describe --always: fall back to showing an abbreviated object name

Some callers may find it useful if "git describe" always gave back a
string that can be used as a shorter name for a commit object, rather than
checking its exit status (while squelching its error message, which could
potentially talk about more grave errors that should not be squelched) and
implementing a fallback themselves.

This teaches describe/name-rev a new option, --always, to use an
abbreviated object name when no tags or refs to use is found.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/builtin-describe.c b/builtin-describe.c
index 2f1e7ba150..2946457e9a 100644
--- a/builtin-describe.c
+++ b/builtin-describe.c
@@ -21,6 +21,7 @@ static int longformat;
 static int abbrev = DEFAULT_ABBREV;
 static int max_candidates = 10;
 const char *pattern = NULL;
+static int always;
 
 struct commit_name {
 	struct tag *tag;
@@ -250,8 +251,14 @@ static void describe(const char *arg, int last_one)
 		}
 	}
 
-	if (!match_cnt)
-		die("cannot describe '%s'", sha1_to_hex(cmit->object.sha1));
+	if (!match_cnt) {
+		const unsigned char *sha1 = cmit->object.sha1;
+		if (always) {
+			printf("%s\n", find_unique_abbrev(sha1, abbrev));
+			return;
+		}
+		die("cannot describe '%s'", sha1_to_hex(sha1));
+	}
 
 	qsort(all_matches, match_cnt, sizeof(all_matches[0]), compare_pt);
 
@@ -305,6 +312,8 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 			    "consider <n> most recent tags (default: 10)"),
 		OPT_STRING(0, "match",       &pattern, "pattern",
 			   "only consider tags matching <pattern>"),
+		OPT_BOOLEAN(0, "always",     &always,
+			   "show abbreviated commit object as fallback"),
 		OPT_END(),
 	};
 
@@ -320,11 +329,13 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 		die("--long is incompatible with --abbrev=0");
 
 	if (contains) {
-		const char **args = xmalloc((6 + argc) * sizeof(char*));
+		const char **args = xmalloc((7 + argc) * sizeof(char*));
 		int i = 0;
 		args[i++] = "name-rev";
 		args[i++] = "--name-only";
 		args[i++] = "--no-undefined";
+		if (always)
+			args[i++] = "--always";
 		if (!all) {
 			args[i++] = "--tags";
 			if (pattern) {