[PATCH 3/3] docs, parallelism: Rearrange how jobserver reservations are made

[Kees Cook]

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@xxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/eb25959a-9ec4-3530-2031-d9d716b40b20@xxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 Documentation/Makefile                      |  5 +-
 Documentation/sphinx/parallel-wrapper.sh    | 25 +++++++
 scripts/{jobserver-count => jobserver-exec} | 73 ++++++++++++---------
 3 files changed, 68 insertions(+), 35 deletions(-)
 create mode 100644 Documentation/sphinx/parallel-wrapper.sh
 rename scripts/{jobserver-count => jobserver-exec} (50%)
 mode change 100755 => 100644

diff --git a/Documentation/Makefile b/Documentation/Makefile
index ce8eb63b523a..30554a2fbdd7 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -33,8 +33,6 @@ ifeq ($(HAVE_SPHINX),0)
 
 else # HAVE_SPHINX
 
-export SPHINX_PARALLEL = $(shell perl -e 'open IN,"sphinx-build --version 2>&1 |"; while (<IN>) { if (m/([\d\.]+)/) { print "auto" if ($$1 >= "1.7") } ;} close IN')
-
 # User-friendly check for pdflatex and latexmk
 HAVE_PDFLATEX := $(shell if which $(PDFLATEX) >/dev/null 2>&1; then echo 1; else echo 0; fi)
 HAVE_LATEXMK := $(shell if which latexmk >/dev/null 2>&1; then echo 1; else echo 0; fi)
@@ -67,8 +65,9 @@ quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
       cmd_sphinx = $(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=Documentation/media $2 && \
 	PYTHONDONTWRITEBYTECODE=1 \
 	BUILDDIR=$(abspath $(BUILDDIR)) SPHINX_CONF=$(abspath $(srctree)/$(src)/$5/$(SPHINX_CONF)) \
+	$(PYTHON) $(srctree)/scripts/jobserver-exec \
+	$(SHELL) $(srctree)/Documentation/sphinx/parallel-wrapper.sh \
 	$(SPHINXBUILD) \
-	-j $(shell python $(srctree)/scripts/jobserver-count $(SPHINX_PARALLEL)) \
 	-b $2 \
 	-c $(abspath $(srctree)/$(src)) \
 	-d $(abspath $(BUILDDIR)/.doctrees/$3) \
diff --git a/Documentation/sphinx/parallel-wrapper.sh b/Documentation/sphinx/parallel-wrapper.sh
new file mode 100644
index 000000000000..a416dbfd2025
--- /dev/null
+++ b/Documentation/sphinx/parallel-wrapper.sh
@@ -0,0 +1,25 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Figure out if we should follow a specific parallelism from the make
+# environment (as exported by scripts/jobserver-exec), or fall back to
+# the "auto" parallelism when "-jN" is not specified at the top-level
+# "make" invocation.
+
+sphinx="$1"
+shift || true
+
+parallel="${PARALLELISM:-1}"
+if [ ${parallel} -eq 1 ] ; then
+	auto=$(perl -e 'open IN,"'"$sphinx"' --version 2>&1 |";
+			while (<IN>) {
+				if (m/([\d\.]+)/) {
+					print "auto" if ($1 >= "1.7")
+				}
+			}
+			close IN')
+	if [ -n "$auto" ] ; then
+		parallel="$auto"
+	fi
+fi
+exec "$sphinx" "-j$parallel" "$@"
diff --git a/scripts/jobserver-count b/scripts/jobserver-exec
old mode 100755
new mode 100644
similarity index 50%
rename from scripts/jobserver-count
rename to scripts/jobserver-exec
index a68a04ad304f..4593b2a1e36d
--- a/scripts/jobserver-count
+++ b/scripts/jobserver-exec
@@ -2,17 +2,16 @@
 # SPDX-License-Identifier: GPL-2.0+
 #
 # This determines how many parallel tasks "make" is expecting, as it is
-# not exposed via an special variables.
+# 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, fcntl, errno
-
-# Default parallelism is "1" unless overridden on the command-line.
-default="1"
-if len(sys.argv) > 1:
-	default=sys.argv[1]
+import subprocess
 
 # Extract and prepare jobserver file descriptors from envirnoment.
+jobs = b""
 try:
 	# Fetch the make environment options.
 	flags = os.environ['MAKEFLAGS']
@@ -30,31 +29,41 @@ try:
 	reader = os.open("/proc/self/fd/%d" % (reader), os.O_RDONLY)
 	flags = fcntl.fcntl(reader, fcntl.F_GETFL)
 	fcntl.fcntl(reader, fcntl.F_SETFL, flags | os.O_NONBLOCK)
-except (KeyError, IndexError, ValueError, IOError, OSError) as e:
-	print(e, file=sys.stderr)
+
+	# Read out as many jobserver slots as possible.
+	while True:
+		try:
+			slot = os.read(reader, 1)
+			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
+except (KeyError, IndexError, ValueError, OSError, IOError) as e:
 	# Any missing environment strings or bad fds should result in just
-	# using the default specified parallelism.
-	print(default)
-	sys.exit(0)
+	# not being parallel.
+	pass
 
-# Read out as many jobserver slots as possible.
-jobs = b""
-while True:
-	try:
-		slot = os.read(reader, 1)
-		jobs += slot
-	except (OSError, IOError) as e:
-		if e.errno == errno.EWOULDBLOCK:
-			# Stop when reach the end of the jobserver queue.
-			break
-		raise e
-# Return all the reserved slots.
-os.write(writer, jobs)
-
-# If the jobserver was (impossibly) full or communication failed, use default.
-if len(jobs) < 1:
-	print(default)
-	sys.exit(0)
-
-# Report available slots (with a bump for our caller's reserveration).
-print(len(jobs) + 1)
+claim = len(jobs)
+if claim < 1:
+	# If the jobserver was (impossibly) full or communication failed
+	# in some way do not use parallelism.
+	claim = 0
+
+# Launch command with a bump for our caller's reserveration,
+# since we're just going to sit here blocked on our child.
+claim += 1
+
+os.unsetenv('MAKEFLAGS')
+os.environ['PARALLELISM'] = '%d' % (claim)
+rc = subprocess.call(sys.argv[1:])
+
+# Return all the actually reserved slots.
+if len(jobs):
+	os.write(writer, jobs)
+
+sys.exit(rc)
-- 
2.17.1