builtin-grep: workaround for non GNU grep.

Some implementations do not know what to do with -H; define
NO_H_OPTION_IN_GREP when you build git if your grep lacks -H.

Most of the time, it can be worked around by prepending
/dev/null to the argument list, but that causes -L and -c to
slightly misbehave (they both expose /dev/null is given), so
when these options are given, do not run external grep that does
not understand -H.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -46,6 +46,8 @@ all:
 #
 # Define NO_MMAP if you want to avoid mmap.
 #
+# Define NO_H_OPTION_IN_GREP if your grep does not understand -H.
+#
 # Define WITH_OWN_SUBPROCESS_PY if you want to use with python 2.3.
 #
 # Define NO_IPV6 if you lack IPv6 support and getaddrinfo().
@@ -444,6 +446,12 @@ ifdef NO_ACCURATE_DIFF
 	ALL_CFLAGS += -DNO_ACCURATE_DIFF
 endif
 
+ifdef NO_H_OPTION_IN_GREP
+	NO_H_OPTION_IN_GREP=1
+else
+	NO_H_OPTION_IN_GREP=0
+endif
+
 # Shell quote (do not use $(call) to accomodate ancient setups);
 
 SHA1_HEADER_SQ = $(subst ','\'',$(SHA1_HEADER))
@@ -526,6 +534,9 @@ git$X git.spec \
 %.o: %.S
 	$(CC) -o $*.o -c $(ALL_CFLAGS) $<
 
+builtin-grep.o: builtin-grep.c
+	$(CC) -o $*.o -c $(ALL_CFLAGS) -DNO_H_OPTION_IN_GREP=$(NO_H_OPTION_IN_GREP) $<
+
 exec_cmd.o: exec_cmd.c
 	$(CC) -o $*.o -c $(ALL_CFLAGS) '-DGIT_EXEC_PATH="$(gitexecdir_SQ)"' $<
 

builtin-grep.c

@@ -453,7 +453,6 @@ static int external_grep(struct grep_opt *opt, const char **paths, int cached)
 
 	len = nr = 0;
 	push_arg("grep");
-	push_arg("-H");
 	if (opt->fixed)
 		push_arg("-F");
 	if (opt->linenum)
@@ -503,7 +502,13 @@ static int external_grep(struct grep_opt *opt, const char **paths, int cached)
 		push_arg("-e");
 		push_arg(p->pattern);
 	}
-	push_arg("--");
+
+	if (NO_H_OPTION_IN_GREP)
+		push_arg("/dev/null");
+	else {
+		push_arg("-H");
+		push_arg("--");
+	}
 
 	hit = 0;
 	argc = nr;
@@ -535,8 +540,19 @@ static int grep_cache(struct grep_opt *opt, const char **paths, int cached)
 	 * Use the external "grep" command for the case where
 	 * we grep through the checked-out files. It tends to
 	 * be a lot more optimized
+	 *
+	 * Some grep implementations do not understand -H nor --
+	 * but /dev/null can be used as a substitution in most
+	 * cases.
+	 *
+	 * However -L and -c would slightly misbehave (-L would
+	 * list /dev/null as a hit, and -c would report 0 hits
+	 * from /dev/null); so do not use the external one on
+	 * such platforms.
 	 */
-	if (!cached) {
+	if (!cached &&
+	    (!NO_H_OPTION_IN_GREP ||
+	     (!opt->count && !opt->unmatch_name_only))) {
 		hit = external_grep(opt, paths, cached);
 		if (hit >= 0)
 			return hit;