index : cgit

-- Server-side about-page rendering for cgit, used with the about-filter setting
-- in cgitrc and the lua: prefix so it runs in cgit's embedded interpreter with
-- no per-request process.
--
--     about-filter=lua:/usr/lib/cgit/filters/about-render.lua
--
-- One filter renders the three baseline about formats, chosen by the readme's
-- file extension.
--
--     markdown   .md .markdown .mkd .mdown
--     man page   .1 through .9 and .man
--     plain text everything else, and the fallback for anything that fails
--
-- Add a format by giving it a render function and a row in the handler table
-- near the foot of this file. Nothing else needs to change.
--
-- SUPPORTED LUA
--
-- Lua 5.1 through 5.4 and LuaJIT.
--
-- REQUIREMENTS
--
-- lpeg, the parsing module, for the Lua cgit is linked against. Markdown and
-- man pages are parsed with lpeg grammars, so without lpeg both fall back to
-- escaped plain text. It ships with cgit's syntax highlighter too, so a cgit
-- that colours source already has it.
--
--     # Debian and Ubuntu
--     sudo apt install lua-lpeg
--     # Fedora
--     sudo dnf install lua-lpeg
--     # Alpine
--     sudo apk add lua5.1-lpeg
--     # or with LuaRocks, matched to your Lua version
--     sudo luarocks --lua-version 5.1 install lpeg
--
-- Plain text needs only Lua itself.
--
-- SECURITY
--
-- The about page renders untrusted repository content, so the safety rule is
-- that every run of text reaches the page through cgit's own html_txt, every
-- attribute value through html_attr, and every link or image target through
-- the safe_url scheme allowlist below before html_attr. Only http, https,
-- mailto and relative targets survive, so a hostile readme cannot inject markup
-- or a javascript: url. Because all output is routed through those sinks by
-- construction, a bug in the parser can only mis-render, never inject.
--
-- LIMITATIONS
--
-- Markdown is a deliberate subset, not CommonMark. Headings, thematic breaks,
-- fenced and inline code, blockquotes, pipe tables, single-level lists, links,
-- images and emphasis. No reference links, no raw HTML passthrough, no nested
-- lists, no setext headings. Emphasis does not span a hard line break inside a
-- paragraph. Man rendering covers the common macros (section headings, filled
-- and no-fill paragraphs, bold and italic, font escapes) and drops the rest.
-- cgit sends filter output through a C string sink that stops at the first NUL
-- byte, so content past a NUL is truncated.
--
-- OUTPUT
--
-- Markdown is wrapped in <div class='markdown'>, man pages in
-- <div class='markdown manpage'>, plain text in <pre class='plaintext'>, all
-- styled by assets/cgit.css.

local ok_lpeg, lpeg = pcall(require, "lpeg")


-- ===== configuration ===================================================

-- Readmes larger than this are served as escaped plain text rather than parsed.
local max_bytes = 512 * 1024

-- Ceiling on blockquote and emphasis nesting, so a hostile readme of stacked
-- markers cannot drive the recursive parser into a stack overflow.
local max_depth = 24

local filename, chunks = "", {}


-- ===== helpers =========================================================

local function trim(s)
	return (s:gsub("^%s+", ""):gsub("%s+$", ""))
end

-- Split on newlines after normalising CRLF and CR, returning the lines without
-- their terminators. A trailing newline yields a final empty line, which every
-- caller treats as blank.
local function split_lines(s)
	s = s:gsub("\r\n?", "\n")
	local lines, start = {}, 1
	while true do
		local nl = s:find("\n", start, true)
		if not nl then
			lines[#lines + 1] = s:sub(start)
			return lines
		end
		lines[#lines + 1] = s:sub(start, nl - 1)
		start = nl + 1
	end
end

-- Split a table row into trimmed cells, dropping one optional leading and
-- trailing pipe.
local function split_cells(row)
	row = trim(row):gsub("^|", ""):gsub("|$", "")
	local cells = {}
	for c in (row .. "|"):gmatch("(.-)|") do
		cells[#cells + 1] = trim(c)
	end
	return cells
end

-- Return the url if its scheme is safe, else nil. Control and whitespace bytes
-- are stripped anywhere first because a browser ignores them when resolving the
-- scheme, so "java\nscript:" must still be caught as javascript. The class is
-- %c%s rather than a literal 0x00 range so it is safe on Lua 5.1, where an
-- embedded zero byte ends a pattern.
local function safe_url(url)
	url = url:gsub("[%c%s]", "")
	-- A leading "//" or "/\" is scheme-relative, which a browser resolves to
	-- another origin, so it is not the local target the parser meant to allow.
	if url:sub(1, 2) == "//" or url:sub(1, 2) == "/\\" then
		return nil
	end
	local scheme = url:match("^(%a[%w%+%.%-]*):")
	if scheme then
		scheme = scheme:lower()
		if scheme ~= "http" and scheme ~= "https" and scheme ~= "mailto" then
			return nil
		end
	end
	return url
end


