selftests, bpf: Make docs tests fail more reliably

Previously, if rst2man caught errors, then these would be ignored and
the output file would be written anyway. This would allow developers to
introduce regressions in the docs comments in the BPF headers.

Additionally, even if you instruct rst2man to fail out, it will still
write out to the destination target file, so if you ran the tests twice
in a row it would always pass. Use a temporary file for the initial run
to ensure that if rst2man fails out under "--strict" mode, subsequent
runs will not automatically pass.

Tested via ./tools/testing/selftests/bpf/test_doc_build.sh

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20210608015756.340385-1-joe@cilium.io

tools/testing/selftests/bpf/.gitignore

@@ -37,5 +37,6 @@ test_cpp
 /runqslower
 /bench
 *.ko
+*.tmp
 xdpxceiver
 xdp_redirect_multi

tools/testing/selftests/bpf/Makefile.docs

@@ -52,7 +52,8 @@ $(OUTPUT)%.$2: $(OUTPUT)%.rst
 ifndef RST2MAN_DEP
 	$$(error "rst2man not found, but required to generate man pages")
 endif
-	$$(QUIET_GEN)rst2man $$< > $$@
+	$$(QUIET_GEN)rst2man --exit-status=1 $$< > $$@.tmp
+	$$(QUIET_GEN)mv $$@.tmp $$@
 
 docs-clean-$1:
 	$$(call QUIET_CLEAN, eBPF_$1-manpage)

tools/testing/selftests/bpf/test_doc_build.sh

@@ -1,5 +1,6 @@
 #!/bin/bash
 # SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+set -e
 
 # Assume script is located under tools/testing/selftests/bpf/. We want to start
 # build attempts from the top of kernel repository.