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git-commit-vandalism/Documentation/fetch-options.txt
Patrick Steinhardt c7b190dabd fetch: implement support for atomic reference updates 
When executing a fetch, then git will currently allocate one reference
transaction per reference update and directly commit it. This means that
fetches are non-atomic: even if some of the reference updates fail,
others may still succeed and modify local references.

This is fine in many scenarios, but this strategy has its downsides.

- The view of remote references may be inconsistent and may show a
  bastardized state of the remote repository.

- Batching together updates may improve performance in certain
  scenarios. While the impact probably isn't as pronounced with loose
  references, the upcoming reftable backend may benefit as it needs to
  write less files in case the update is batched.

- The reference-update hook is currently being executed twice per
  updated reference. While this doesn't matter when there is no such
  hook, we have seen severe performance regressions when doing a
  git-fetch(1) with reference-transaction hook when the remote
  repository has hundreds of thousands of references.

Similar to `git push --atomic`, this commit thus introduces atomic
fetches. Instead of allocating one reference transaction per updated
reference, it causes us to only allocate a single transaction and commit
it as soon as all updates were received. If locking of any reference
fails, then we abort the complete transaction and don't update any
reference, which gives us an all-or-nothing fetch.

Note that this may not completely fix the first of above downsides, as
the consistent view also depends on the server-side. If the server
doesn't have a consistent view of its own references during the
reference negotiation phase, then the client would get the same
inconsistent view the server has. This is a separate problem though and,
if it actually exists, can be fixed at a later point.

This commit also changes the way we write FETCH_HEAD in case `--atomic`
is passed. Instead of writing changes as we go, we need to accumulate
all changes first and only commit them at the end when we know that all
reference updates succeeded. Ideally, we'd just do so via a temporary
file so that we don't need to carry all updates in-memory. This isn't
trivially doable though considering the `--append` mode, where we do not
truncate the file but simply append to it. And given that we support
concurrent processes appending to FETCH_HEAD at the same time without
any loss of data, seeding the temporary file with current contents of
FETCH_HEAD initially and then doing a rename wouldn't work either. So
this commit implements the simple strategy of buffering all changes and
appending them to the file on commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-12 12:06:15 -08:00