-- ===== emit ============================================================
-- html, html_txt and html_attr are injected by cgit's lua filter host. Tag
-- scaffolding is a literal argument to html; every value that came from the
-- repository goes through html_txt, html_attr or safe_url.

local emit_inline

emit_inline = function(list)
	for _, nd in ipairs(list) do
		local t = nd.t
		if t == "text" then
			html_txt(nd.v)
		elseif t == "code" then
			html("<code>"); html_txt(nd.v); html("</code>")
		elseif t == "strong" then
			html("<strong>"); emit_inline(nd.kids); html("</strong>")
		elseif t == "em" then
			html("<em>"); emit_inline(nd.kids); html("</em>")
		elseif t == "link" then
			local u = safe_url(nd.url)
			if u then
				html("<a href='"); html_attr(u); html("'>")
				emit_inline(nd.kids)
				html("</a>")
			else
				emit_inline(nd.kids)
			end
		elseif t == "image" then
			local u = safe_url(nd.url)
			if u then
				html("<img src='"); html_attr(u)
				html("' alt='"); html_attr(nd.alt); html("'/>")
			else
				html_txt("![" .. nd.alt .. "]")
			end
		end
	end
end

local emit_blocks

emit_blocks = function(blocks)
	for _, b in ipairs(blocks) do
		local t = b.t
		if t == "heading" then
			html("<h" .. b.level .. ">")
			emit_inline(b.kids)
			html("</h" .. b.level .. ">")
		elseif t == "hr" then
			html("<hr/>")
		elseif t == "code" then
			html("<pre><code")
			if b.lang and b.lang ~= "" then
				html(" data-lang='"); html_attr(b.lang); html("'")
			end
			html(">"); html_txt(b.text); html("</code></pre>")
		elseif t == "blockquote" then
			html("<blockquote>"); emit_blocks(b.blocks); html("</blockquote>")
		elseif t == "table" then
			html("<table><thead><tr>")
			for _, c in ipairs(b.head) do
				html("<th>"); emit_inline(c); html("</th>")
			end
			html("</tr></thead><tbody>")
			for _, row in ipairs(b.rows) do
				html("<tr>")
				for _, c in ipairs(row) do
					html("<td>"); emit_inline(c); html("</td>")
				end
				html("</tr>")
			end
			html("</tbody></table>")
		elseif t == "list" then
			local tag = b.ordered and "ol" or "ul"
			html("<" .. tag .. ">")
			for _, item in ipairs(b.items) do
				html("<li>"); emit_inline(item); html("</li>")
			end
			html("</" .. tag .. ">")
		elseif t == "para" then
			html("<p>")
			for k, line in ipairs(b.lines) do
				if k > 1 then html("<br/>") end
				emit_inline(line)
			end
			html("</p>")
		end
	end
end


-- ===== plain text ======================================================

local function render_plaintext(text)
	html("<pre class='plaintext'>")
	html_txt(text)
	html("</pre>")
end


-- ===== markdown and man ================================================
-- Both are parsed with lpeg into the block/inline node trees emit_blocks and
-- emit_inline above consume. Parsing is pure and builds a tree; emit is the
-- only step that writes output, so a parse failure falls back to plain text
-- without leaving half a page behind.

local render_markdown, render_man

if ok_lpeg then
	local P, R, S, C, Ct = lpeg.P, lpeg.R, lpeg.S, lpeg.C, lpeg.Ct

	local sp = S(" \t")
	local ws = S(" \t\r\n\f\v")
	local eol = P(-1)

