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git-commit-vandalism/Documentation/git-cat-file.txt
Jeff King 5c5b29b459 cat-file: disable refs/replace with --batch-all-objects 
When we're enumerating all objects in the object database, it doesn't
make sense to respect refs/replace. The point of this option is to
enumerate all of the objects in the database at a low level. By
definition we'd already show the replacement object's contents (under
its real oid), and showing those contents under another oid is almost
certainly working against what the user is trying to do.

Note that you could make the same argument for something like:

  git show-index <foo.idx |
  awk '{print $2}' |
  git cat-file --batch

but there we can't know in cat-file exactly what the user intended,
because we don't know the source of the input. They could be trying to
do low-level debugging, or they could be doing something more high-level
(e.g., imagine a porcelain built around cat-file for its object
accesses). So in those cases, we'll have to rely on the user specifying
"git --no-replace-objects" to tell us what to do.

One _could_ make an argument that "cat-file --batch" is sufficiently
low-level plumbing that it should not respect replace-objects at all
(and the caller should do any replacement if they want it).  But we have
been doing so for some time. The history is a little tangled:

  - looking back as far as v1.6.6, we would not respect replace refs for
    --batch-check, but would for --batch (because the former used
    sha1_object_info(), and the replace mechanism only affected actual
    object reads)

  - this discrepancy was made even weirder by 98e2092b50 (cat-file:
    teach --batch to stream blob objects, 2013-07-10), where we always
    output the header using the --batch-check code, and then printed the
    object separately. This could lead to "cat-file --batch" dying (when
    it notices the size or type changed for a non-blob object) or even
    producing bogus output (in streaming mode, we didn't notice that we
    wrote the wrong number of bytes).

  - that persisted until 1f7117ef7a (sha1_file: perform object
    replacement in sha1_object_info_extended(), 2013-12-11), which then
    respected replace refs for both forms.

So it has worked reliably this way for over 7 years, and we should make
sure it continues to do so. That could also be an argument that
--batch-all-objects should not change behavior (which this patch is
doing), but I really consider the current behavior to be an unintended
bug. It's a side effect of how the code is implemented (feeding the oids
back into oid_object_info() rather than looking at what we found while
reading the loose and packed object storage).

The implementation is straight-forward: we just disable the global
read_replace_refs flag when we're in --batch-all-objects mode. It would
perhaps be a little cleaner to change the flag we pass to
oid_object_info_extended(), but that's not enough. We also read objects
via read_object_file() and stream_blob_to_fd(). The former could switch
to its _extended() form, but the streaming code has no mechanism for
disabling replace refs. Setting the global flag works, and as a bonus,
it's impossible to have any "oops, we're sometimes replacing the object
and sometimes not" bugs in the output (like the ones caused by
98e2092b50 above).

The tests here cover the regular-input and --batch-all-objects cases,
for both --batch-check and --batch. There is a test in t6050 that covers
the regular-input case with --batch already, but this new one goes much
further in actually verifying the output (plus covering --batch-check
explicitly). This is perhaps a little overkill and the tests would be
simpler just covering --batch-check, but I wanted to make sure we're
checking that --batch output is consistent between the header and the
content. The global-flag technique used here makes that easy to get
right, but this is future-proofing us against regressions.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-10-08 15:45:14 -07:00

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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' (-t [--allow-unknown-type]| -s [--allow-unknown-type]| -e | -p | <type> | --textconv | --filters ) [--path=<path>] <object>
'git cat-file' (--batch[=<format>] | --batch-check[=<format>]) [ --textconv | --filters ] [--follow-symlinks]
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-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` 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].
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, 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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