docs, parallelism: Rearrange how jobserver reservations are made

Rasmus correctly observed that the existing jobserver reservation only
worked if no other build targets were specified. The correct approach
is to hold the jobserver slots until sphinx has finished. To fix this,
the following changes are made:

- refactor (and rename) scripts/jobserver-exec to set an environment
  variable for the maximally reserved jobserver slots and exec a
  child, to release the slots on exit.

- create Documentation/scripts/parallel-wrapper.sh which examines both
  $PARALLELISM and the detected "-jauto" logic from Documentation/Makefile
  to decide sphinx's final -j argument.

- chain these together in Documentation/Makefile

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/lkml/eb25959a-9ec4-3530-2031-d9d716b40b20@rasmusvillemoes.dk
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20191121205929.40371-4-keescook@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 66 insertions, 0 deletions

diff --git a/scripts/jobserver-exec b/scripts/jobserver-exec
new file mode 100755
index 000000000000..0fdb31a790a8
--- /dev/null
+++ b/scripts/jobserver-exec
@@ -0,0 +1,66 @@
+#!/usr/bin/env python
+# SPDX-License-Identifier: GPL-2.0+
+#
+# This determines how many parallel tasks "make" is expecting, as it is
+# not exposed via an special variables, reserves them all, runs a subprocess
+# with PARALLELISM environment variable set, and releases the jobs back again.
+#
+# https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html#POSIX-Jobserver
+from __future__ import print_function
+import os, sys, errno
+import subprocess
+
+# Extract and prepare jobserver file descriptors from envirnoment.
+claim = 0
+jobs = b""
+try:
+	# Fetch the make environment options.
+	flags = os.environ['MAKEFLAGS']
+
+	# Look for "--jobserver=R,W"
+	# Note that GNU Make has used --jobserver-fds and --jobserver-auth
+	# so this handles all of them.
+	opts = [x for x in flags.split(" ") if x.startswith("--jobserver")]
+
+	# Parse out R,W file descriptor numbers and set them nonblocking.
+	fds = opts[0].split("=", 1)[1]
+	reader, writer = [int(x) for x in fds.split(",", 1)]
+	# Open a private copy of reader to avoid setting nonblocking
+	# on an unexpecting process with the same reader fd.
+	reader = os.open("/proc/self/fd/%d" % (reader),
+			 os.O_RDONLY | os.O_NONBLOCK)
+
+	# Read out as many jobserver slots as possible.
+	while True:
+		try:
+			slot = os.read(reader, 8)
+			jobs += slot
+		except (OSError, IOError) as e:
+			if e.errno == errno.EWOULDBLOCK:
+				# Stop at the end of the jobserver queue.
+				break
+			# If something went wrong, give back the jobs.
+			if len(jobs):
+				os.write(writer, jobs)
+			raise e
+	# Add a bump for our caller's reserveration, since we're just going
+	# to sit here blocked on our child.
+	claim = len(jobs) + 1
+except (KeyError, IndexError, ValueError, OSError, IOError) as e:
+	# Any missing environment strings or bad fds should result in just
+	# not being parallel.
+	pass
+
+# We can only claim parallelism if there was a jobserver (i.e. a top-level
+# "-jN" argument) and there were no other failures. Otherwise leave out the
+# environment variable and let the child figure out what is best.
+if claim > 0:
+	os.environ['PARALLELISM'] = '%d' % (claim)
+
+rc = subprocess.call(sys.argv[1:])
+
+# Return all the reserved slots.
+if len(jobs):
+	os.write(writer, jobs)
+
+sys.exit(rc)