	-- ----- markdown inline -----
	local marker = S("`![*_")
	local urlchar = 1 - S(")") - ws
	-- Bound the link and image inner scans. Without a cap a readme of unclosed
	-- brackets ("[[[[...") makes each position scan to end of line for a "]"
	-- that never comes, which is quadratic. Real link text and urls sit far
	-- under this, and anything longer simply renders as plain text.
	local CAP = 512

	local parse_inline
	local inline_depth = 0

	local function node_text(s) return { t = "text", v = s } end
	local function node_code(s) return { t = "code", v = s } end
	local function node_strong(s) return { t = "strong", kids = parse_inline(s) } end
	local function node_em(s) return { t = "em", kids = parse_inline(s) } end
	local function node_link(text, url) return { t = "link", kids = parse_inline(text), url = url } end
	local function node_image(alt, url) return { t = "image", alt = alt, url = url } end

	local inline_grammar = Ct((
		  (P("`") * C((1 - P("`")) ^ 1) * P("`")) / node_code
		+ (P("![") * C((1 - P("]")) ^ (-CAP)) * P("]") * P("(") * sp ^ 0
			* C(urlchar * urlchar ^ (-CAP)) * (1 - P(")")) ^ (-CAP) * P(")")) / node_image
		+ (P("[") * C((1 - P("]")) ^ (-CAP)) * P("]") * P("(") * sp ^ 0
			* C(urlchar * urlchar ^ (-CAP)) * (1 - P(")")) ^ (-CAP) * P(")")) / node_link
		+ (P("**") * C((1 - P("**")) ^ 1) * P("**")) / node_strong
		+ (P("__") * C((1 - P("__")) ^ 1) * P("__")) / node_strong
		+ (P("*") * C((1 - ws) * (1 - P("*")) ^ 0) * P("*")) / node_em
		+ (P("_") * C((1 - ws) * (1 - P("_")) ^ 0) * P("_")) / node_em
		+ C((1 - marker) ^ 1) / node_text
		+ C(P(1)) / node_text
	) ^ 0)

	parse_inline = function(s)
		if inline_depth >= max_depth then
			return { node_text(s) }
		end
		inline_depth = inline_depth + 1
		local nodes = inline_grammar:match(s) or { node_text(s) }
		inline_depth = inline_depth - 1
		return nodes
	end

	-- ----- markdown blocks -----
	-- Line matchers. Each is anchored at the start of a single line and returns
	-- its captures, or nil when the line is not of that kind.
	local langchar = R("az", "AZ", "09") + S("_.+#-")
	local m_heading = C(P("#") * P("#") ^ -5) * sp ^ 1 * C(P(1) ^ 0)
	local function rule(c) return sp ^ 0 * P(c) * (sp ^ 0 * P(c)) ^ 2 * sp ^ 0 * eol end
	local m_hr = rule("-") + rule("*") + rule("_")
	local m_fence = sp ^ 0 * (C(P("`") ^ 3) + C(P("~") ^ 3)) * sp ^ 0 * C(langchar ^ 0)
	local m_close_bt = sp ^ 0 * P("`") ^ 3 * sp ^ 0 * eol
	local m_close_ti = sp ^ 0 * P("~") ^ 3 * sp ^ 0 * eol
	local m_bq = sp ^ 0 * P(">")
	local bullet = S("-*+")
	local number = R("09") ^ 1 * S(".)")
	local m_item = sp ^ 0 * C(bullet + number) * sp ^ 1 * C(P(1) ^ 0)
	local dcell = sp ^ 0 * P(":") ^ -1 * P("-") ^ 1 * P(":") ^ -1 * sp ^ 0
	-- A delimiter row is dash-cells joined by pipes. Require at least one pipe,
	-- a leading one or one between cells, so a bare rule of dashes stays a
	-- thematic break and an ordinary paragraph line is never taken for a table.
	local pipe = P("|")
	local m_tdelim = sp ^ 0 * (
		  pipe * dcell * (pipe * dcell) ^ 0 * pipe ^ -1
		+ dcell * (pipe * dcell) ^ 1 * pipe ^ -1
	) * sp ^ 0 * eol
	local m_blockstart = sp ^ 0 * ((P("#") * P("#") ^ -5 * sp) + P(">")
		+ P("`") ^ 3 + P("~") ^ 3 + ((bullet + number) * sp))

