Re: [PATCH 03/20] scripts/make-erofs.sh: Avoid unneeded calls to awk and chmod

[Alyssa Ross <hi@alyssa.is>]

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Demi Marie Obenour <demiobenour@gmail.com> writes:

> On 9/8/25 04:28, Alyssa Ross wrote:
>> Demi Marie Obenour <demiobenour@gmail.com> writes:
>> 
>>> These calls were made to work around permission problems, but it is much
>>> cleaner to solve these problems by making every directory in the new
>>> filesystem image writable so that cp can write to it.
>>>
>>> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
>>> ---
>>>  scripts/make-erofs.sh | 22 +++++++++++-----------
>>>  1 file changed, 11 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/scripts/make-erofs.sh b/scripts/make-erofs.sh
>>> index 3f211d848b938405510d0dbf6b11cf5512c9ef5d..e63bcbed9c3028f0f2b55431d46ba9ec67bc26ef 100755
>>> --- a/scripts/make-erofs.sh
>>> +++ b/scripts/make-erofs.sh
>>> @@ -37,18 +37,18 @@ while read -r arg1; do
>>>  	fi
>>>  	echo
>>>  
>>> -	parent="$(dirname "$arg2")"
>>> -	awk -v parent="$parent" -v root="$root" 'BEGIN {
>>> -		n = split(parent, components, "/")
>>> -		for (i = 1; i <= n; i++) {
>>> -			printf "%s/", root
>>> -			for (j = 1; j <= i; j++)
>>> -				printf "%s/", components[j]
>>> -			print
>>> -		}
>>> -	}' | xargs -rd '\n' chmod +w -- 2>/dev/null || :
>>> -	mkdir -p -- "$root/$parent"
>>> +	if [ "$arg2" = / ]; then
>>> +		cp -RT -- "$arg1" "$root"
>>> +		# Nix store paths are read-only, so fix up permissions
>>> +		# so that subsequent copies can write to directories
>>> +		# created by the above copy.  This means giving all
>>> +		# directories 0755 permissions.
>>> +		find "$root" -type d -exec chmod 0755 -- '{}' +
>> 
>> Won't this be much slower, since it runs across the whole root every
>> time?  We're going from one chmod() per path component to one for each
>> directory in root, aren't we?
>
> The root directory is always the first one populated.  Most of the
> root filesystem is the Nix store, which this skips.  The call to find
> operates on only the stuff *not* in the Nix store.  Also, there are
> significantly fewer calls to fork() and execve().  chmod is called with
> many arguments at once by find.

I suppose it is.  I think when writing make-erofs.sh, my intention was
to have order not matter, though, and if a directory in my Spectrum
source tree is somehow read-only, I'd want that chmod-ed too.

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