internal/pkg: zero atime and mtime

This is significantly more practical than keeping track of them in directory flattening format and setting this in every non-artifact implementation. Only tarArtifact can have meaningful deterministic checksums that are not zero and zeroing them still keeps autotools happy.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg.go

@@ -19,6 +19,7 @@ import (
 	"strings"
 	"sync"
 	"syscall"
+	"unsafe"
 
 	"hakurei.app/container/check"
 	"hakurei.app/message"
@@ -801,6 +802,33 @@ func removeAll(pathname *check.Absolute) (chmodErr, removeErr error) {
 	return
 }
 
+// zeroTimes zeroes atime and mtime for the named file.
+func zeroTimes(path string) (err error) {
+	// include/uapi/linux/fcntl.h
+	const (
+		AT_FDCWD            = -100
+		AT_SYMLINK_NOFOLLOW = 0x100
+	)
+	_AT_FDCWD := AT_FDCWD
+
+	var _p0 *byte
+	_p0, err = syscall.BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	if _, _, errno := syscall.Syscall6(
+		syscall.SYS_UTIMENSAT,
+		uintptr(_AT_FDCWD),
+		uintptr(unsafe.Pointer(_p0)),
+		uintptr(unsafe.Pointer(new([2]syscall.Timespec))),
+		AT_SYMLINK_NOFOLLOW,
+		0, 0,
+	); errno != 0 {
+		return os.NewSyscallError("utimensat", errno)
+	}
+	return
+}
+
 // overrideFileInfo overrides the permission bits of [fs.FileInfo] to 0500 and
 // is the concrete type returned by overrideFile.Stat.
 type overrideFileInfo struct{ fs.FileInfo }
@@ -914,6 +942,9 @@ func (c *Cache) Cure(a Artifact) (
 				checksumLinknamePrefix+checksums,
 				pathname.String(),
 			)
+			if err == nil {
+				err = zeroTimes(pathname.String())
+			}
 		}
 	}()
 
@@ -1009,10 +1040,15 @@ func (c *Cache) Cure(a Artifact) (
 		}
 		_, err = w.Write(data)
 		closeErr := w.Close()
+		timeErr := zeroTimes(checksumPathname.String())
+		c.checksumMu.Unlock()
+
+		if err == nil {
+			err = timeErr
+		}
 		if err == nil {
 			err = closeErr
 		}
-		c.checksumMu.Unlock()
 
 		return
 	}
@@ -1125,6 +1161,14 @@ func (c *Cache) Cure(a Artifact) (
 	if err = os.Chmod(t.work.String(), 0700); err != nil {
 		return
 	}
+	if err = filepath.WalkDir(t.work.String(), func(path string, _ fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		return zeroTimes(path)
+	}); err != nil {
+		return
+	}
 	c.checksumMu.Lock()
 	if err = os.Rename(
 		t.work.String(),