> The commit map dir is populated from the state branch assuming a
> "to_commit:from_commit" format, but the state branch is written with a
> "from_commit:to_commit" format, resulting in an inverted mapping when the
> map is populated from the state branch. This is especially evident when
> --prune-empty is used and creates commits which map to nothing; when the
> map dir is populated from this state on subsequent runs, git-filter-branch
> outputs many errors while trying to create files with empty names, like:
> 
> > /usr/lib/git-core/git-filter-branch: line 305: ../map/: Is a directory
> 
> This change corrects the population of the commit map dir to match the
> "from_commit:to_commit" format.

> diff --git a/git-filter-branch.sh b/git-filter-branch.sh
> index 24fa317aaa..9aa07be6e1 100755
> --- a/git-filter-branch.sh
> +++ b/git-filter-branch.sh
> @@ -302,7 +302,9 @@ then
>  		do
>  			case "$line" in
>  			*:*)
> -				echo "${line%:*}" >../map/"${line#*:}";;
> +				from_commit=${line%:*}
> +				to_commit=${line#*:}
> +				echo "$to_commit" >../map/"$from_commit";;
>  			*)
>  				die "Unable to load state from $state_branch:filter.map";;
>  			esac

> diff --git a/t/t7003-filter-branch.sh b/t/t7003-filter-branch.sh
> index 86011e7b1f..3934cc4a11 100755
> --- a/t/t7003-filter-branch.sh
> +++ b/t/t7003-filter-branch.sh
> @@ -121,7 +121,7 @@ W=$(git rev-parse HEAD)
>  test_expect_success 'using --state-branch to skip already rewritten commits' '
>  	test_when_finished git reset --hard $V &&
>  	git reset --hard $V &&
> -	git filter-branch --state-branch state -f --tree-filter "touch file || :" HEAD &&
> +	git filter-branch --state-branch state -f --tree-filter "exit 1" HEAD &&
>  	test_cmp_rev $W HEAD
>  '

>>  test_expect_success 'using --state-branch to skip already rewritten commits' '
>>  test_when_finished git reset --hard $V &&
>>  git reset --hard $V &&
>> - git filter-branch --state-branch state -f --tree-filter "touch file || :" HEAD &&
>> + git filter-branch --state-branch state -f --tree-filter "exit 1" HEAD &&
>>  test_cmp_rev $W HEAD
>>  '
>
>So does this now detect the issue? If so, it feels somewhat roundabout.

> On September 30, 2026 10:40:06 AM EDT, Patrick Steinhardt <ps@pks.im> wrote:
> > Okay. A simpler fix could've been the following diff:
> > 
> > - echo "${line%:*}" >../map/"${line#*:}";;
> > + echo "${line#*:}" >../map/"${line%:*}";;
> 
> Yep, that's equivalent and was my first version, but I decided to err on the
> side of clearer intent. Plus, the more verbose version mirrors how the state
> branch is constructed further down in the script.
> 
> > So... does this mean that we don't have test coverage for this case at
> > all?
> 
> Yeah, the test for this case was broken.
> 
> > >  test_expect_success 'using --state-branch to skip already rewritten commits' '
> > >  test_when_finished git reset --hard $V &&
> > >  git reset --hard $V &&
> > > - git filter-branch --state-branch state -f --tree-filter "touch file || :" HEAD &&
> > > + git filter-branch --state-branch state -f --tree-filter "exit 1" HEAD &&
> > >  test_cmp_rev $W HEAD
> > >  '
> > 
> > So does this now detect the issue? If so, it feels somewhat roundabout.
> 
> The current test seemingly intends to check if the --tree-filter filter was
> run on already processed commits, but it only checks that the filter
> produces the same final result. Since the filter is deterministic, the final
> result is the same whether or not the filter is re-run on already processed
> commits.
> 
> The proposed change makes the test fail immediately if the tree-filter
> is re-run. I looked around other tests for a test_* command or
> conventions to fail with a message, but I didn't find anything. I've
> checked that the test fails without the filter-branch change, and passes
> with it.

