 				if attr.Key != "src" {
 					continue
 				}
@@ -377,6 +377,7 @@ func visitNode(ctx *RenderContext, procs []processor, node *html.Node, visitText
 
 					attr.Val = util.URLJoin(prefix, attr.Val)
 				}
+				node.Attr[i] = attr
 			}
 		} else if node.Data == "a" {
 			visitText = false

modules/markup/html_test.go

@@ -425,6 +425,41 @@ func TestRender_ShortLinks(t *testing.T) {
 		`<p><a href="https://example.org" rel="nofollow">[[foobar]]</a></p>`)
 }
 
+func TestRender_RelativeImages(t *testing.T) {
+	setting.AppURL = AppURL
+	setting.AppSubURL = AppSubURL
+	tree := util.URLJoin(AppSubURL, "src", "master")
+
+	test := func(input, expected, expectedWiki string) {
+		buffer, err := markdown.RenderString(&RenderContext{
+			URLPrefix: tree,
+			Metas:     localMetas,
+		}, input)
+		assert.NoError(t, err)
+		assert.Equal(t, strings.TrimSpace(expected), strings.TrimSpace(buffer))
+		buffer, err = markdown.RenderString(&RenderContext{
+			URLPrefix: setting.AppSubURL,
+			Metas:     localMetas,
+			IsWiki:    true,
+		}, input)
+		assert.NoError(t, err)
+		assert.Equal(t, strings.TrimSpace(expectedWiki), strings.TrimSpace(buffer))
+	}
+
+	rawwiki := util.URLJoin(AppSubURL, "wiki", "raw")
+	mediatree := util.URLJoin(AppSubURL, "media", "master")
+
+	test(
+		`<img src="Link">`,
+		`<img src="`+util.URLJoin(mediatree, "Link")+`"/>`,
+		`<img src="`+util.URLJoin(rawwiki, "Link")+`"/>`)
+
+	test(
+		`<img src="./icon.png">`,
+		`<img src="`+util.URLJoin(mediatree, "icon.png")+`"/>`,
+		`<img src="`+util.URLJoin(rawwiki, "icon.png")+`"/>`)
+}
+
 func Test_ParseClusterFuzz(t *testing.T) {
 	setting.AppURL = AppURL
 	setting.AppSubURL = AppSubURL