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

One ZFS commit can take a file through several sizes, and unix.Sync can
return before the blocks are counted. A test that expected the size on
disk to be settled therefore 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. It then rescans until the size reads the same before and
after a rescan, and compares disk_size exactly, within the same 30
seconds. The other two tests fail only on a disk_size outside their two
reads. They call unix.Sync once first, so a wrong 512 usually fails them.

Model: opus-5-5

README.md

@@ -407,9 +407,9 @@ operands:
   is what makes a daily rescan cheap.
 - When such a file's size on disk differs from its recorded `disk_size`, `scan`
   updates `disk_size` alone. It still does not read the file, and counts it as
-  unchanged. ZFS, for example, counts a new file's blocks up to seconds after
-  the write. A block is the unit in which a filesystem allocates disk space. A
-  file scanned within those seconds gets its real `disk_size` at the next scan.
+  unchanged. ZFS, for example, counts a new file's blocks only seconds after the
+  write. A block is the unit in which a filesystem allocates disk space. A file
+  scanned within those seconds gets its real `disk_size` at the next scan.
 - `scan` hashes a file again when its size differs or its mtime is newer. The
   mtime is compared to the nanosecond, as finely as the filesystem records it. A
   same-size rewrite counts as a change whenever the filesystem gives it a later

TODO.md

@@ -29,11 +29,6 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
-- `README.md` and a comment in `scan.go` no longer say ZFS counts a new file's
-  blocks only seconds after the write. The count can land in under a second.
-  Both now say up to seconds after the write, as the comment on `diskSizeOf` in
-  `scan_test.go` does (2026-11-03, https://git.eeqj.de/sneak/sfdupes/issues/152)
-
 - 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

scan.go

@@ -554,7 +554,7 @@ func (s *scanState) walkPhase(events <-chan walkEvent, roots []string) {
 
 		// The space a file takes on disk can change while its size and
 		// mtime stay the same. ZFS, for one, counts a new file's blocks
-		// up to seconds after the write.
+		// only seconds after the write.
 		if old.diskSize != ev.rec.diskSize {
 			s.diskSizes = append(s.diskSizes, ev.rec)
 		}