archive: simplify archive format guessing

The code to guess an output archive's format consumed any --format
options and built a new one.  Jonathan noticed that it does so in an
unsafe way, risking to overflow the static buffer fmt_opt.

Change the code to keep the existing --format options intact and to only
add a new one if a format could be guessed based on the output file name.
The new option is added as the first one, allowing the existing ones to
overrule it, i.e. explicit --format options given on the command line win
over format guesses, as before.

To simplify the code further, format_from_name() is changed to return the
full --format option, thus no potentially dangerous sprintf() calls are
needed any more.

Reported-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-archive.c

@@ -67,7 +67,7 @@ static const char *format_from_name(const char *filename)
 		return NULL;
 	ext++;
 	if (!strcasecmp(ext, "zip"))
-		return "zip";
+		return "--format=zip";
 	return NULL;
 }
 
@@ -81,41 +81,39 @@ int cmd_archive(int argc, const char **argv, const char *prefix)
 	const char *exec = "git-upload-archive";
 	const char *output = NULL;
 	const char *remote = NULL;
-	const char *format = NULL;
+	const char *format_option = NULL;
 	struct option local_opts[] = {
 		OPT_STRING('o', "output", &output, "file",
 			"write the archive to this file"),
 		OPT_STRING(0, "remote", &remote, "repo",
 			"retrieve the archive from remote repository <repo>"),
 		OPT_STRING(0, "exec", &exec, "cmd",
 			"path to the remote git-upload-archive command"),
-		OPT_STRING(0, "format", &format, "fmt", "archive format"),
 		OPT_END()
 	};
-	char fmt_opt[32];
 
 	argc = parse_options(argc, argv, prefix, local_opts, NULL,
 			     PARSE_OPT_KEEP_ALL);
 
 	if (output) {
 		create_output_file(output);
-		if (!format)
-			format = format_from_name(output);
+		format_option = format_from_name(output);
 	}
 
-	if (format) {
-		sprintf(fmt_opt, "--format=%s", format);
-		/*
-		 * We have enough room in argv[] to muck it in place,
-		 * because either --format and/or --output must have
-		 * been given on the original command line if we get
-		 * to this point, and parse_options() must have eaten
-		 * it, i.e. we can add back one element to the array.
-		 * But argv[] may contain "--"; we should make it the
-		 * first option.
-		 */
+	/*
+	 * We have enough room in argv[] to muck it in place, because
+	 * --output must have been given on the original command line
+	 * if we get to this point, and parse_options() must have eaten
+	 * it, i.e. we can add back one element to the array.
+	 *
+	 * We add a fake --format option at the beginning, with the
+	 * format inferred from our output filename.  This way explicit
+	 * --format options can override it, and the fake option is
+	 * inserted before any "--" that might have been given.
+	 */
+	if (format_option) {
 		memmove(argv + 2, argv + 1, sizeof(*argv) * argc);
-		argv[1] = fmt_opt;
+		argv[1] = format_option;
 		argv[++argc] = NULL;
 	}
 

t/t5000-tar-tree.sh

@@ -189,6 +189,16 @@ test_expect_success 'git archive --format=zip with --output' \
     'git archive --format=zip --output=d2.zip HEAD &&
     test_cmp d.zip d2.zip'
 
+test_expect_success 'git archive with --output, inferring format' '
+	git archive --output=d3.zip HEAD &&
+	test_cmp d.zip d3.zip
+'
+
+test_expect_success 'git archive with --output, override inferred format' '
+	git archive --format=tar --output=d4.zip HEAD &&
+	test_cmp b.tar d4.zip
+'
+
 $UNZIP -v >/dev/null 2>&1
 if [ $? -eq 127 ]; then
 	say "Skipping ZIP tests, because unzip was not found"