cat-file: add mailmap support to --batch-check option

Even though the cat-file command with `--batch-check` option does not
complain when `--use-mailmap` option is given, the latter option is
ignored. Compute the size of the object after replacing the idents and
report it instead.

In order to make `--batch-check` option honour the mailmap mechanism we
have to read the contents of the commit/tag object.

There were two ways to do it:

1. Make two calls to `oid_object_info_extended()`. If `--use-mailmap`
   option is given, the first call will get us the type of the object
   and second call will only be made if the object type is either a
   commit or tag to get the contents of the object.

2. Make one call to `oid_object_info_extended()` to get the type of the
   object. Then, if the object type is either of commit or tag, make a
   call to `repo_read_object_file()` to read the contents of the object.

I benchmarked the following command with both the above approaches and
compared against the current implementation where `--use-mailmap`
option is ignored:

`git cat-file --use-mailmap --batch-all-objects --batch-check --buffer
--unordered`

The results can be summarized as follows:
                       Time (mean ± σ)
default               827.7 ms ± 104.8 ms
first approach        6.197 s ± 0.093 s
second approach       1.975 s ± 0.217 s

Since, the second approach is faster than the first one, I implemented
it in this patch.

The command git cat-file can now use the mailmap mechanism to replace
idents with canonical versions for commit and tag objects. There are
several options like `--batch`, `--batch-check` and `--batch-command`
that can be combined with `--use-mailmap`. But the documentation for
`--batch`, `--batch-check` and `--batch-command` doesn't say so. This
patch fixes that documentation.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: John Cai <johncai86@gmail.com>
Helped-by: Taylor Blau <me@ttaylorr.com>
Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index 8a6e2343ec..1a8d90b5f5 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -444,6 +444,9 @@ static void batch_object_write(const char *obj_name,
 	if (!data->skip_object_info) {
 		int ret;
 
+		if (use_mailmap)
+			data->info.typep = &data->type;
+
 		if (pack)
 			ret = packed_object_info(the_repository, pack, offset,
 						 &data->info);
@@ -457,6 +460,18 @@ static void batch_object_write(const char *obj_name,
 			fflush(stdout);
 			return;
 		}
+
+		if (use_mailmap && (data->type == OBJ_COMMIT || data->type == OBJ_TAG)) {
+			size_t s = data->size;
+			char *buf = NULL;
+
+			buf = repo_read_object_file(the_repository, &data->oid, &data->type,
+						    &data->size);
+			buf = replace_idents_using_mailmap(buf, &s);
+			data->size = cast_size_t_to_ulong(s);
+
+			free(buf);
+		}
 	}
 
 	strbuf_reset(scratch);