[PATCH v4 3/4] Makefile/coccicheck: allow for setting xargs concurrency

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Extend the SPATCH_BATCH_SIZE facility added in
960154b9c17 (coccicheck: optionally batch spatch invocations,
2019-05-06) and bcb4edf7af7 (coccicheck: make batch size of 0 mean
"unlimited", 2019-05-08) to allow for both setting
SPATCH_BATCH_SIZE=N, and also to have a more advanced SPATCH_XARGS
argument.

The reason to replace the "$$limit" is that now you actually see under
V=1 what argument your program will get invoked with.

The reason for the "9999" limit is that if you e.g. try to define an
"$(XARGS)" which is conditionally an empty string or not depending on
this setting then e.g.:

    echo $(FILES) | $(XARGS) $(XARGS_FLAGS) $(SPATCH)

Over multiple lines with "\" will error out. I think just setting it
to "xargs -n 9999" as a trivial workaround is the best solution here.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Makefile | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index e43a9618df5..eaac14275bb 100644
--- a/Makefile
+++ b/Makefile
@@ -1206,6 +1206,25 @@ SPATCH_FLAGS = --all-includes --include-headers-for-types --patch .
 # Setting it to 0 will feed all files in a single spatch invocation.
 SPATCH_BATCH_SIZE = 1
 
+# For the 'coccicheck' target; SPATCH_XARGS can be used to manually
+# tweak the xargs invocation. By default we invoke "xargs -n 1", and
+# "xargs -n 9999" under SPATCH_BATCH_SIZE=0.
+#
+# Setting SPATCH_XARGS overrides SPATCH_BATCH_SIZE. To get concurrency
+# when targeting a single contrib/coccinelle/%.patch use e.g. "-P" if
+# your xargs(1) supports it:
+#
+#    make contrib/coccinelle/strbuf.cocci.patch SPATCH_XARGS="xargs -P 8 -n 8"
+#
+# Or a combination -jN and "xargs -P":
+#
+#    make -j4 coccicheck SPATCH_XARGS="xargs -P 2 -n 8"
+ifeq (0,$(SPATCH_BATCH_SIZE))
+SPATCH_XARGS = xargs -n 9999
+else
+SPATCH_XARGS = xargs -n $(SPATCH_BATCH_SIZE)
+endif
+
 include config.mak.uname
 -include config.mak.autogen
 -include config.mak
@@ -2866,12 +2885,7 @@ COCCI_SOURCES = $(filter-out $(THIRD_PARTY_SOURCES),$(FOUND_C_SOURCES))
 
 %.cocci.patch: %.cocci $(COCCI_SOURCES)
 	$(QUIET_SPATCH) \
-	if test $(SPATCH_BATCH_SIZE) = 0; then \
-		limit=; \
-	else \
-		limit='-n $(SPATCH_BATCH_SIZE)'; \
-	fi; \
-	if ! echo $(COCCI_SOURCES) | xargs $$limit \
+	if ! echo $(COCCI_SOURCES) | $(SPATCH_XARGS) \
 		$(SPATCH) --sp-file $< $(SPATCH_FLAGS) \
 		>$@+ 2>$@.log; \
 	then \
-- 
2.31.0.285.gb40d23e604f