	local function ordered(mark) return mark:match("%d") ~= nil end

	local parse_blocks

	parse_blocks = function(lines, depth)
		local blocks, i, n = {}, 1, #lines
		while i <= n do
			local line = lines[i]
			local fence, lang = m_fence:match(line)
			local hashes, htext = m_heading:match(line)
			local mark = m_item:match(line)
			if line:match("^%s*$") then
				i = i + 1
			elseif fence then
				local close = (fence:sub(1, 1) == "`") and m_close_bt or m_close_ti
				local code = {}
				i = i + 1
				while i <= n and not close:match(lines[i]) do
					code[#code + 1] = lines[i]; i = i + 1
				end
				i = i + 1
				local body = table.concat(code, "\n")
				if #code > 0 then body = body .. "\n" end
				blocks[#blocks + 1] = { t = "code", lang = lang, text = body }
			elseif hashes then
				blocks[#blocks + 1] = {
					t = "heading",
					level = #hashes,
					kids = parse_inline((htext:gsub("%s*#*%s*$", ""))),
				}
				i = i + 1
			elseif m_hr:match(line) then
				blocks[#blocks + 1] = { t = "hr" }
				i = i + 1
			elseif m_bq:match(line) then
				local inner = {}
				while i <= n and m_bq:match(lines[i]) do
					inner[#inner + 1] = (lines[i]:gsub("^%s*>%s?", ""))
					i = i + 1
				end
				if depth < max_depth then
					blocks[#blocks + 1] = { t = "blockquote", blocks = parse_blocks(inner, depth + 1) }
				else
					local para = {}
					for _, l in ipairs(inner) do para[#para + 1] = parse_inline(l) end
					blocks[#blocks + 1] = { t = "para", lines = para }
				end
			elseif i + 1 <= n and line:find("|", 1, true) and m_tdelim:match(lines[i + 1]) then
				local head = {}
				for _, c in ipairs(split_cells(line)) do head[#head + 1] = parse_inline(c) end
				local rows = {}
				i = i + 2
				while i <= n and lines[i]:find("|", 1, true) and not lines[i]:match("^%s*$") do
					local row = {}
					for _, c in ipairs(split_cells(lines[i])) do row[#row + 1] = parse_inline(c) end
					rows[#rows + 1] = row
					i = i + 1
				end
				blocks[#blocks + 1] = { t = "table", head = head, rows = rows }
			elseif mark then
				local is_ol = ordered(mark)
				local items = {}
				while i <= n do
					local mk, ct = m_item:match(lines[i])
					if not mk or ordered(mk) ~= is_ol then break end
					items[#items + 1] = parse_inline(ct)
					i = i + 1
				end
				blocks[#blocks + 1] = { t = "list", ordered = is_ol, items = items }
			else
				local para = { parse_inline(line) }
				i = i + 1
				while i <= n and not lines[i]:match("^%s*$") and not m_blockstart:match(lines[i]) do
					para[#para + 1] = parse_inline(lines[i]); i = i + 1
				end
				blocks[#blocks + 1] = { t = "para", lines = para }
			end
		end
		return blocks
	end

	render_markdown = function(text)
		if #text > max_bytes then
			return render_plaintext(text)
		end
		local ok, blocks = pcall(parse_blocks, split_lines(text), 0)
		if not ok then
			return render_plaintext(text)
		end
		html("<div class='markdown'>")
		-- Emit is wrapped so a parser bug can at worst truncate the page, never
		-- error out and let the outer fallback duplicate what was already sent.
		pcall(emit_blocks, blocks)
		html("</div>")
	end

	-- ----- man pages -----
	-- Macro lines are matched with lpeg, and the roff inline font and character
	-- escapes are an lpeg grammar producing a flat token list that a fold turns
	-- into the same inline nodes markdown emits. Bold and italic are the current
	-- font, which carries across a run, so the inline pass tokenises and folds
	-- rather than nesting the way markdown does.
	local m_macro = S(".'") * sp ^ 0 * C((1 - sp) ^ 1) * sp ^ 0 * C(P(1) ^ 0)

	local one_font = { B = "B", I = "I" }
	local two_font = { CB = "B", BI = "B", CI = "I" }
	local man_chars = {
		aq = "'", cq = "'", oq = "'", dq = '"', lq = '"', rq = '"',
		hy = "-", en = "-", em = "-",
	}
	local esc = P("\\")
	local function font_tok(f) return { k = "font", f = f } end
	local function text_tok(v) return { k = "text", v = v } end
	local skip_tok = { k = "skip" }

	local man_inline_grammar = Ct((
		  (esc * P("f") * P("(") * C(P(1) * P(1))) / function(nm) return font_tok(two_font[nm] or "R") end
		+ (esc * P("f") * P("[") * C((1 - P("]")) ^ 0) * P("]")) / function(nm) return font_tok(one_font[nm] or "R") end
		+ (esc * P("f") * C(P(1))) / function(f) return font_tok(one_font[f] or "R") end
		+ (esc * P("-")) / function() return text_tok("-") end
		+ (esc * P("e")) / function() return text_tok("\\") end
		+ (esc * P(" ")) / function() return text_tok(" ") end
		+ (esc * S("&|^")) / function() return skip_tok end
		+ (esc * P("(") * C(P(1) * P(1))) / function(nm) return text_tok(man_chars[nm] or "") end
		+ (esc * P("*") * (P("(") * P(1) * P(1) + P(1))) / function() return skip_tok end
		+ (esc * C(P(1))) / text_tok
		+ esc / function() return skip_tok end
		+ C((1 - esc) ^ 1) / text_tok
	) ^ 0)

