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std.Build.Step.Compile: fix race condition in args file creation #23997

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std.Build.Step.Compile: fix race condition in args file creation #23997

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21 changes: 20 additions & 1 deletion lib/std/Build/Step/Compile.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -1801,7 +1801,26 @@ fn getZigArgs(compile: *Compile, fuzz: bool) ![][]const u8 {
);

const args_file = "args" ++ fs.path.sep_str ++ args_hex_hash;
try b.cache_root.handle.writeFile(.{ .sub_path = args_file, .data = args });
if (b.cache_root.handle.access(args_file, .{})) |_| {
// The args file is already present from a previous run.
} else |err| switch (err) {
error.FileNotFound => {
try b.cache_root.handle.makePath("tmp");
const rand_int = std.crypto.random.int(u64);
const tmp_path = "tmp" ++ fs.path.sep_str ++ std.fmt.hex(rand_int);
try b.cache_root.handle.writeFile(.{ .sub_path = tmp_path, .data = args });
defer b.cache_root.handle.deleteFile(tmp_path) catch {
// It's fine if the temporary file can't be cleaned up.
};
b.cache_root.handle.rename(tmp_path, args_file) catch |rename_err| switch (rename_err) {
error.PathAlreadyExists => {
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It's worth noting that the package fetching code also handles error.AccessDenied:

zig/src/Package/Fetch.zig

Line 1446 in ef35c3d

error.PathAlreadyExists, error.AccessDenied => {
However, based on the documentation for RenameError, it looks like that case is only relevant for directories, not files as in this case:

zig/lib/std/posix.zig

Lines 2595 to 2597 in ef35c3d

/// On Windows, this error may be returned instead of PathAlreadyExists when
/// renaming a directory over an existing directory.
AccessDenied,
so this code doesn't handle it.

// The args file was created by another concurrent build process.
},
else => |other_err| return other_err,
};
},
else => |other_err| return other_err,
}

const resolved_args_file = try mem.concat(arena, u8, &.{
"@",
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