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git-commit-vandalism/Documentation/git-cat-file.txt
John Cai 440c705ea6 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>
2022-02-18 11:21:46 -08:00

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git-cat-file(1)
===============
NAME
----
git-cat-file - Provide content or type and size information for repository objects
SYNOPSIS
--------
[verse]
'git cat-file' <type> <object>
'git cat-file' (-e | -p) <object>
'git cat-file' (-t | -s) [--allow-unknown-type] <object>
'git cat-file' (--batch | --batch-check) [--batch-all-objects]
[--buffer] [--follow-symlinks] [--unordered]
[--textconv | --filters]
'git cat-file' (--textconv | --filters)
[<rev>:<path|tree-ish> | --path=<path|tree-ish> <rev>]
DESCRIPTION
-----------
In its first form, the command provides the content or the type of an object in
the repository. The type is required unless `-t` or `-p` is used to find the
object type, or `-s` is used to find the object size, or `--textconv` or
`--filters` is used (which imply type "blob").
In the second form, a list of objects (separated by linefeeds) is provided on
stdin, and the SHA-1, type, and size of each object is printed on stdout. The
output format can be overridden using the optional `<format>` argument. If
either `--textconv` or `--filters` was specified, the input is expected to
list the object names followed by the path name, separated by a single
whitespace, so that the appropriate drivers can be determined.
OPTIONS
-------
<object>::
The name of the object to show.
For a more complete list of ways to spell object names, see
the "SPECIFYING REVISIONS" section in linkgit:gitrevisions[7].
-t::
Instead of the content, show the object type identified by
`<object>`.
-s::
Instead of the content, show the object size identified by
`<object>`.
-e::
Exit with zero status if `<object>` exists and is a valid
object. If `<object>` is of an invalid format exit with non-zero and
emits an error on stderr.
-p::
Pretty-print the contents of `<object>` based on its type.
<type>::
Typically this matches the real type of `<object>` but asking
for a type that can trivially be dereferenced from the given
`<object>` is also permitted. An example is to ask for a
"tree" with `<object>` being a commit object that contains it,
or to ask for a "blob" with `<object>` being a tag object that
points at it.
--textconv::
Show the content as transformed by a textconv filter. In this case,
`<object>` has to be of the form `<tree-ish>:<path>`, or `:<path>` in
order to apply the filter to the content recorded in the index at
`<path>`.
--filters::
Show the content as converted by the filters configured in
the current working tree for the given `<path>` (i.e. smudge filters,
end-of-line conversion, etc). In this case, `<object>` has to be of
the form `<tree-ish>:<path>`, or `:<path>`.
--path=<path>::
For use with `--textconv` or `--filters`, to allow specifying an object
name and a path separately, e.g. when it is difficult to figure out
the revision from which the blob came.
--batch::
--batch=<format>::
Print object information and contents for each object provided
on stdin. May not be combined with any other options or arguments
except `--textconv` or `--filters`, in which case the input lines
also need to specify the path, separated by whitespace. See the
section `BATCH OUTPUT` below for details.
--batch-check::
--batch-check=<format>::
Print object information for each object provided on stdin. May
not be combined with any other options or arguments except
`--textconv` or `--filters`, in which case the input lines also
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
any alternate object stores (not just reachable objects).
Requires `--batch` or `--batch-check` be specified. By default,
the objects are visited in order sorted by their hashes; see
also `--unordered` below. Objects are presented as-is, without
respecting the "replace" mechanism of linkgit:git-replace[1].
--buffer::
Normally batch output is flushed after each object is output, so
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` or `--batch-command` on a large number of objects.
--unordered::
When `--batch-all-objects` is in use, visit objects in an
order which may be more efficient for accessing the object
contents than hash order. The exact details of the order are
unspecified, but if you do not require a specific order, this
should generally result in faster output, especially with
`--batch`. Note that `cat-file` will still show each object
only once, even if it is stored multiple times in the
repository.
--allow-unknown-type::
Allow `-s` or `-t` to query broken/corrupt objects of unknown type.
--follow-symlinks::
With `--batch` or `--batch-check`, follow symlinks inside the
repository when requesting objects with extended SHA-1
expressions of the form tree-ish:path-in-tree. Instead of
providing output about the link itself, provide output about
the linked-to object. If a symlink points outside the
tree-ish (e.g. a link to `/foo` or a root-level link to `../foo`),
the portion of the link which is outside the tree will be
printed.