> The commit map dir is populated from the state branch assuming a
> "to_commit:from_commit" format, but the state branch is written with a
> "from_commit:to_commit" format, resulting in an inverted mapping when the
> map is populated from the state branch. This is especially evident when
> --prune-empty is used and creates commits which map to nothing; when the
> map dir is populated from this state on subsequent runs, git-filter-branch
> outputs many errors while trying to create files with empty names, like:

> > /usr/lib/git-core/git-filter-branch: line 305: ../map/: Is a directory
> 
> This change corrects the population of the commit map dir to match the
> "from_commit:to_commit" format and adds/updates tests to check that the
> state branch is written correctly.

> diff --git a/t/t7003-filter-branch.sh b/t/t7003-filter-branch.sh
> index 86011e7b1f..cf225b0f0f 100755
> --- a/t/t7003-filter-branch.sh
> +++ b/t/t7003-filter-branch.sh
> @@ -121,10 +121,32 @@ W=$(git rev-parse HEAD)
>  test_expect_success 'using --state-branch to skip already rewritten commits' '
>  	test_when_finished git reset --hard $V &&
>  	git reset --hard $V &&
> -	git filter-branch --state-branch state -f --tree-filter "touch file || :" HEAD &&
> +	git filter-branch --state-branch state -f --tree-filter "exit 1" HEAD &&
>  	test_cmp_rev $W HEAD
>  '
>  
> +test_expect_success '--state-branch incremental rewrite uses the rewritten parents' '

> +	git init incremental &&
> +	(
> +		cd incremental &&
> +		mkdir sub &&
> +		test_commit first sub/file &&
> +		test_commit outside root-file &&
> +		git filter-branch --state-branch refs/state \
> +			--prune-empty --subdirectory-filter sub -- HEAD &&
> +		rewritten_first=$(git rev-parse HEAD) &&
> +		git reset --hard outside &&
> +		test_commit second sub/file &&
> +		git filter-branch -f --state-branch refs/state \
> +			--prune-empty --subdirectory-filter sub -- outside..HEAD &&
> +		test_cmp_rev $rewritten_first HEAD^ &&
> +		git show refs/state:filter.map >map &&
> +		echo "$(git rev-parse second):$(git rev-parse HEAD)" >expect &&
> +		grep "^$(git rev-parse second):" map >actual &&
> +		test_cmp expect actual
> +	)
> +'

> The commit map dir is populated from the state branch assuming a
> "to_commit:from_commit" format, but the state branch is written with a
> "from_commit:to_commit" format, resulting in an inverted mapping when the
> map is populated from the state branch. This is especially evident when
> --prune-empty is used and creates commits which map to nothing; when the
> map dir is populated from this state on subsequent runs, git-filter-branch
> outputs many errors while trying to create files with empty names, like:
>
>> /usr/lib/git-core/git-filter-branch: line 305: ../map/: Is a directory
>
> This change corrects the population of the commit map dir to match the
> "from_commit:to_commit" format and adds/updates tests to check that the
> state branch is written correctly.
>
> Signed-off-by: Grant Moyer <dev@grantmoyer.com>
> Tested-by: Michele Locati <michele@locati.it>
> Co-authored-by: Michele Locati <michele@locati.it>
> ---
>  git-filter-branch.sh     |  4 +++-
>  t/t7003-filter-branch.sh | 24 +++++++++++++++++++++++-
>  2 files changed, 26 insertions(+), 2 deletions(-)
>
> diff --git a/git-filter-branch.sh b/git-filter-branch.sh
> index 24fa317aaa..9aa07be6e1 100755
> --- a/git-filter-branch.sh
> +++ b/git-filter-branch.sh
> @@ -302,7 +302,9 @@ then
>  		do
>  			case "$line" in
>  			*:*)
> -				echo "${line%:*}" >../map/"${line#*:}";;
> +				from_commit=${line%:*}
> +				to_commit=${line#*:}
> +				echo "$to_commit" >../map/"$from_commit";;
>  			*)
>  				die "Unable to load state from $state_branch:filter.map";;
>  			esac

