Carry the README changes from next into the rewrite

`next` changed the old README after this branch was cut, in
#146 (the `totals` table, commit
98193b2) and #147 (`disk_size`,
commit 4ed2e37). This commit states the same facts in the rewritten
README, in the places #140 (comment)
lists, and changes nothing else. Every statement was checked against the
code on `next`.

Model: fable-5-1

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

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)
 		}

scan_test.go

@@ -43,16 +43,10 @@ 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.
+// own lstat: st_blocks counts 512-byte units. ZFS counts a new file's
+// blocks only seconds after the write, and fsync does not hasten that.
+// A test therefore calls unix.Sync, which waits for it, before the scan
+// whose disk_size it compares with this.
 func diskSizeOf(t *testing.T, path string) int64 {
 	t.Helper()
 
@@ -1454,26 +1448,20 @@ func TestScanStoresDiskSize(t *testing.T) {
 
 	unix.Sync() // see diskSizeOf
 
-	size := diskSizeOf(t, p)
-	if size == 0 || size >= length {
+	want := diskSizeOf(t, p)
+	if want == 0 || want >= length {
 		t.Fatalf("the file takes %d bytes on disk, want more than none "+
-			"and fewer than its length %d", size, length)
+			"and fewer than its length %d", want, length)
 	}
 
 	for _, operand := range []string{filepath.Dir(p), p} {
 		db := openTestDB(t)
-
-		// ZFS may count the blocks during the scan; see diskSizeOf.
-		before := diskSizeOf(t, p)
-
 		syncTree(t, db, operand)
 
-		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",
-				operand, got, before, after)
+		if got != want {
+			t.Errorf("scan of %s: disk_size = %d, want %d",
+				operand, got, want)
 		}
 	}
 }
@@ -1491,7 +1479,7 @@ func TestSyncScanRewriteUpdatesDiskSize(t *testing.T) {
 
 	syncTree(t, db, dir)
 
-	recorded := recordByPath(t, dbRecords(t, db), p).diskSize
+	before := recordByPath(t, dbRecords(t, db), p).diskSize
 
 	pokeAt(t, p, 512*1024, pattern(7, 64*1024))
 
@@ -1504,53 +1492,22 @@ func TestSyncScanRewriteUpdatesDiskSize(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	// Wait for ZFS to count the write; see diskSizeOf.
-	deadline := time.Now().Add(30 * time.Second)
-	for diskSizeOf(t, p) == recorded {
-		if time.Now().After(deadline) {
-			t.Fatalf("the file still takes %d bytes on disk 30 seconds "+
-				"after the write into its hole", recorded)
-		}
+	unix.Sync() // see diskSizeOf
 
-		unix.Sync()
+	want := diskSizeOf(t, p)
+	if want == before {
+		t.Fatalf("the file still takes %d bytes on disk after the write "+
+			"into its hole", want)
 	}
 
-	// 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)
+	st := syncTree(t, db, dir)
+	if st != (scanStats{walked: 1, updated: 1}) {
+		t.Fatalf("rescan stats = %+v, want 1 updated", st)
+	}
 
-		st := syncTree(t, db, dir)
-
-		after := diskSizeOf(t, p)
-
-		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 != want {
+		t.Errorf("disk_size = %d, want %d", got, want)
 	}
 }
 
@@ -1576,21 +1533,15 @@ func TestSyncScanUnchangedUpdatesDiskSize(t *testing.T) {
 
 	unix.Sync() // see diskSizeOf
 
-	// ZFS may count the blocks during the rescan; see diskSizeOf.
-	before := diskSizeOf(t, p)
-
 	// A file read again would count as updated.
 	st := syncTree(t, db, dir)
-
-	after := diskSizeOf(t, p)
-
 	if st != (scanStats{walked: 1, unchanged: 1}) {
 		t.Fatalf("rescan stats = %+v, want 1 unchanged", st)
 	}
 
 	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 want := diskSizeOf(t, p); got != want {
+		t.Errorf("disk_size = %d, want %d", got, want)
 	}
 }