cat-file: add --batch-command mode

Add a new flag --batch-command that accepts commands and arguments
from stdin, similar to git-update-ref --stdin.

At GitLab, we use a pair of long running cat-file processes when
accessing object content. One for iterating over object metadata with
--batch-check, and the other to grab object contents with --batch.

However, if we had --batch-command, we wouldn't need to keep both
processes around, and instead just have one --batch-command process
where we can flip between getting object info, and getting object
contents. Since we have a pair of cat-file processes per repository,
this means we can get rid of roughly half of long lived git cat-file
processes. Given there are many repositories being accessed at any given
time, this can lead to huge savings.

git cat-file --batch-command

will enter an interactive command mode whereby the user can enter in
commands and their arguments that get queued in memory:

<command1> [arg1] [arg2] LF
<command2> [arg1] [arg2] LF

When --buffer mode is used, commands will be queued in memory until a
flush command is issued that execute them:

flush LF

The reason for a flush command is that when a consumer process (A)
talks to a git cat-file process (B) and interactively writes to and
reads from it in --buffer mode, (A) needs to be able to control when
the buffer is flushed to stdout.

Currently, from (A)'s perspective, the only way is to either

1. kill (B)'s process
2. send an invalid object to stdin.

1. is not ideal from a performance perspective as it will require
spawning a new cat-file process each time, and 2. is hacky and not a
good long term solution.

With this mechanism of queueing up commands and letting (A) issue a
flush command, process (A) can control when the buffer is flushed and
can guarantee it will receive all of the output when in --buffer mode.
--batch-command also will not allow (B) to flush to stdout until a flush
is received.

This patch adds the basic structure for adding command which can be
extended in the future to add more commands. It also adds the following
two commands (on top of the flush command):

contents <object> LF
info <object> LF

The contents command takes an <object> argument and prints out the object
contents.

The info command takes an <object> argument and prints out the object
metadata.

These can be used in the following way with --buffer:

info <object> LF
contents <object> LF
contents <object> LF
info <object> LF
flush LF
info <object> LF
flush LF

When used without --buffer:

info <object> LF
contents <object> LF
contents <object> LF
info <object> LF
info <object> LF

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 38 insertions, 4 deletions

diff --git a/Documentation/git-cat-file.txt b/Documentation/git-cat-file.txt
index bef76f4dd0..70c5b4f12d 100644
--- a/Documentation/git-cat-file.txt
+++ b/Documentation/git-cat-file.txt
@@ -96,6 +96,33 @@ OPTIONS
 	need to specify the path, separated by whitespace.  See the
 	section `BATCH OUTPUT` below for details.
 
+--batch-command::
+--batch-command=<format>::
+	Enter a command mode that reads commands and arguments from stdin. May
+	only be combined with `--buffer`, `--textconv` or `--filters`. In the
+	case of `--textconv` or `--filters`, the input lines also need to specify
+	the path, separated by whitespace. See the section `BATCH OUTPUT` below
+	for details.
++
+`--batch-command` recognizes the following commands:
++
+--
+contents <object>::
+	Print object contents for object reference `<object>`. This corresponds to
+	the output of `--batch`.
+
+info <object>::
+	Print object info for object reference `<object>`. This corresponds to the
+	output of `--batch-check`.
+
+flush::
+	Used with `--buffer` to execute all preceding commands that were issued
+	since the beginning or since the last flush was issued. When `--buffer`
+	is used, no output will come until a `flush` is issued. When `--buffer`
+	is not used, commands are flushed each time without issuing `flush`.
+--
++
+
 --batch-all-objects::
 	Instead of reading a list of objects on stdin, perform the
 	requested batch operation on all objects in the repository and
@@ -110,7 +137,7 @@ OPTIONS
 	that a process can interactively read and write from
 	`cat-file`. With this option, the output uses normal stdio
 	buffering; this is much more efficient when invoking
-	`--batch-check` on a large number of objects.
+	`--batch-check` or `--batch-command` on a large number of objects.
 
 --unordered::
 	When `--batch-all-objects` is in use, visit objects in an
@@ -202,6 +229,13 @@ from stdin, one per line, and print information about them. By default,
 the whole line is considered as an object, as if it were fed to
 linkgit:git-rev-parse[1].
 
+When `--batch-command` is given, `cat-file` will read commands from stdin,
+one per line, and print information based on the command given. With
+`--batch-command`, the `info` command followed by an object will print
+information about the object the same way `--batch-check` would, and the
+`contents` command followed by an object prints contents in the same way
+`--batch` would.
+
 You can specify the information shown for each object by using a custom
 `<format>`. The `<format>` is copied literally to stdout for each
 object, with placeholders of the form `%(atom)` expanded, followed by a
@@ -237,9 +271,9 @@ newline. The available atoms are:
 If no format is specified, the default format is `%(objectname)
 %(objecttype) %(objectsize)`.
 
-If `--batch` is specified, the object information is followed by the
-object contents (consisting of `%(objectsize)` bytes), followed by a
-newline.
+If `--batch` is specified, or if `--batch-command` is used with the `contents`
+command, the object information is followed by the object contents (consisting
+of `%(objectsize)` bytes), followed by a newline.
 
 For example, `--batch` without a custom format would produce: