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Recommend git-filter-repo instead of git-filter-branch

filter-branch suffers from a deluge of disguised dangers that disfigure
history rewrites (i.e. deviate from the deliberate changes).  Many of
these problems are unobtrusive and can easily go undiscovered until the
new repository is in use.  This can result in problems ranging from an
even messier history than what led folks to filter-branch in the first
place, to data loss or corruption.  These issues cannot be backward
compatibly fixed, so add a warning to both filter-branch and its manpage
recommending that another tool (such as filter-repo) be used instead.

Also, update other manpages that referenced filter-branch.  Several of
these needed updates even if we could continue recommending
filter-branch, either due to implying that something was unique to
filter-branch when it applied more generally to all history rewriting
tools (e.g. BFG, reposurgeon, fast-import, filter-repo), or because
something about filter-branch was used as an example despite other more
commonly known examples now existing.  Reword these sections to fix
these issues and to avoid recommending filter-branch.

Finally, remove the section explaining BFG Repo Cleaner as an
alternative to filter-branch.  I feel somewhat bad about this,
especially since I feel like I learned so much from BFG that I put to
good use in filter-repo (which is much more than I can say for
filter-branch), but keeping that section presented a few problems:
  * In order to recommend that people quit using filter-branch, we need
    to provide them a recomendation for something else to use that
    can handle all the same types of rewrites.  To my knowledge,
    filter-repo is the only such tool.  So it needs to be mentioned.
  * I don't want to give conflicting recommendations to users
  * If we recommend two tools, we shouldn't expect users to learn both
    and pick which one to use; we should explain which problems one
    can solve that the other can't or when one is much faster than
    the other.
  * BFG and filter-repo have similar performance
  * All filtering types that BFG can do, filter-repo can also do.  In
    fact, filter-repo comes with a reimplementation of BFG named
    bfg-ish which provides the same user-interface as BFG but with
    several bugfixes and new features that are hard to implement in
    BFG due to its technical underpinnings.
While I could still mention both tools, it seems like I would need to
provide some kind of comparison and I would ultimately just say that
filter-repo can do everything BFG can, so ultimately it seems that it
is just better to remove that section altogether.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
tags/v2.24.0-rc0
Elijah Newren Junio C Hamano 2 months ago
parent
commit
9df53c5de6
9 changed files with 288 additions and 59 deletions
  1. +3
    -3
      Documentation/git-fast-export.txt
  2. +243
    -30
      Documentation/git-filter-branch.txt
  3. +8
    -9
      Documentation/git-gc.txt
  4. +2
    -1
      Documentation/git-rebase.txt
  5. +5
    -5
      Documentation/git-replace.txt
  6. +5
    -5
      Documentation/git-svn.txt
  7. +6
    -4
      Documentation/githooks.txt
  8. +2
    -2
      contrib/svn-fe/svn-fe.txt
  9. +14
    -0
      git-filter-branch.sh

+ 3
- 3
Documentation/git-fast-export.txt View File

@@ -17,9 +17,9 @@ This program dumps the given revisions in a form suitable to be piped
into 'git fast-import'.

You can use it as a human-readable bundle replacement (see
linkgit:git-bundle[1]), or as a kind of an interactive
'git filter-branch'.
linkgit:git-bundle[1]), or as a format that can be edited before being
fed to 'git fast-import' in order to do history rewrites (an ability
relied on by tools like 'git filter-repo').

OPTIONS
-------


+ 243
- 30
Documentation/git-filter-branch.txt View File

@@ -16,6 +16,19 @@ SYNOPSIS
[--original <namespace>] [-d <directory>] [-f | --force]
[--state-branch <branch>] [--] [<rev-list options>...]

WARNING
-------
'git filter-branch' has a plethora of pitfalls that can produce non-obvious
manglings of the intended history rewrite (and can leave you with little
time to investigate such problems since it has such abysmal performance).
These safety and performance issues cannot be backward compatibly fixed and
as such, its use is not recommended. Please use an alternative history
filtering tool such as https://github.com/newren/git-filter-repo/[git
filter-repo]. If you still need to use 'git filter-branch', please
carefully read <<SAFETY>> (and <<PERFORMANCE>>) to learn about the land
mines of filter-branch, and then vigilantly avoid as many of the hazards
listed there as reasonably possible.

