[PATCH 01/12] grep: add ability to disable threading with --threads=0 or grep.threads=0

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

Add the ability to entirely disable threading by having grep.threads=0
in the config or --threads=0 on the command-line.

This was made configurable in commit 89f09dd34e ("grep: add
--threads=<num> option and grep.threads configuration",
2015-12-15). Before that change there was no way to disable threaded
grep other than to recompile Git.

It's very useful for testing & debugging to be able to entirely
disable threading without recompiling with NO_PTHREADS=YesPlease, so
support setting the value to 0 to disable threading.

There was no reason this wasn't the case already other than an
implementation detail in how OPT_INTEGER() works. When it's used
there's no way to tell the difference between an unset value & the
default value. Use OPT_CALLBACK() instead using the same pattern as in
commit b16a991c1b ("cherry-pick: detect bogus arguments to
--mainline", 2017-03-15).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/git-grep.txt |  4 ++--
 builtin/grep.c             | 26 ++++++++++++++++++++++----
 t/t7810-grep.sh            | 10 ++++++++++
 3 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/Documentation/git-grep.txt b/Documentation/git-grep.txt
index 71f32f3508..7b52e3fbc4 100644
--- a/Documentation/git-grep.txt
+++ b/Documentation/git-grep.txt
@@ -56,8 +56,8 @@ grep.extendedRegexp::
 	other than 'default'.
 
 grep.threads::
-	Number of grep worker threads to use.  If unset (or set to 0),
-	8 threads are used by default (for now).
+	Number of grep worker threads to use.  If unset, 8 threads are
+	used by default (for now). Set to 0 to disable threading.
 
 grep.fullName::
 	If set to true, enable `--full-name` option by default.
diff --git a/builtin/grep.c b/builtin/grep.c
index 65070c52fc..9478ab5dff 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -35,7 +35,7 @@ static int grep_submodule_launch(struct grep_opt *opt,
 				 const struct grep_source *gs);
 
 #define GREP_NUM_THREADS_DEFAULT 8
-static int num_threads;
+static int num_threads = -1;
 
 #ifndef NO_PTHREADS
 static pthread_t *threads;
@@ -897,6 +897,24 @@ static int context_callback(const struct option *opt, const char *arg,
 	return 0;
 }
 
+static int thread_callback(const struct option *opt,
+			   const char *arg, int unset)
+{
+	int *threads = (int*)opt->value;
+	char *end;
+
+	if (unset) {
+		*threads = GREP_NUM_THREADS_DEFAULT;
+		return 0;
+	}
+
+	*threads = strtol(arg, &end, 10);
+	if (*end || *threads < 0)
+		return opterror(opt, "invalid number of threads specified", 0);
+
+	return 0;
+}
+
 static int file_callback(const struct option *opt, const char *arg, int unset)
 {
 	struct grep_opt *grep_opt = opt->value;
@@ -1049,8 +1067,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			N_("show <n> context lines before matches")),
 		OPT_INTEGER('A', "after-context", &opt.post_context,
 			N_("show <n> context lines after matches")),
-		OPT_INTEGER(0, "threads", &num_threads,
-			N_("use <n> worker threads")),
+		OPT_CALLBACK(0, "threads", &num_threads, N_("n"),
+			N_("use <n> worker threads"), thread_callback),
 		OPT_NUMBER_CALLBACK(&opt, N_("shortcut for -C NUM"),
 			context_callback),
 		OPT_BOOL('p', "show-function", &opt.funcname,
@@ -1222,7 +1240,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 #ifndef NO_PTHREADS
 	if (list.nr || cached || show_in_pager)
 		num_threads = 0;
-	else if (num_threads == 0)
+	else if (num_threads == -1)
 		num_threads = GREP_NUM_THREADS_DEFAULT;
 	else if (num_threads < 0)
 		die(_("invalid number of threads specified (%d)"), num_threads);
diff --git a/t/t7810-grep.sh b/t/t7810-grep.sh
index cee42097b0..53c2ca05c4 100755
--- a/t/t7810-grep.sh
+++ b/t/t7810-grep.sh
@@ -1505,4 +1505,14 @@ test_expect_success 'grep does not report i-t-a and assume unchanged with -L' '
 	test_cmp expected actual
 '
 
+test_expect_success 'grep with thread options' '
+	git -c grep.threads=4 grep st.*dio &&
+	git grep --threads=4 st.*dio &&
+	git -c grep.threads=4 grep --threads=6 st.*dio &&
+	test_must_fail git -c grep.threads=-1 grep st.*dio &&
+	test_must_fail git -c grep.threads=-1 grep --threads=-1 st.*dio &&
+	test_must_fail git -c grep.threads=-1 grep --threads=1 st.*dio &&
+	test_must_fail git -c grep.threads=1 grep --threads=-1 st.*dio
+'
+
 test_done
-- 
2.11.0