describe: teach describe negative pattern matches

Teach git-describe the `--exclude` option which will allow specifying
a glob pattern of tags to ignore. This can be combined with the
`--match` patterns to enable more flexibility in determining which tags
to consider.

For example, suppose you wish to find the first official release tag
that contains a certain commit. If we assume that official release tags
are of the form "v*" and pre-release candidates include "*rc*" in their
name, we can now find the first release tag that introduces the commit
abcdef:

  git describe --contains --match="v*" --exclude="*rc*" abcdef

Add documentation, tests, and completion for this change.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -88,6 +88,16 @@ OPTIONS
 	patterns will be considered. Use `--no-match` to clear and reset the
 	list of patterns.
 
+--exclude <pattern>::
+	Do not consider tags matching the given `glob(7)` pattern, excluding
+	the "refs/tags/" prefix. This can be used to narrow the tag space and
+	find only tags matching some meaningful criteria. If given multiple
+	times, a list of patterns will be accumulated and tags matching any
+	of the patterns will be excluded. When combined with --match a tag will
+	be considered when it matches at least one --match pattern and does not
+	match any of the --exclude patterns. Use `--no-exclude` to clear and
+	reset the list of patterns.
+
 --always::
 	Show uniquely abbreviated commit object as fallback.
 

builtin/describe.c

@@ -29,6 +29,7 @@ static int max_candidates = 10;
 static struct hashmap names;
 static int have_util;
 static struct string_list patterns = STRING_LIST_INIT_NODUP;
+static struct string_list exclude_patterns = STRING_LIST_INIT_NODUP;
 static int always;
 static const char *dirty;
 
@@ -129,6 +130,22 @@ static int get_name(const char *path, const struct object_id *oid, int flag, voi
 	if (!all && !is_tag)
 		return 0;
 
+	/*
+	 * If we're given exclude patterns, first exclude any tag which match
+	 * any of the exclude pattern.
+	 */
+	if (exclude_patterns.nr) {
+		struct string_list_item *item;
+
+		if (!is_tag)
+			return 0;
+
+		for_each_string_list_item(item, &exclude_patterns) {
+			if (!wildmatch(item->string, path + 10, 0, NULL))
+				return 0;
+		}
+	}
+
 	/*
 	 * If we're given patterns, accept only tags which match at least one
 	 * pattern.
@@ -421,6 +438,8 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 			    N_("consider <n> most recent tags (default: 10)")),
 		OPT_STRING_LIST(0, "match", &patterns, N_("pattern"),
 			   N_("only consider tags matching <pattern>")),
+		OPT_STRING_LIST(0, "exclude", &exclude_patterns, N_("pattern"),
+			   N_("do not consider tags matching <pattern>")),
 		OPT_BOOL(0, "always",        &always,
 			N_("show abbreviated commit object as fallback")),
 		{OPTION_STRING, 0, "dirty",  &dirty, N_("mark"),
@@ -458,6 +477,8 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 			argv_array_push(&args, "--tags");
 			for_each_string_list_item(item, &patterns)
 				argv_array_pushf(&args, "--refs=refs/tags/%s", item->string);
+			for_each_string_list_item(item, &exclude_patterns)
+				argv_array_pushf(&args, "--exclude=refs/tags/%s", item->string);
 		}
 		if (argc)
 			argv_array_pushv(&args, argv);

contrib/completion/git-completion.bash

@@ -1198,6 +1198,7 @@ _git_describe ()
 		__gitcomp "
 			--all --tags --contains --abbrev= --candidates=
 			--exact-match --debug --long --match --always
+			--exclude
 			"
 		return
 	esac

t/t6120-describe.sh

@@ -218,6 +218,14 @@ test_expect_success 'describe --contains and --match' '
 	test_cmp expect actual
 '
 
+test_expect_success 'describe --exclude' '
+	echo "c~1" >expect &&
+	tagged_commit=$(git rev-parse "refs/tags/A^0") &&
+	test_must_fail git describe --contains --match="B" $tagged_commit &&
+	git describe --contains --match="?" --exclude="A" $tagged_commit >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'describe --contains and --no-match' '
 	echo "A^0" >expect &&
 	tagged_commit=$(git rev-parse "refs/tags/A^0") &&