DESCRIPTION
-----------
Lets you rewrite Git revision history by rewriting the branches mentioned
@@ -445,36 +458,236 @@ warned.
(or if your git-gc is not new enough to support arguments to
`--prune`, use `git repack -ad; git prune` instead).

NOTES
-----

git-filter-branch allows you to make complex shell-scripted rewrites
of your Git history, but you probably don't need this flexibility if
you're simply _removing unwanted data_ like large files or passwords.
For those operations you may want to consider
http://rtyley.github.io/bfg-repo-cleaner/[The BFG Repo-Cleaner],
a JVM-based alternative to git-filter-branch, typically at least
10-50x faster for those use-cases, and with quite different
characteristics:

* Any particular version of a file is cleaned exactly _once_. The BFG,
unlike git-filter-branch, does not give you the opportunity to
handle a file differently based on where or when it was committed
within your history. This constraint gives the core performance
benefit of The BFG, and is well-suited to the task of cleansing bad
data - you don't care _where_ the bad data is, you just want it
_gone_.

* By default The BFG takes full advantage of multi-core machines,
cleansing commit file-trees in parallel. git-filter-branch cleans
commits sequentially (i.e. in a single-threaded manner), though it
_is_ possible to write filters that include their own parallelism,
in the scripts executed against each commit.

* The http://rtyley.github.io/bfg-repo-cleaner/#examples[command options]
are much more restrictive than git-filter branch, and dedicated just
to the tasks of removing unwanted data- e.g:
`--strip-blobs-bigger-than 1M`.
[[PERFORMANCE]]
PERFORMANCE
-----------

The performance of git-filter-branch is glacially slow; its design makes it
impossible for a backward-compatible implementation to ever be fast:

* In editing files, git-filter-branch by design checks out each and
every commit as it existed in the original repo. If your repo has 10\^5
files and 10\^5 commits, but each commit only modifies 5 files, then
git-filter-branch will make you do 10\^10 modifications, despite only
having (at most) 5*10^5 unique blobs.

* If you try and cheat and try to make git-filter-branch only work on
files modified in a commit, then two things happen

** you run into problems with deletions whenever the user is simply
trying to rename files (because attempting to delete files that
don't exist looks like a no-op; it takes some chicanery to remap
deletes across file renames when the renames happen via arbitrary
user-provided shell)

** even if you succeed at the map-deletes-for-renames chicanery, you
still technically violate backward compatibility because users are
allowed to filter files in ways that depend upon topology of
commits instead of filtering solely based on file contents or names
(though this has not been observed in the wild).

* Even if you don't need to edit files but only want to e.g. rename or
remove some and thus can avoid checking out each file (i.e. you can use
--index-filter), you still are passing shell snippets for your filters.
This means that for every commit, you have to have a prepared git repo
where those filters can be run. That's a significant setup.

* Further, several additional files are created or updated per commit by
git-filter-branch. Some of these are for supporting the convenience
functions provided by git-filter-branch (such as map()), while others
are for keeping track of internal state (but could have also been
accessed by user filters; one of git-filter-branch's regression tests
does so). This essentially amounts to using the filesystem as an IPC
mechanism between git-filter-branch and the user-provided filters.
Disks tend to be a slow IPC mechanism, and writing these files also
effectively represents a forced synchronization point between separate
processes that we hit with every commit.

* The user-provided shell commands will likely involve a pipeline of
commands, resulting in the creation of many processes per commit.
Creating and running another process takes a widely varying amount of
time between operating systems, but on any platform it is very slow
relative to invoking a function.

* git-filter-branch itself is written in shell, which is kind of slow.
This is the one performance issue that could be backward-compatibly
fixed, but compared to the above problems that are intrinsic to the
design of git-filter-branch, the language of the tool itself is a
relatively minor issue.

** Side note: Unfortunately, people tend to fixate on the
written-in-shell aspect and periodically ask if git-filter-branch
could be rewritten in another language to fix the performance
issues. Not only does that ignore the bigger intrinsic problems
with the design, it'd help less than you'd expect: if
git-filter-branch itself were not shell, then the convenience
functions (map(), skip_commit(), etc) and the `--setup` argument
could no longer be executed once at the beginning of the program
but would instead need to be prepended to every user filter (and
thus re-executed with every commit).

