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cwd causes build scripts to be recompiled #35500

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opened 2026年05月27日 22:59:25 +02:00 by andrewrk · 3 comments

Zig Version

0.17.0-dev.640+ea044a4b8

Steps to Reproduce, Observed Behavior, and Expected Behavior

mkdir -p ~/tmp/foo
cd ~/tmp/foo
zig init
zig build run # observe that it compiles configurer
cd src/
zig build run # observe that it compiles configurer again

I expect the cwd to not affect whether the configurer needs to be recompiled.

This happens because in the first one, the cache manifest contains:

8056 57443266 1779594529138626853 5a83591f61e2713a38200dd649d689b9 0 build.zig

i.e. it has a cwd-relative reference to build.zig. That's what the last 0 there means, it's a base directory of cwd.

In the second one, the manifest contains:

8056 57443266 1779594529138626853 5a83591f61e2713a38200dd649d689b9 0 /home/andy/tmp/abc/build.zig

This is a cwd-relative absolute path to build.zig. Since those don't match, it's a cache miss.

What I would expect instead is that file entry to have a project-relative path to build.zig. In this case it would be a cache hit, plus if you moved the ~/tmp/foo directory to somewhere else, it would stay cached.

Related: #32097

### Zig Version 0.17.0-dev.640+ea044a4b8 ### Steps to Reproduce, Observed Behavior, and Expected Behavior ``` mkdir -p ~/tmp/foo cd ~/tmp/foo zig init zig build run # observe that it compiles configurer cd src/ zig build run # observe that it compiles configurer again ``` I expect the cwd to not affect whether the configurer needs to be recompiled. This happens because in the first one, the cache manifest contains: ``` 8056 57443266 1779594529138626853 5a83591f61e2713a38200dd649d689b9 0 build.zig ``` i.e. it has a cwd-relative reference to `build.zig`. That's what the last `0` there means, it's a base directory of cwd. In the second one, the manifest contains: ``` 8056 57443266 1779594529138626853 5a83591f61e2713a38200dd649d689b9 0 /home/andy/tmp/abc/build.zig ``` This is a cwd-relative absolute path to `build.zig`. Since those don't match, it's a cache miss. What I would expect instead is that file entry to have a project-relative path to build.zig. In this case it would be a cache hit, plus if you moved the ~/tmp/foo directory to somewhere else, it would stay cached. Related: #32097
andrewrk added this to the Urgent milestone 2026年05月27日 22:59:25 +02:00
andrewrk modified the milestone from Urgent to 0.17.0 2026年05月28日 00:07:14 +02:00

Would it be ok to solve this by having the build process (the maker?) chdir to the directory with build.zig ? (I use that after determining which zig version to chain, and it seems to work without problems.)
I propose to implement that chdir() and have the code guarded by an environment variable (so if that is env var is set, the code will not be executed). That way it is relatively easy for other programs to switch this code off, in case it causes interoperability problems (which than can then be addressed in either zig or in the other program later on).

I can work out a PR for this using the above approach if OK, and would also like to address #35291 with that PR as well.

Would it be ok to solve this by having the build process (the maker?) chdir to the directory with `build.zig` ? (I use that after determining which zig version to chain, and it seems to work without problems.) I propose to implement that `chdir()` and have the code guarded by an environment variable (so if that is env var is set, the code will not be executed). That way it is relatively easy for other programs to switch this code off, in case it causes interoperability problems (which than can then be addressed in either `zig` or in the other program later on). I can work out a PR for this using the above approach if OK, and would also like to address https://codeberg.org/ziglang/zig/issues/35291 with that PR as well.
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@andrewrk wrote in #35500 (comment):

What I would expect instead is that file entry to have a project-relative path to build.zig.

The caching for the configurer compilation is done by a zig build-exe invocation instead of the build system, right? So there is no concept of a project-relative prefix that could be used by the caching system. Wouldn't it suffice to compile the configurer with a cwd that is the project's build root instead of inheriting the cwd of the maker process?

@andrewrk wrote in https://codeberg.org/ziglang/zig/issues/35500#issue-5357321: > What I would expect instead is that file entry to have a project-relative path to build.zig. The caching for the configurer compilation is done by a `zig build-exe` invocation instead of the build system, right? So there is no concept of a project-relative prefix that could be used by the caching system. Wouldn't it suffice to compile the configurer with a cwd that is the project's build root instead of inheriting the cwd of the maker process?
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In general I think it's almost always buggy to change the cwd of child process, because suddenly paths between the child and the parent are not compatible. I think the compiler should become aware of project relative paths instead. Consider this would integrate well with the issue that I didn't type up yet about bypassing the CLI and passing Configuration (and a Compile Step index) directly to the compiler. There's more cool directions this can go too by increasing the semantic awareness of file paths like this.

In general I think it's almost always buggy to change the cwd of child process, because suddenly paths between the child and the parent are not compatible. I think the compiler should become aware of project relative paths instead. Consider this would integrate well with the issue that I didn't type up yet about bypassing the CLI and passing Configuration (and a Compile Step index) directly to the compiler. There's more cool directions this can go too by increasing the semantic awareness of file paths like this.
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