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-fincremental silently fails inside /tmp #35365

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opened 2026年05月20日 08:52:18 +02:00 by pancelor · 2 comments
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Zig Version

0.16.0 and 0.17.0-dev.313+27be3b069

Steps to Reproduce and Observed Behavior

When using zig build --watch, if I (1) add -fincremental and (2) am running a project inside the system's /tmp directory, then the compiler notices my edits when I save, but it runs an old version of the code.

~ $ mkdir /tmp/dir1; cd /tmp/dir1
/tmp/dir1 $ zig env | grep target
 .target = "x86_64-linux.6.12.68...6.12.68-gnu.2.42",
/tmp/dir1 $ zig init
info: created build.zig
info: created build.zig.zon
info: created src/main.zig
info: created src/root.zig
info: see `zig build --help` for a menu of options
/tmp/dir1 $ zig build run --watch -fincremental
All your codebase are belong to us.
info: arg: /tmp/dir1/zig-out/bin/dir1
Run `zig build test` to run the tests.
Build Summary: 5/5 steps succeeded
All your codebase are belong to us.
info: arg: /tmp/dir1/zig-out/bin/dir1
Run `zig build test` to run the tests.
Build Summary: 5/5 steps succeeded
All your codebase are belong to us.
info: arg: /tmp/dir1/zig-out/bin/dir1
Run `zig build test` to run the tests.
Build Summary: 5/5 steps succeeded
^Ctching 77 directories, 1 processes

Those re-runs are from me changing one line of src/main.zig: I changed the args in std.debug.print("All your {s} are belong to us.\n", .{"codebase"}); to .{"linker"} and other variations, but the incremental compiler kept outputting "All your codebase are belong to us.", instead of the expected "All your linker are belong to us."

If I drop the -fincremental, it behaves as expected:

/tmp/dir1 $ zig build run --watch
All your linker are belong to us.
info: arg: /tmp/dir1/zig-out/bin/dir1
Run `zig build test` to run the tests.
Build Summary: 5/5 steps succeeded
All your compiler are belong to us.
info: arg: /tmp/dir1/zig-out/bin/dir1
Run `zig build test` to run the tests.
Build Summary: 5/5 steps succeeded
^Ctching 78 directories, 0 processes

It also behaves as expected if I do these exact same steps (including -fincremental) in a non-temporary directory, e.g. mkdir /home/pancelor/dir2; cd /home/pancelor/dir2

Expected Behavior

zig build --watch -fincremental inside /tmp/dir1 should run my up-to-date code (printing All your linker instead of All your codebase, etc). Or there should be a visible error instead of a silent miscompilation

### Zig Version 0.16.0 and 0.17.0-dev.313+27be3b069 ### Steps to Reproduce and Observed Behavior When using `zig build --watch`, if I (1) add `-fincremental` and (2) am running a project inside the system's `/tmp` directory, then the compiler notices my edits when I save, but it runs an old version of the code. ```shell ~ $ mkdir /tmp/dir1; cd /tmp/dir1 /tmp/dir1 $ zig env | grep target .target = "x86_64-linux.6.12.68...6.12.68-gnu.2.42", /tmp/dir1 $ zig init info: created build.zig info: created build.zig.zon info: created src/main.zig info: created src/root.zig info: see `zig build --help` for a menu of options /tmp/dir1 $ zig build run --watch -fincremental All your codebase are belong to us. info: arg: /tmp/dir1/zig-out/bin/dir1 Run `zig build test` to run the tests. Build Summary: 5/5 steps succeeded All your codebase are belong to us. info: arg: /tmp/dir1/zig-out/bin/dir1 Run `zig build test` to run the tests. Build Summary: 5/5 steps succeeded All your codebase are belong to us. info: arg: /tmp/dir1/zig-out/bin/dir1 Run `zig build test` to run the tests. Build Summary: 5/5 steps succeeded ^Ctching 77 directories, 1 processes ``` Those re-runs are from me changing one line of `src/main.zig`: I changed the args in ` std.debug.print("All your {s} are belong to us.\n", .{"codebase"});` to `.{"linker"}` and other variations, but the incremental compiler kept outputting "All your codebase are belong to us.", instead of the expected "All your linker are belong to us." If I drop the `-fincremental`, it behaves as expected: ```shell /tmp/dir1 $ zig build run --watch All your linker are belong to us. info: arg: /tmp/dir1/zig-out/bin/dir1 Run `zig build test` to run the tests. Build Summary: 5/5 steps succeeded All your compiler are belong to us. info: arg: /tmp/dir1/zig-out/bin/dir1 Run `zig build test` to run the tests. Build Summary: 5/5 steps succeeded ^Ctching 78 directories, 0 processes ``` It also behaves as expected if I do these exact same steps (including `-fincremental`) in a non-temporary directory, e.g. `mkdir /home/pancelor/dir2; cd /home/pancelor/dir2` ### Expected Behavior `zig build --watch -fincremental` inside `/tmp/dir1` should run my up-to-date code (printing `All your linker` instead of `All your codebase`, etc). Or there should be a visible error instead of a silent miscompilation
mlugg added this to the Urgent milestone 2026年05月20日 09:52:13 +02:00
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Seems to be caused by this if:

