patch(1): Exit successfully if we're fed a 0-length patch

This change is made in the name of GNU patch compatibility. If GNU patch is
fed a zero-length patch, it will exit successfully with no output. This is
used in at least one port to date (comms/wsjtx), and we break on this usage.

It seems unlikely that anyone relies on patch(1) calling their completely
empty patch garbage and failing, and GNU compatibility is a plus if it helps
with porting, so make the switch.

Reported by:	db
MFC after:	2 weeks

usr.bin/patch/common.h

@@ -64,6 +64,7 @@ extern size_t	buf_size;	/* size of general purpose buffer */
 
 extern bool	using_plan_a;	/* try to keep everything in memory */
 extern bool	out_of_mem;	/* ran out of memory in plan a */
+extern bool	nonempty_patchf_seen;	/* seen a non-zero-length patch file? */
 
 #define	MAXFILEC 2
 

usr.bin/patch/patch.c

@@ -53,6 +53,7 @@ size_t		buf_size;		/* size of the general purpose buffer */
 
 bool		using_plan_a = true;	/* try to keep everything in memory */
 bool		out_of_mem = false;	/* ran out of memory in plan a */
+bool		nonempty_patchf_seen = false;	/* seen nonempty patch file? */
 
 #define MAXFILEC 2
 
@@ -419,7 +420,7 @@ main(int argc, char *argv[])
 		set_signals(1);
 	}
 
-	if (!patch_seen)
+	if (!patch_seen && nonempty_patchf_seen)
 		error = 2;
 
 	my_exit(error);

usr.bin/patch/pch.c

@@ -179,6 +179,9 @@ there_is_another_patch(void)
 			say("done\n");
 		return false;
 	}
+	if (p_filesize == 0)
+		return false;
+	nonempty_patchf_seen = true;
 	if (verbose)
 		say("Hmm...");
 	diff_type = intuit_diff_type();