+
This option does not (currently) work correctly when an object in the
index is specified (e.g. `:link` instead of `HEAD:link`) rather than
one in the tree.
+
This option cannot (currently) be used unless `--batch` or
`--batch-check` is used.
+
For example, consider a git repository containing:
+
--
f: a file containing "hello\n"
link: a symlink to f
dir/link: a symlink to ../f
plink: a symlink to ../f
alink: a symlink to /etc/passwd
--
+
For a regular file `f`, `echo HEAD:f | git cat-file --batch` would print
+
--
ce013625030ba8dba906f756967f9e9ca394464a blob 6
--
+
And `echo HEAD:link | git cat-file --batch --follow-symlinks` would
print the same thing, as would `HEAD:dir/link`, as they both point at
`HEAD:f`.
+
Without `--follow-symlinks`, these would print data about the symlink
itself. In the case of `HEAD:link`, you would see
+
--
4d1ae35ba2c8ec712fa2a379db44ad639ca277bd blob 1
--
+
Both `plink` and `alink` point outside the tree, so they would
respectively print:
+
--
symlink 4
../f
symlink 11
/etc/passwd
--
OUTPUT
------
If `-t` is specified, one of the `<type>`.
If `-s` is specified, the size of the `<object>` in bytes.
If `-e` is specified, no output, unless the `<object>` is malformed.
If `-p` is specified, the contents of `<object>` are pretty-printed.
If `<type>` is specified, the raw (though uncompressed) contents of the `<object>`
will be returned.
BATCH OUTPUT
------------
If `--batch` or `--batch-check` is given, `cat-file` will read objects
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
newline. The available atoms are:
`objectname`::
The full hex representation of the object name.
`objecttype`::
The type of the object (the same as `cat-file -t` reports).
`objectsize`::
The size, in bytes, of the object (the same as `cat-file -s`
reports).
`objectsize:disk`::
The size, in bytes, that the object takes up on disk. See the
note about on-disk sizes in the `CAVEATS` section below.
`deltabase`::
If the object is stored as a delta on-disk, this expands to the
full hex representation of the delta base object name.
Otherwise, expands to the null OID (all zeroes). See `CAVEATS`
below.
`rest`::
If this atom is used in the output string, input lines are split
at the first whitespace boundary. All characters before that
whitespace are considered to be the object name; characters
after that first run of whitespace (i.e., the "rest" of the
line) are output in place of the `%(rest)` atom.
If no format is specified, the default format is `%(objectname)
%(objecttype) %(objectsize)`.
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:
------------
<oid> SP <type> SP <size> LF
<contents> LF
------------
Whereas `--batch-check='%(objectname) %(objecttype)'` would produce:
------------
<oid> SP <type> LF
------------
If a name is specified on stdin that cannot be resolved to an object in
the repository, then `cat-file` will ignore any custom format and print:
------------
<object> SP missing LF
------------
If a name is specified that might refer to more than one object (an ambiguous short sha), then `cat-file` will ignore any custom format and print:
------------
<object> SP ambiguous LF
------------
If `--follow-symlinks` is used, and a symlink in the repository points
outside the repository, then `cat-file` will ignore any custom format
and print:
------------
symlink SP <size> LF
<symlink> LF
------------
The symlink will either be absolute (beginning with a `/`), or relative
to the tree root. For instance, if dir/link points to `../../foo`, then
`<symlink>` will be `../foo`. `<size>` is the size of the symlink in bytes.
If `--follow-symlinks` is used, the following error messages will be
displayed:
------------
<object> SP missing LF
------------
is printed when the initial symlink requested does not exist.
------------
dangling SP <size> LF
<object> LF
------------
is printed when the initial symlink exists, but something that
it (transitive-of) points to does not.
------------
loop SP <size> LF
<object> LF
------------
is printed for symlink loops (or any symlinks that
require more than 40 link resolutions to resolve).
------------
notdir SP <size> LF
<object> LF
------------
is printed when, during symlink resolution, a file is used as a
directory name.
CAVEATS
-------
Note that the sizes of objects on disk are reported accurately, but care
should be taken in drawing conclusions about which refs or objects are
responsible for disk usage. The size of a packed non-delta object may be
much larger than the size of objects which delta against it, but the
choice of which object is the base and which is the delta is arbitrary
and is subject to change during a repack.
Note also that multiple copies of an object may be present in the object
database; in this case, it is undefined which copy's size or delta base
will be reported.
GIT
---
Part of the linkgit:git[1] suite
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