if(!size_matchor!mtime_matchor!inode_match){

After an incremental update, the file is assumed to have the same content compared to before the incremental update, because the size, mtime and inode match. A hash to check if the contents are equal is only calculated inside this if block.

Seems to be caused by this `if`: https://codeberg.org/ziglang/zig/src/commit/65073f3dc5244941914e7c4d1897a74ac8adcd69/lib/std/Build/Cache.zig#L808 After an incremental update, the file is assumed to have the same content compared to before the incremental update, because the size, mtime and inode match. A hash to check if the contents are equal is only calculated inside this `if` block.
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This if seems to not be the only cause of this issue. Adding a or true to the if does not fix the issue, when doing zig build run --watch -fincremental after a zig init.

Adding a or true does however fix the following:

zig build test --watch -fincremental

with

build.zig:

conststd=@import("std");pubfnbuild(b:*std.Build)!void{consttarget=b.standardTargetOptions(.{});constoptimize=b.standardOptimizeOption(.{});constmod=b.createModule(.{.root_source_file=b.path("test.zig"),.optimize=optimize,.target=target,});consttest_art=b.addTest(.{.root_module=mod,});consttest_run=b.addRunArtifact(test_art);consttest_step=b.step("test","test");test_step.dependOn(&test_run.step);}

test.zig before update:

conststd=@import("std");test"a"{}

test.zig after update:

conststd=@import("std");test"a"{returnerror.Err;}

Without the or true in the if, this works as expected on non tmpfs file systems and similar the the observed behavior described in the issue description on tmpfs.

This `if` seems to not be the only cause of this issue. Adding a `or true` to the `if` does not fix the issue, when doing `zig build run --watch -fincremental` after a `zig init`. Adding a `or true` does however fix the following: ``` zig build test --watch -fincremental ``` with <details> <summary> `build.zig`: </summary> ```zig const std = @import("std"); pub fn build(b: *std.Build) !void { const target = b.standardTargetOptions(.{}); const optimize = b.standardOptimizeOption(.{}); const mod = b.createModule(.{ .root_source_file = b.path("test.zig"), .optimize = optimize, .target = target, }); const test_art = b.addTest(.{ .root_module = mod, }); const test_run = b.addRunArtifact(test_art); const test_step = b.step("test", "test"); test_step.dependOn(&test_run.step); } ``` </details> <details> <summary> `test.zig` before update: </summary> ```zig const std = @import("std"); test "a" {} ``` </details> <details> <summary> `test.zig` after update: </summary> ```zig const std = @import("std"); test "a" { return error.Err; } ``` </details> Without the `or true` in the if, this works as expected on non `tmpfs` file systems and similar the the observed behavior described in the issue description on `tmpfs`.
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