[PATCH v2] docs: Use make invocation's -j argument for parallelism

[Kees Cook]

While sphinx 1.7 and later supports "-jauto" for parallelism, this
effectively ignores the "-j" flag used in the "make" invocation, which
may cause confusion for build systems. Instead, extract the available
parallelism from "make"'s job server (since it is not exposed in any
special variables) and use that for the "sphinx-build" run. Now things
work correctly for builds where -j is specified at the top-level:

	make -j16 htmldocs

If -j is not specified, continue to fallback to "-jauto" if available.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v2: retain "-jauto" default behavior with top-level -j is missing.
---
 Documentation/Makefile  |  3 ++-
 scripts/jobserver-count | 53 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 1 deletion(-)
 create mode 100755 scripts/jobserver-count

diff --git a/Documentation/Makefile b/Documentation/Makefile
index e145e4db508b..8bfd38a865ff 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -33,7 +33,7 @@ ifeq ($(HAVE_SPHINX),0)
 
 else # HAVE_SPHINX
 
-export SPHINXOPTS = $(shell perl -e 'open IN,"sphinx-build --version 2>&1 |"; while (<IN>) { if (m/([\d\.]+)/) { print "-jauto" if ($$1 >= "1.7") } ;} close IN')
+export SPHINX_PARALLEL = $(shell perl -e 'open IN,"sphinx-build --version 2>&1 |"; while (<IN>) { if (m/([\d\.]+)/) { print "auto" if ($$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)
@@ -68,6 +68,7 @@ quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
 	PYTHONDONTWRITEBYTECODE=1 \
 	BUILDDIR=$(abspath $(BUILDDIR)) SPHINX_CONF=$(abspath $(srctree)/$(src)/$5/$(SPHINX_CONF)) \
 	$(SPHINXBUILD) \
+	-j $(shell python3 $(srctree)/scripts/jobserver-count $(SPHINX_PARALLEL)) \
 	-b $2 \
 	-c $(abspath $(srctree)/$(src)) \
 	-d $(abspath $(BUILDDIR)/.doctrees/$3) \
diff --git a/scripts/jobserver-count b/scripts/jobserver-count
new file mode 100755
index 000000000000..ff6ebe6b0194
--- /dev/null
+++ b/scripts/jobserver-count
@@ -0,0 +1,53 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# This determines how many parallel tasks "make" is expecting, as it is
+# not exposed via an special variables.
+# https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html#POSIX-Jobserver
+import os, sys, fcntl
+
+# Default parallelism is "1" unless overridden on the command-line.
+default="1"
+if len(sys.argv) > 1:
+	default=sys.argv[1]
+
+# Set non-blocking for a given file descriptor.
+def nonblock(fd):
+	flags = fcntl.fcntl(fd, fcntl.F_GETFL)
+	fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK)
+	return fd
+
+# Extract and prepare jobserver file descriptors from envirnoment.
+try:
+	# Fetch the make environment options.
+	flags = os.environ['MAKEFLAGS']
+
+	# Look for "--jobserver=R,W"
+	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 = [nonblock(int(x)) for x in fds.split(",", 1)]
+except:
+	# Any failures here should result in just using the default
+	# specified parallelism.
+	print(default)
+	sys.exit(0)
+
+# Read out as many jobserver slots as possible.
+jobs = b""
+while True:
+	try:
+		slot = os.read(reader, 1)
+		jobs += slot
+	except:
+		break
+# 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)
+
+# Report available slots (with a bump for our caller's reserveration).
+print(len(jobs) + 1)
-- 
2.17.1


-- 
Kees Cook