Re: [PATCH 3/9] t4141: fix inefficient use of dd(1)

["SZEDER Gábor" <szeder.dev@gmail.com>]

On Thu, Jul 02, 2026 at 02:00:56PM +0200, Patrick Steinhardt wrote:
> In t4141 we generate a patch that is roughly 1GB in size to verify that
> git-apply(1) indeed rejects that patch. We generate that patch by
> prepending a patch header and then executing `test-tool genzeros`
> without a limit. This causes us to print infinitely many zeros, and we
> limit the overall amount of generated bytes via `test_copy_bytes`.
> 
> This test setup is extremely expensive, as `test_copy_bytes` is
> implemented via `dd ibs=1 count="$1"`, which copies data one byte at a
> time. So as we write 1GB of data, we end up doing 1 billion reads and
> writes. This naturally takes a while: it takes 6 minutes on my system,
> and around 40 minutes in some CI jobs!
> 
> We can do much better though, as genzeros already knows to handle an
> optional limit of how much data it is supposed to write, which allows us
> to remove the call to `test_copy_bytes`. Furthermore, it has already
> been optimized to generate the data fast.
> 
> And indeed, doing this conversion drops the test execution to less than
> a second on my machine, so that we can drop the EXPENSIVE prerequisite.

EXPENSIVE is not only about execution time, but about resources in
general.  While the modified test finishes quite fast indeed, 'git
apply' uses over 1GB of RSS.  Therefore, the EXPENSIVE prerequisite
should be kept.

> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  t/t4141-apply-too-large.sh | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/t/t4141-apply-too-large.sh b/t/t4141-apply-too-large.sh
> index eac6f7e151..dad67779ed 100755
> --- a/t/t4141-apply-too-large.sh
> +++ b/t/t4141-apply-too-large.sh
> @@ -4,8 +4,7 @@ test_description='git apply with too-large patch'
>  
>  . ./test-lib.sh
>  
> -test_expect_success EXPENSIVE 'git apply rejects patches that are too large' '
> -	sz=$((1024 * 1024 * 1023)) &&
> +test_expect_success 'git apply rejects patches that are too large' '
>  	{
>  		cat <<-\EOF &&
>  		diff --git a/file b/file
> @@ -14,8 +13,8 @@ test_expect_success EXPENSIVE 'git apply rejects patches that are too large' '
>  		+++ b/file
>  		@@ -0,0 +1 @@
>  		EOF
> -		test-tool genzeros
> -	} | test_copy_bytes $sz | test_must_fail git apply 2>err &&
> +		test-tool genzeros $((1024 * 1024 * 1023))
> +	} | test_must_fail git apply 2>err &&
>  	grep "patch too large" err
>  '
>  
> 
> -- 
> 2.55.0.795.g602f6c329a.dirty
>