Stop the disk size tests failing at random on ZFS (closes #150)

unix.Sync can return before ZFS has counted a file's blocks, so a test
that expected the size on disk to be settled after it failed at random.
The test that needs the size to change now calls unix.Sync in a loop
until it does, for at most 30 seconds. All three tests read the size
before and after the scan and accept either value.

Model: opus-5-5

TODO.md

@@ -30,12 +30,10 @@ Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
 - The three disk size tests in `scan_test.go` no longer fail at random on ZFS.
-  `unix.Sync` can return before ZFS has counted a file's blocks, and one ZFS
-  commit can take a file through several sizes. The test that needs the size to
-  change waits for it, for at most 30 seconds. Each test reads the size before
-  and after its scan. The rewrite test rescans until the two reads match, then
-  compares exactly. The other two fail only on a `disk_size` outside the two
-  reads (2026-11-02, https://git.eeqj.de/sneak/sfdupes/issues/150)
+  `unix.Sync` can return before ZFS has counted a file's blocks. The test that
+  needs the size to change waits for it, for at most 30 seconds. Each test reads
+  the size before and after its scan and accepts either value (2026-11-02,
+  https://git.eeqj.de/sneak/sfdupes/issues/150)
 
 - `TODO.md` rewritten from scratch by the same rules as `README.md`, sentence by
   sentence. Every completed step is kept, with its date and its issue or branch.

scan_test.go

@@ -44,15 +44,12 @@ func writeFile(t *testing.T, dir, name string, data []byte) string {
 
 // diskSizeOf returns the bytes the file at path takes on disk, from its
 // own lstat: st_blocks counts 512-byte units. ZFS counts a write's
-// blocks only when it commits the write, up to seconds later, and one
-// commit can take the file through several growing sizes. So this value
-// can change at any moment. Neither fsync nor unix.Sync hastens that,
-// but a unix.Sync call often takes long enough that the blocks are
-// counted when it returns. A test that needs the value to change waits
-// for the change. A test that compares it with a scan's disk_size reads
-// it before and after the scan. The rewrite test rescans until the two
-// reads match, then compares exactly. The other two tests fail only on
-// a disk_size outside the two reads.
+// blocks only when it commits the write, seconds later and sometimes in
+// steps, so this value can change at any moment. fsync does not hasten
+// that. unix.Sync usually does, but it can return before the blocks are
+// counted. A test that needs the value to change therefore waits for
+// that, and a test that compares it with a scan's disk_size reads it
+// before and after the scan and accepts either.
 func diskSizeOf(t *testing.T, path string) int64 {
 	t.Helper()
 
@@ -1452,8 +1449,6 @@ func TestScanStoresDiskSize(t *testing.T) {
 	p := sparseFile(t, t.TempDir(), "sparse", length)
 	pokeAt(t, p, 0, []byte("x"))
 
-	unix.Sync() // see diskSizeOf
-
 	size := diskSizeOf(t, p)
 	if size == 0 || size >= length {
 		t.Fatalf("the file takes %d bytes on disk, want more than none "+
@@ -1471,8 +1466,8 @@ func TestScanStoresDiskSize(t *testing.T) {
 		after := diskSizeOf(t, p)
 
 		got := recordByPath(t, dbRecords(t, db), p).diskSize
-		if got < before || got > after {
-			t.Errorf("scan of %s: disk_size = %d, want %d to %d",
+		if got != before && got != after {
+			t.Errorf("scan of %s: disk_size = %d, want %d or %d",
 				operand, got, before, after)
 		}
 	}
@@ -1515,42 +1510,20 @@ func TestSyncScanRewriteUpdatesDiskSize(t *testing.T) {
 		unix.Sync()
 	}
 
-	// ZFS may still be counting, so the size may change during a rescan.
-	// Only a rescan with the same size on both sides of it can be compared
-	// exactly. A rescan that keeps the old size still fails, because the
-	// wait made before differ from it.
-	for {
-		before := diskSizeOf(t, p)
+	// ZFS may still be counting, so the size may change during the rescan.
+	before := diskSizeOf(t, p)
 
-		st := syncTree(t, db, dir)
+	st := syncTree(t, db, dir)
 
-		after := diskSizeOf(t, p)
+	after := diskSizeOf(t, p)
 
-		if st != (scanStats{walked: 1, updated: 1}) {
-			t.Fatalf("rescan stats = %+v, want 1 updated", st)
-		}
+	if st != (scanStats{walked: 1, updated: 1}) {
+		t.Fatalf("rescan stats = %+v, want 1 updated", st)
+	}
 
-		if before == after {
-			got := recordByPath(t, dbRecords(t, db), p).diskSize
-			if got != after {
-				t.Errorf("disk_size = %d, want %d", got, after)
-			}
-
-			return
-		}
-
-		if time.Now().After(deadline) {
-			t.Fatalf("the size on disk was still changing 30 seconds after "+
-				"the write into its hole: %d, then %d", before, after)
-		}
-
-		// A later mtime makes the next rescan read the file again.
-		later = later.Add(time.Second)
-
-		err = os.Chtimes(p, later, later)
-		if err != nil {
-			t.Fatal(err)
-		}
+	got := recordByPath(t, dbRecords(t, db), p).diskSize
+	if got != before && got != after {
+		t.Errorf("disk_size = %d, want %d or %d", got, before, after)
 	}
 }
 
@@ -1574,8 +1547,6 @@ func TestSyncScanUnchangedUpdatesDiskSize(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	unix.Sync() // see diskSizeOf
-
 	// ZFS may count the blocks during the rescan; see diskSizeOf.
 	before := diskSizeOf(t, p)
 
@@ -1589,8 +1560,8 @@ func TestSyncScanUnchangedUpdatesDiskSize(t *testing.T) {
 	}
 
 	got := recordByPath(t, dbRecords(t, db), p).diskSize
-	if got < before || got > after {
-		t.Errorf("disk_size = %d, want %d to %d", got, before, after)
+	if got != before && got != after {
+		t.Errorf("disk_size = %d, want %d or %d", got, before, after)
 	}
 }