	local function man_inline(s)
		local toks = man_inline_grammar:match(s) or {}
		local nodes, buf, font = {}, {}, "R"
		local function flush()
			if #buf == 0 then return end
			local txt = table.concat(buf); buf = {}
			if txt == "" then return end
			if font == "B" then
				nodes[#nodes + 1] = { t = "strong", kids = { { t = "text", v = txt } } }
			elseif font == "I" then
				nodes[#nodes + 1] = { t = "em", kids = { { t = "text", v = txt } } }
			else
				nodes[#nodes + 1] = { t = "text", v = txt }
			end
		end
		for _, tk in ipairs(toks) do
			if tk.k == "text" then
				buf[#buf + 1] = tk.v
			elseif tk.k == "font" then
				flush(); font = tk.f
			end
		end
		flush()
		return nodes
	end

	local function parse_man(text)
		local lines = split_lines(text)
		local blocks = {}
		local para, pre, nofill = nil, nil, false
		local function flush_para()
			if para and #para > 0 then
				blocks[#blocks + 1] = { t = "para", lines = para }
			end
			para = nil
		end
		local function flush_pre()
			if pre then
				local body = table.concat(pre, "\n")
				if #pre > 0 then body = body .. "\n" end
				blocks[#blocks + 1] = { t = "code", text = body }
				pre = nil
			end
		end
		local function add_line(nodes)
			para = para or {}
			para[#para + 1] = nodes
		end
		for _, line in ipairs(lines) do
			local macro, rest = m_macro:match(line)
			if rest then rest = (rest:gsub('^"(.*)"$', "%1")) end
			if nofill then
				if macro == "fi" then
					nofill = false; flush_pre()
				else
					pre[#pre + 1] = line
				end
			elseif line:match("^%s*$") then
				flush_para()
			elseif macro and macro:sub(1, 1) == "\\" then
				-- roff comment, dropped
			elseif macro == "SH" or macro == "SS" then
				flush_para()
				blocks[#blocks + 1] = {
					t = "heading",
					level = (macro == "SH") and 2 or 3,
					kids = man_inline(rest),
				}
			elseif macro == "nf" then
				flush_para(); nofill = true; pre = {}
			elseif macro == "fi" then
				flush_pre()
			elseif macro == "B" or macro == "BR" or macro == "RB"
			    or macro == "BI" or macro == "IB" then
				add_line({ { t = "strong", kids = man_inline(rest) } })
			elseif macro == "I" or macro == "IR" or macro == "RI" then
				add_line({ { t = "em", kids = man_inline(rest) } })
			elseif macro == "TH" or macro == "PP" or macro == "LP"
			    or macro == "P" or macro == "TP" or macro == "IP"
			    or macro == "HP" or macro == "RS" or macro == "RE"
			    or macro == "sp" or macro == "br" then
				flush_para()
			elseif macro then
				if rest ~= "" then add_line(man_inline(rest)) end
			else
				add_line(man_inline(line))
			end
		end
		flush_para(); flush_pre()
		return blocks
	end

	render_man = function(text)
		if #text > max_bytes then
			return render_plaintext(text)
		end
		local ok, blocks = pcall(parse_man, text)
		if not ok then
			return render_plaintext(text)
		end
		html("<div class='markdown manpage'>")
		pcall(emit_blocks, blocks)
		html("</div>")
	end
else
	-- No lpeg, so markdown and man readmes read as escaped source, not nothing.
	render_markdown = render_plaintext
	render_man = render_plaintext
end


-- ===== dispatch ========================================================

local handlers = {}
for _, ext in ipairs({ "md", "markdown", "mkd", "mdown" }) do
	handlers[ext] = render_markdown
end
for _, ext in ipairs({ "1", "2", "3", "4", "5", "6", "7", "8", "9", "man" }) do
	handlers[ext] = render_man
end

function filter_open(name)
	filename = name or ""
	chunks = {}
end

function filter_write(str)
	chunks[#chunks + 1] = str
end

function filter_close()
	local text = table.concat(chunks)
	chunks = {}
	local ext = (filename:match("%.([^.]+)$") or ""):lower()
	local render = handlers[ext] or render_plaintext
	local ok = pcall(render, text)
	if not ok then
		render_plaintext(text)
	end
	return 0
end