The https://github.com/newren/git-filter-repo/[git filter-repo] tool is
an alternative to git-filter-branch which does not suffer from these
performance problems or the safety problems (mentioned below). For those
with existing tooling which relies upon git-filter-branch, 'git
repo-filter' also provides
https://github.com/newren/git-filter-repo/blob/master/contrib/filter-repo-demos/filter-lamely[filter-lamely],
a drop-in git-filter-branch replacement (with a few caveats). While
filter-lamely suffers from all the same safety issues as
git-filter-branch, it at least ameloriates the performance issues a
little.

[[SAFETY]]
SAFETY
------

git-filter-branch is riddled with gotchas resulting in various ways to
easily corrupt repos or end up with a mess worse than what you started
with:

* Someone can have a set of "working and tested filters" which they
document or provide to a coworker, who then runs them on a different OS
where the same commands are not working/tested (some examples in the
git-filter-branch manpage are also affected by this). BSD vs. GNU
userland differences can really bite. If lucky, error messages are
spewed. But just as likely, the commands either don't do the filtering
requested, or silently corrupt by making some unwanted change. The
unwanted change may only affect a few commits, so it's not necessarily
obvious either. (The fact that problems won't necessarily be obvious
means they are likely to go unnoticed until the rewritten history is in
use for quite a while, at which point it's really hard to justify
another flag-day for another rewrite.)

* Filenames with spaces are often mishandled by shell snippets since
they cause problems for shell pipelines. Not everyone is familiar with
find -print0, xargs -0, git-ls-files -z, etc. Even people who are
familiar with these may assume such flags are not relevant because
someone else renamed any such files in their repo back before the person
doing the filtering joined the project. And often, even those familiar
with handling arguments with spaces may not do so just because they
aren't in the mindset of thinking about everything that could possibly
go wrong.

* Non-ascii filenames can be silently removed despite being in a desired
directory. Keeping only wanted paths is often done using pipelines like
`git ls-files | grep -v ^WANTED_DIR/ | xargs git rm`. ls-files will
only quote filenames if needed, so folks may not notice that one of the
files didn't match the regex (at least not until it's much too late).
Yes, someone who knows about core.quotePath can avoid this (unless they
have other special characters like \t, \n, or "), and people who use
ls-files -z with something other than grep can avoid this, but that
doesn't mean they will.

* Similarly, when moving files around, one can find that filenames with
non-ascii or special characters end up in a different directory, one
that includes a double quote character. (This is technically the same
issue as above with quoting, but perhaps an interesting different way
that it can and has manifested as a problem.)

* It's far too easy to accidentally mix up old and new history. It's
still possible with any tool, but git-filter-branch almost invites it.
If lucky, the only downside is users getting frustrated that they don't
know how to shrink their repo and remove the old stuff. If unlucky,
they merge old and new history and end up with multiple "copies" of each
commit, some of which have unwanted or sensitive files and others which
don't. This comes about in multiple different ways:

** the default to only doing a partial history rewrite ('--all' is not
the default and few examples show it)

** the fact that there's no automatic post-run cleanup

** the fact that --tag-name-filter (when used to rename tags) doesn't
remove the old tags but just adds new ones with the new name

** the fact that little educational information is provided to inform
users of the ramifications of a rewrite and how to avoid mixing old
and new history. For example, this man page discusses how users
need to understand that they need to rebase their changes for all
their branches on top of new history (or delete and reclone), but
that's only one of multiple concerns to consider. See the
"DISCUSSION" section of the git filter-repo manual page for more
details.

* Annotated tags can be accidentally converted to lightweight tags, due
to either of two issues:

** Someone can do a history rewrite, realize they messed up, restore
from the backups in refs/original/, and then redo their
git-filter-branch command. (The backup in refs/original/ is not a
real backup; it dereferences tags first.)

** Running git-filter-branch with either --tags or --all in your
<rev-list options>. In order to retain annotated tags as
annotated, you must use --tag-name-filter (and must not have
restored from refs/original/ in a previously botched rewrite).

* Any commit messages that specify an encoding will become corrupted
by the rewrite; git-filter-branch ignores the encoding, takes the original
bytes, and feeds it to commit-tree without telling it the proper
encoding. (This happens whether or not --msg-filter is used.)

* Commit messages (even if they are all UTF-8) by default become
corrupted due to not being updated -- any references to other commit
hashes in commit messages will now refer to no-longer-extant commits.

