md2html: render linked .md via the about view, fix relative base

Links to markdown files now point at /<repo>/about/<path> (rendered
through the about-filter) instead of /<repo>/tree/<path> (raw blob), so
clicking a README link to another .md shows rendered markdown.

Also resolve relative links against the rendered file's own directory:
about-formatting.sh now forwards the filename cgit passes ($1) to
md2html as argv[1], and md2html uses dirname(argv[1]) as the base. This
makes links *inside* a rendered subpage (docs/foo.md -> ./bar.md,
../README.md, images) resolve correctly. Non-markdown files and
directories still go to tree/; images to plain/; about/ links carry no
?h= since the about view renders against the configured readme ref.

2 files changed, 45 insertions, 26 deletions

diff --git a/filters/about-formatting.sh b/filters/about-formatting.sh
index 85daf9c..c6c4f9c 100755
--- a/filters/about-formatting.sh
+++ b/filters/about-formatting.sh
@@ -19,7 +19,7 @@
 
 cd "$(dirname $0)/html-converters/"
 case "$(printf '%s' "$1" | tr '[:upper:]' '[:lower:]')" in
-	*.markdown|*.mdown|*.md|*.mkd) exec ./md2html; ;;
+	*.markdown|*.mdown|*.md|*.mkd) exec ./md2html "$1"; ;;
 	*.rst) exec ./rst2html; ;;
 	*.[1-9]) exec ./man2html; ;;
 	*.htm|*.html) exec cat; ;;
diff --git a/filters/html-converters/md2html b/filters/html-converters/md2html
index 90021a7..79d4a17 100755
--- a/filters/html-converters/md2html
+++ b/filters/html-converters/md2html
@@ -358,17 +358,21 @@ html = markdown.markdown(
 		"markdown.extensions.codehilite":{"css_class":"highlight"}})
 
 # A README's relative paths point at repo content, but a browser resolves them
-# against the page the readme is shown on (/<repo>/about/ or the summary page
-# /<repo>/), not against the repo root — so they 404. cgit serves blobs only via
-# its `plain`/`tree` commands, so rewrite relative references to absolute cgit
-# URLs (absolute so it works both on the about page and the summary page):
-#   <img src>   -> /<repo>/plain/<path>?h=<branch>   (raw bytes for the browser)
-#   <a href>    -> /<repo>/tree/<path>?h=<branch>     (browsable file/dir view)
-# Relative hrefs are resolved as paths against the repo root; a href that climbs
-# out of the repo with ".." (e.g. ../sibling) is resolved against the repo's URL
-# location instead (-> /sibling), matching how sibling repos are addressed.
-# cgit exports the repo context to about-filters via these environment vars; if
-# they're absent we leave the references untouched.
+# against the page the readme is shown on (/<repo>/about/... or the summary page
+# /<repo>/), not against the file's own location — so they 404 or hit the wrong
+# file. cgit serves this content through three commands, so rewrite relative
+# references to absolute cgit URLs (absolute so they work on the about page and
+# the summary page alike):
+#   <img src>          -> /<repo>/plain/<path>?h=<branch>   (raw bytes)
+#   <a href> *.md      -> /<repo>/about/<path>              (rendered markdown)
+#   <a href> other/dir -> /<repo>/tree/<path>?h=<branch>    (browsable blob view)
+# cgit hands the about-filter the path of the file being rendered as argv[1]
+# (README.md, or docs/foo.md for a rendered subpage), so relative links resolve
+# against *that* file's directory and links inside a rendered subpage work too.
+# A href that climbs out of the repo with ".." (e.g. ../sibling) is resolved
+# against the repo's URL location (-> /sibling). The about view renders against
+# the configured readme ref, so about/ links carry no ?h=. If the repo context
+# env vars are absent we leave the references untouched.
 import os, re
 from urllib.parse import urljoin
 repo = os.environ.get("CGIT_REPO_URL", "").strip("/")
@@ -377,16 +381,16 @@ if repo:
 	repo_root = "/" + repo + "/"
 	plain = repo_root + "plain/"
 	query = ("?h=" + branch) if branch else ""
+	# directory of the file being rendered; relative links resolve against it.
+	# argv[1] is the repo-relative readme/subpage path cgit passes the filter.
+	cur = sys.argv[1] if len(sys.argv) > 1 else ""
+	base_dir = "" if cur.startswith("/") else os.path.dirname(cur)
+	# markdown files are rendered via the about view; everything else is a blob
+	md_ext = re.compile(r"\.(?:md|markdown|mdown|mkd)$", re.I)
 	# leave absolute, protocol-relative, root-relative, anchor, query-only and
 	# scheme (mailto:, http:, data:, ...) URLs alone
 	skip = re.compile(r"^(?:[a-z][a-z0-9+.\-]*:|//|/|[#?])", re.I)
 
-	def rewrite_img(m):
-		url = m.group(2)
-		if not url or skip.match(url):
-			return m.group(0)
-		return m.group(1) + plain + re.sub(r"^\./", "", url) + query + m.group(3)
-
 	def _split_frag(url):
 		# split off a trailing #fragment (kept verbatim on the rewritten URL)
 		if "#" in url:
@@ -394,11 +398,13 @@ if repo:
 			return path, "#" + frag
 		return url, ""
 
-	def _normalize(path):
-		# resolve "." / ".." against the repo root; returns (climbs, cleaned)
-		# where `climbs` counts ".." segments that escaped above the repo root.
+	def _resolve(path):
+		# resolve `path` (relative to base_dir) into a repo-root-relative path,
+		# collapsing "." / ".."; returns (climbs, cleaned) where `climbs` counts
+		# ".." segments that escaped above the repo root.
+		combined = (base_dir + "/" + path) if base_dir else path
 		out, climbs = [], 0
-		for seg in path.split("/"):
+		for seg in combined.split("/"):
 			if seg in ("", "."):
 				continue
 			if seg == "..":
@@ -410,19 +416,32 @@ if repo:
 				out.append(seg)
 		return climbs, "/".join(out)
 
+	def rewrite_img(m):
+		url = m.group(2)
+		if not url or skip.match(url):
+			return m.group(0)
+		path, frag = _split_frag(url)
+		climbs, cleaned = _resolve(path)
+		if climbs or not cleaned:
+			return m.group(0)
+		return m.group(1) + plain + cleaned + query + frag + m.group(3)
+
 	def rewrite_href(m):
 		url = m.group(2)
 		if not url or skip.match(url):
 			return m.group(0)
 		path, frag = _split_frag(url)
-		climbs, cleaned = _normalize(path)
+		climbs, cleaned = _resolve(path)
 		if climbs:
 			# escapes the repo (sibling repo etc.): resolve against repo location
 			dest = urljoin(repo_root, "../" * climbs + cleaned)
-		elif cleaned:
-			dest = repo_root + "tree/" + cleaned + query
-		else:
+		elif not cleaned:
 			dest = repo_root + (query or "")
+		elif md_ext.search(cleaned):
+			# rendered through the about-filter (renders against the readme ref)
+			dest = repo_root + "about/" + cleaned
+		else:
+			dest = repo_root + "tree/" + cleaned + query
 		return m.group(1) + dest + frag + m.group(3)
 
 	html = re.sub(r'(<img\b[^>]*?\bsrc=")([^"]*)(")', rewrite_img, html, flags=re.I)