builtin/for-each-ref.c: add `--exclude` option

When using `for-each-ref`, it is sometimes convenient for the caller to
be able to exclude certain parts of the references.

For example, if there are many `refs/__hidden__/*` references, the
caller may want to emit all references *except* the hidden ones.
Currently, the only way to do this is to post-process the output, like:

    $ git for-each-ref --format='%(refname)' | grep -v '^refs/hidden/'

Which is do-able, but requires processing a potentially large quantity
of references.

Teach `git for-each-ref` a new `--exclude=<pattern>` option, which
excludes references from the results if they match one or more excluded
patterns.

This patch provides a naive implementation where the `ref_filter` still
sees all references (including ones that it will discard) and is left to
check whether each reference matches any excluded pattern(s) before
emitting them.

By culling out references we know the caller doesn't care about, we can
avoid allocating memory for their storage, as well as spending time
sorting the output (among other things). Even the naive implementation
provides a significant speed-up on a modified copy of linux.git (that
has a hidden ref pointing at each commit):

    $ hyperfine \
      'git.compile for-each-ref --format="%(objectname) %(refname)" | grep -vE "[0-9a-f]{40} refs/pull/"' \
      'git.compile for-each-ref --format="%(objectname) %(refname)" --exclude refs/pull/'
    Benchmark 1: git.compile for-each-ref --format="%(objectname) %(refname)" | grep -vE "[0-9a-f]{40} refs/pull/"
      Time (mean ± σ):     820.1 ms ±   2.0 ms    [User: 703.7 ms, System: 152.0 ms]
      Range (min … max):   817.7 ms … 823.3 ms    10 runs

    Benchmark 2: git.compile for-each-ref --format="%(objectname) %(refname)" --exclude refs/pull/
      Time (mean ± σ):     106.6 ms ±   1.1 ms    [User: 99.4 ms, System: 7.1 ms]
      Range (min … max):   104.7 ms … 109.1 ms    27 runs

    Summary
      'git.compile for-each-ref --format="%(objectname) %(refname)" --exclude refs/pull/' ran
        7.69 ± 0.08 times faster than 'git.compile for-each-ref --format="%(objectname) %(refname)" | grep -vE "[0-9a-f]{40} refs/pull/"'

Subsequent patches will improve on this by avoiding visiting excluded
sections of the `packed-refs` file in certain cases.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/ref-filter.h b/ref-filter.h
index 160b807224..1524bc463a 100644
--- a/ref-filter.h
+++ b/ref-filter.h
@@ -6,6 +6,7 @@
 #include "refs.h"
 #include "commit.h"
 #include "string-list.h"
+#include "strvec.h"
 
 /* Quoting styles */
 #define QUOTE_NONE 0
@@ -59,6 +60,7 @@ struct ref_array {
 
 struct ref_filter {
 	const char **name_patterns;
+	struct strvec exclude;
 	struct oid_array points_at;
 	struct commit_list *with_commit;
 	struct commit_list *no_commit;
@@ -94,6 +96,7 @@ struct ref_format {
 
 #define REF_FILTER_INIT { \
 	.points_at = OID_ARRAY_INIT, \
+	.exclude = STRVEC_INIT, \
 }
 #define REF_FORMAT_INIT {             \
 	.use_color = -1,              \
@@ -112,6 +115,9 @@ struct ref_format {
 #define OPT_REF_SORT(var) \
 	OPT_STRING_LIST(0, "sort", (var), \
 			N_("key"), N_("field name to sort on"))
+#define OPT_REF_FILTER_EXCLUDE(var) \
+	OPT_STRVEC(0, "exclude", &(var)->exclude, \
+		   N_("pattern"), N_("exclude refs which match pattern"))
 
 /*
  * API for filtering a set of refs. Based on the type of refs the user