* There are no facilities for helping users find what unwanted crud they
should delete, which means they are much more likely to have incomplete
or partial cleanups that sometimes result in confusion and people
wasting time trying to understand. (For example, folks tend to just
look for big files to delete instead of big directories or extensions,
and once they do so, then sometime later folks using the new repository
who are going through history will notice a build artifact directory
that has some files but not others, or a cache of dependencies
(node_modules or similar) which couldn't have ever been functional since
it's missing some files.)

* If --prune-empty isn't specified, then the filtering process can
create hoards of confusing empty commits

* If --prune-empty is specified, then intentionally placed empty
commits from before the filtering operation are also pruned instead of
just pruning commits that became empty due to filtering rules.

* If --prune empty is specified, sometimes empty commits are missed
and left around anyway (a somewhat rare bug, but it happens...)

* A minor issue, but users who have a goal to update all names and
emails in a repository may be led to --env-filter which will only update
authors and committers, missing taggers.

* If the user provides a --tag-name-filter that maps multiple tags to
the same name, no warning or error is provided; git-filter-branch simply
overwrites each tag in some undocumented pre-defined order resulting in
only one tag at the end. (A git-filter-branch regression test requires
this surprising behavior.)

Also, the poor performance of git-filter-branch often leads to safety
issues:

* Coming up with the correct shell snippet to do the filtering you want
is sometimes difficult unless you're just doing a trivial modification
such as deleting a couple files. Unfortunately, people often learn if
the snippet is right or wrong by trying it out, but the rightness or
wrongness can vary depending on special circumstances (spaces in
filenames, non-ascii filenames, funny author names or emails, invalid
timezones, presence of grafts or replace objects, etc.), meaning they
may have to wait a long time, hit an error, then restart. The
performance of git-filter-branch is so bad that this cycle is painful,
reducing the time available to carefully re-check (to say nothing about
what it does to the patience of the person doing the rewrite even if
they do technically have more time available). This problem is extra
compounded because errors from broken filters may not be shown for a
long time and/or get lost in a sea of output. Even worse, broken
filters often just result in silent incorrect rewrites.

* To top it all off, even when users finally find working commands, they
naturally want to share them. But they may be unaware that their repo
didn't have some special cases that someone else's does. So, when
someone else with a different repository runs the same commands, they
get hit by the problems above. Or, the user just runs commands that
really were vetted for special cases, but they run it on a different OS
where it doesn't work, as noted above.

GIT
---


+ 8
- 9
Documentation/git-gc.txt View File

@@ -115,15 +115,14 @@ NOTES
-----

'git gc' tries very hard not to delete objects that are referenced
anywhere in your repository. In
particular, it will keep not only objects referenced by your current set
of branches and tags, but also objects referenced by the index,
remote-tracking branches, refs saved by 'git filter-branch' in
refs/original/, reflogs (which may reference commits in branches
that were later amended or rewound), and anything else in the refs/* namespace.
If you are expecting some objects to be deleted and they aren't, check
all of those locations and decide whether it makes sense in your case to
remove those references.
anywhere in your repository. In particular, it will keep not only
objects referenced by your current set of branches and tags, but also
objects referenced by the index, remote-tracking branches, notes saved
by 'git notes' under refs/notes/, reflogs (which may reference commits
in branches that were later amended or rewound), and anything else in
the refs/* namespace. If you are expecting some objects to be deleted
and they aren't, check all of those locations and decide whether it
makes sense in your case to remove those references.

On the other hand, when 'git gc' runs concurrently with another process,
there is a risk of it deleting an object that the other process is using


+ 2
- 1
Documentation/git-rebase.txt View File

@@ -832,7 +832,8 @@ Hard case: The changes are not the same.::
This happens if the 'subsystem' rebase had conflicts, or used
`--interactive` to omit, edit, squash, or fixup commits; or
if the upstream used one of `commit --amend`, `reset`, or
`filter-branch`.
a full history rewriting command like
https://github.com/newren/git-filter-repo[`filter-repo`].


The easy case


+ 5
- 5
Documentation/git-replace.txt View File

@@ -123,10 +123,10 @@ The following format are available:
CREATING REPLACEMENT OBJECTS
----------------------------

linkgit:git-filter-branch[1], linkgit:git-hash-object[1] and
linkgit:git-rebase[1], among other git commands, can be used to create
replacement objects from existing objects. The `--edit` option can
also be used with 'git replace' to create a replacement object by
linkgit:git-hash-object[1], linkgit:git-rebase[1], and
https://github.com/newren/git-filter-repo[git-filter-repo], among other git commands, can be used to
create replacement objects from existing objects. The `--edit` option
can also be used with 'git replace' to create a replacement object by
editing an existing object.