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--all::
Fetch all remotes.
-a::
--append::
Append ref names and object names of fetched refs to the
existing contents of `.git/FETCH_HEAD`. Without this
option old data in `.git/FETCH_HEAD` will be overwritten.
--atomic::
Use an atomic transaction to update local refs. Either all refs are
updated, or on error, no refs are updated.
--depth=<depth>::
Limit fetching to the specified number of commits from the tip of
each remote branch history. If fetching to a 'shallow' repository
created by `git clone` with `--depth=<depth>` option (see
linkgit:git-clone[1]), deepen or shorten the history to the specified
number of commits. Tags for the deepened commits are not fetched.
--deepen=<depth>::
Similar to --depth, except it specifies the number of commits
from the current shallow boundary instead of from the tip of
each remote branch history.
--shallow-since=<date>::
Deepen or shorten the history of a shallow repository to
include all reachable commits after <date>.
--shallow-exclude=<revision>::
Deepen or shorten the history of a shallow repository to
exclude commits reachable from a specified remote branch or tag.
This option can be specified multiple times.
--unshallow::
If the source repository is complete, convert a shallow
repository to a complete one, removing all the limitations
imposed by shallow repositories.
+
If the source repository is shallow, fetch as much as possible so that
the current repository has the same history as the source repository.
--update-shallow::
By default when fetching from a shallow repository,
`git fetch` refuses refs that require updating
.git/shallow. This option updates .git/shallow and accept such
refs.
--negotiation-tip=<commit|glob>::
By default, Git will report, to the server, commits reachable
from all local refs to find common commits in an attempt to
reduce the size of the to-be-received packfile. If specified,
Git will only report commits reachable from the given tips.
This is useful to speed up fetches when the user knows which
local ref is likely to have commits in common with the
upstream ref being fetched.
+
This option may be specified more than once; if so, Git will report
commits reachable from any of the given commits.
+
The argument to this option may be a glob on ref names, a ref, or the (possibly
abbreviated) SHA-1 of a commit. Specifying a glob is equivalent to specifying
this option multiple times, one for each matching ref name.
+
See also the `fetch.negotiationAlgorithm` configuration variable
documented in linkgit:git-config[1].
--dry-run::
Show what would be done, without making any changes.
ifndef::git-pull[]
--[no-]write-fetch-head::
Write the list of remote refs fetched in the `FETCH_HEAD`
file directly under `$GIT_DIR`. This is the default.
Passing `--no-write-fetch-head` from the command line tells
Git not to write the file. Under `--dry-run` option, the
file is never written.
endif::git-pull[]
-f::
--force::
When 'git fetch' is used with `<src>:<dst>` refspec it may
refuse to update the local branch as discussed
ifdef::git-pull[]
in the `<refspec>` part of the linkgit:git-fetch[1]
documentation.
endif::git-pull[]
ifndef::git-pull[]
in the `<refspec>` part below.
endif::git-pull[]
This option overrides that check.
-k::
--keep::
Keep downloaded pack.
ifndef::git-pull[]
--multiple::
Allow several <repository> and <group> arguments to be
specified. No <refspec>s may be specified.
--[no-]auto-maintenance::
--[no-]auto-gc::
Run `git maintenance run --auto` at the end to perform automatic
repository maintenance if needed. (`--[no-]auto-gc` is a synonym.)
This is enabled by default.
--[no-]write-commit-graph::
Write a commit-graph after fetching. This overrides the config
setting `fetch.writeCommitGraph`.
endif::git-pull[]
-p::
--prune::
Before fetching, remove any remote-tracking references that no
longer exist on the remote. Tags are not subject to pruning
if they are fetched only because of the default tag
auto-following or due to a --tags option. However, if tags
are fetched due to an explicit refspec (either on the command
line or in the remote configuration, for example if the remote
was cloned with the --mirror option), then they are also
subject to pruning. Supplying `--prune-tags` is a shorthand for
providing the tag refspec.
ifndef::git-pull[]
+
See the PRUNING section below for more details.
-P::
--prune-tags::
Before fetching, remove any local tags that no longer exist on
the remote if `--prune` is enabled. This option should be used
more carefully, unlike `--prune` it will remove any local
references (local tags) that have been created. This option is
a shorthand for providing the explicit tag refspec along with
`--prune`, see the discussion about that in its documentation.
+
See the PRUNING section below for more details.
endif::git-pull[]
ifndef::git-pull[]
-n::
endif::git-pull[]
--no-tags::
By default, tags that point at objects that are downloaded
from the remote repository are fetched and stored locally.
This option disables this automatic tag following. The default
behavior for a remote may be specified with the remote.<name>.tagOpt
setting. See linkgit:git-config[1].
--refmap=<refspec>::