> diff --git a/t/t7003-filter-branch.sh b/t/t7003-filter-branch.sh
> index 86011e7b1f..cf225b0f0f 100755
> --- a/t/t7003-filter-branch.sh
> +++ b/t/t7003-filter-branch.sh
> @@ -121,10 +121,32 @@ W=$(git rev-parse HEAD)
>  test_expect_success 'using --state-branch to skip already rewritten commits' '
>  	test_when_finished git reset --hard $V &&
>  	git reset --hard $V &&
> -	git filter-branch --state-branch state -f --tree-filter "touch file || :" HEAD &&
> +	git filter-branch --state-branch state -f --tree-filter "exit 1" HEAD &&
>  	test_cmp_rev $W HEAD
>  '

> +	git init incremental &&
> +	(
> +		cd incremental &&
> +		mkdir sub &&
> +		test_commit first sub/file &&
> +		test_commit outside root-file &&
> +		git filter-branch --state-branch refs/state \
> +			--prune-empty --subdirectory-filter sub -- HEAD &&
> +		rewritten_first=$(git rev-parse HEAD) &&
> +		git reset --hard outside &&
> +		test_commit second sub/file &&
> +		git filter-branch -f --state-branch refs/state \
> +			--prune-empty --subdirectory-filter sub -- outside..HEAD &&
> +		test_cmp_rev $rewritten_first HEAD^ &&
> +		git show refs/state:filter.map >map &&
> +		echo "$(git rev-parse second):$(git rev-parse HEAD)" >expect &&
> +		grep "^$(git rev-parse second):" map >actual &&
> +		test_cmp expect actual
> +	)
> +'
> +
>  git tag oldD HEAD~4
>  test_expect_success 'rewrite one branch, keeping a side branch' '
>  	git branch modD oldD &&

> The "--state-branch" option asks git-filter-branch(1) to write a
> mapping from old to new objects into a branch to enable incremental
> processing of large histories. This object mapping is stored as
> a simple blob at "$state_branch:filter.map" with one object pair
> per line in the format "$from_commit:$to_commit". Before processing
> commits in a subsequent run, the state branch is used to populate an
> object mapping directory, where each file is named "map/$from_commit"
> and has contents "$to_commit".
> 
> In f6d855091e (filter-branch: stop depending on Perl, 2025-04-16),
> we refactored git-filter-branch(1) to no longer require Perl,
> but accidentally started to interpret object pairs in reverse (as
> "$to_commit:$from_commit") when populating the map directory from
> the state branch. This can cause all kinds of bad behavior from the
> state-branch being effectively ignored to previously filtered objects
> accidentally being mapped back to unfiltered objects. One especially
> evident case occurs when "git filter-branch --prune-empty ..." maps
> some commits to nothing, then on subsequent runs outputs many errors
> while trying to create files with empty names, like:
> 
> > /usr/lib/git-core/git-filter-branch: line 305: ../map/: Is a directory
> 
> This regression went unnoticed because, since the introduction of
> the only test for "--state-branch" in 709cfe848a (filter-branch: skip
> commits present on --state-branch, 2018-06-26), the test accidentally
> passes even if commits are not skipped. The test checks that after
> populating a state branch with git-filter-branch(1), then running
> it again with that state branch, the resulting filtered commits for
> the first run and second run match. However since the filter used is
> deterministic, the commits always match, even if the commits in the
> state branch are re-filtered.
> 
> Fix the population of the object mapping dir by interpreting
> object pairs as "$from_commit:$to_commit". Also fix the existing
> "--state-branch" test by directly exiting with a non-zero code if
> any commits from the state branch aren't skipped. Finally, add a new
> "--state-branch" test which directly checks that a commit from the
> state branch is used when incrementally filtering a repo.
> 
> Tested-by: Michele Locati <michele@locati.it>
> Co-authored-by: Michele Locati <michele@locati.it>
> Signed-off-by: Michele Locati <michele@locati.it>
> Signed-off-by: Grant Moyer <dev@grantmoyer.com>