If you want to replace many blobs, trees or commits that are part of a
@@ -148,13 +148,13 @@ pending objects.
SEE ALSO
--------
linkgit:git-hash-object[1]
linkgit:git-filter-branch[1]
linkgit:git-rebase[1]
linkgit:git-tag[1]
linkgit:git-branch[1]
linkgit:git-commit[1]
linkgit:git-var[1]
linkgit:git[1]
https://github.com/newren/git-filter-repo[git-filter-repo]

GIT
---


+ 5
- 5
Documentation/git-svn.txt View File

@@ -769,11 +769,11 @@ option for (hopefully) obvious reasons.
+
This option is NOT recommended as it makes it difficult to track down
old references to SVN revision numbers in existing documentation, bug
reports and archives. If you plan to eventually migrate from SVN to Git
and are certain about dropping SVN history, consider
linkgit:git-filter-branch[1] instead. filter-branch also allows
reformatting of metadata for ease-of-reading and rewriting authorship
info for non-"svn.authorsFile" users.
reports, and archives. If you plan to eventually migrate from SVN to
Git and are certain about dropping SVN history, consider
https://github.com/newren/git-filter-repo[git-filter-repo] instead.
filter-repo also allows reformatting of metadata for ease-of-reading
and rewriting authorship info for non-"svn.authorsFile" users.

svn.useSvmProps::
svn-remote.<name>.useSvmProps::


+ 6
- 4
Documentation/githooks.txt View File

@@ -425,10 +425,12 @@ post-rewrite

This hook is invoked by commands that rewrite commits
(linkgit:git-commit[1] when called with `--amend` and
linkgit:git-rebase[1]; currently `git filter-branch` does 'not' call
it!). Its first argument denotes the command it was invoked by:
currently one of `amend` or `rebase`. Further command-dependent
arguments may be passed in the future.
linkgit:git-rebase[1]; however, full-history (re)writing tools like
linkgit:git-fast-import[1] or
https://github.com/newren/git-filter-repo[git-filter-repo] typically
do not call it!). Its first argument denotes the command it was
invoked by: currently one of `amend` or `rebase`. Further
command-dependent arguments may be passed in the future.

The hook receives a list of the rewritten commits on stdin, in the
format


+ 2
- 2
contrib/svn-fe/svn-fe.txt View File

@@ -56,7 +56,7 @@ line. This line has the form `git-svn-id: URL@REVNO UUID`.

The resulting repository will generally require further processing
to put each project in its own repository and to separate the history
of each branch. The 'git filter-branch --subdirectory-filter' command
of each branch. The 'git filter-repo --subdirectory-filter' command
may be useful for this purpose.

BUGS
@@ -67,5 +67,5 @@ The exit status does not reflect whether an error was detected.

SEE ALSO
--------
git-svn(1), svn2git(1), svk(1), git-filter-branch(1), git-fast-import(1),
git-svn(1), svn2git(1), svk(1), git-filter-repo(1), git-fast-import(1),
https://svn.apache.org/repos/asf/subversion/trunk/notes/dump-load-format.txt

+ 14
- 0
git-filter-branch.sh View File

@@ -83,6 +83,20 @@ set_ident () {
finish_ident COMMITTER
}

if test -z "$FILTER_BRANCH_SQUELCH_WARNING$GIT_TEST_DISALLOW_ABBREVIATED_OPTIONS"
then
cat <<EOF
WARNING: git-filter-branch has a glut of gotchas generating mangled history
rewrites. Hit Ctrl-C before proceeding to abort, then use an
alternative filtering tool such as 'git filter-repo'
(https://github.com/newren/git-filter-repo/) instead. See the
filter-branch manual page for more details; to squelch this warning,
set FILTER_BRANCH_SQUELCH_WARNING=1.
EOF
sleep 10
printf "Proceeding with filter-branch...\n\n"
fi

USAGE="[--setup <command>] [--subdirectory-filter <directory>] [--env-filter <command>]
[--tree-filter <command>] [--index-filter <command>]
[--parent-filter <command>] [--msg-filter <command>]


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