When fetching refs listed on the command line, use the
specified refspec (can be given more than once) to map the
refs to remote-tracking branches, instead of the values of
`remote.*.fetch` configuration variables for the remote
repository. Providing an empty `<refspec>` to the
`--refmap` option causes Git to ignore the configured
refspecs and rely entirely on the refspecs supplied as
command-line arguments. See section on "Configured Remote-tracking
Branches" for details.
-t::
--tags::
Fetch all tags from the remote (i.e., fetch remote tags
`refs/tags/*` into local tags with the same name), in addition
to whatever else would otherwise be fetched. Using this
option alone does not subject tags to pruning, even if --prune
is used (though tags may be pruned anyway if they are also the
destination of an explicit refspec; see `--prune`).
ifndef::git-pull[]
--recurse-submodules[=yes|on-demand|no]::
This option controls if and under what conditions new commits of
populated submodules should be fetched too. It can be used as a
boolean option to completely disable recursion when set to 'no' or to
unconditionally recurse into all populated submodules when set to
'yes', which is the default when this option is used without any
value. Use 'on-demand' to only recurse into a populated submodule
when the superproject retrieves a commit that updates the submodule's
reference to a commit that isn't already in the local submodule
clone. By default, 'on-demand' is used, unless
`fetch.recurseSubmodules` is set (see linkgit:git-config[1]).
endif::git-pull[]
-j::
--jobs=<n>::
Number of parallel children to be used for all forms of fetching.
+
If the `--multiple` option was specified, the different remotes will be fetched
in parallel. If multiple submodules are fetched, they will be fetched in
parallel. To control them independently, use the config settings
`fetch.parallel` and `submodule.fetchJobs` (see linkgit:git-config[1]).
+
Typically, parallel recursive and multi-remote fetches will be faster. By
default fetches are performed sequentially, not in parallel.
ifndef::git-pull[]
--no-recurse-submodules::
Disable recursive fetching of submodules (this has the same effect as
using the `--recurse-submodules=no` option).
endif::git-pull[]
--set-upstream::
If the remote is fetched successfully, add upstream
(tracking) reference, used by argument-less
linkgit:git-pull[1] and other commands. For more information,
see `branch.<name>.merge` and `branch.<name>.remote` in
linkgit:git-config[1].
ifndef::git-pull[]
--submodule-prefix=<path>::
Prepend <path> to paths printed in informative messages
such as "Fetching submodule foo". This option is used
internally when recursing over submodules.
--recurse-submodules-default=[yes|on-demand]::
This option is used internally to temporarily provide a
non-negative default value for the --recurse-submodules
option. All other methods of configuring fetch's submodule
recursion (such as settings in linkgit:gitmodules[5] and
linkgit:git-config[1]) override this option, as does
specifying --[no-]recurse-submodules directly.
-u::
--update-head-ok::
By default 'git fetch' refuses to update the head which
corresponds to the current branch. This flag disables the
check. This is purely for the internal use for 'git pull'
to communicate with 'git fetch', and unless you are
implementing your own Porcelain you are not supposed to
use it.
endif::git-pull[]
--upload-pack <upload-pack>::
When given, and the repository to fetch from is handled
by 'git fetch-pack', `--exec=<upload-pack>` is passed to
the command to specify non-default path for the command
run on the other end.
ifndef::git-pull[]
-q::
--quiet::
Pass --quiet to git-fetch-pack and silence any other internally
used git commands. Progress is not reported to the standard error
stream.
-v::
--verbose::
Be verbose.
endif::git-pull[]
--progress::
Progress status is reported on the standard error stream
by default when it is attached to a terminal, unless -q
is specified. This flag forces progress status even if the
standard error stream is not directed to a terminal.
-o <option>::
--server-option=<option>::
Transmit the given string to the server when communicating using
protocol version 2. The given string must not contain a NUL or LF
character. The server's handling of server options, including
unknown ones, is server-specific.
When multiple `--server-option=<option>` are given, they are all
sent to the other side in the order listed on the command line.
--show-forced-updates::
By default, git checks if a branch is force-updated during
fetch. This can be disabled through fetch.showForcedUpdates, but
the --show-forced-updates option guarantees this check occurs.
See linkgit:git-config[1].
--no-show-forced-updates::
By default, git checks if a branch is force-updated during
fetch. Pass --no-show-forced-updates or set fetch.showForcedUpdates
to false to skip this check for performance reasons. If used during
'git-pull' the --ff-only option will still check for forced updates
before attempting a fast-forward update. See linkgit:git-config[1].
-4::
--ipv4::
Use IPv4 addresses only, ignoring IPv6 addresses.
-6::
--ipv6::
Use IPv6 addresses only, ignoring IPv4 addresses.
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