Reject scan --workers below 1 as a usage error (closes #10)

A --workers value of 0 or less used to be quietly raised to 1, so a
typo ran the whole scan on one worker with nothing on stderr to say
why. scan now refuses it before anything is scanned: one line on
stderr and exit 2, like the other usage errors. The clamp in runScan
is gone, and README states the rule and the default.

Model: opus-5-5

TODO.md

@@ -33,6 +33,9 @@
   running single-threaded (2026-10-04,
   https://git.eeqj.de/sneak/sfdupes/issues/10)
 
+- a test fails when either walk cancellation check in `scan.go` is
+  removed (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/81)
+
 - test that `scan` refuses a database with another schema version
   (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/64)
 

cancel_test.go

@@ -586,20 +586,49 @@ func TestSendEventAbandonsBlockedSend(t *testing.T) {
 	awaitReturn(t, done, "sendEvent")
 }
 
-// TestWalkWorkersDropQueuedDirs checks that cancelled walk workers keep
+// TestWalkOneDirStopsWhenCancelled checks that a cancelled scan stops
-// reading jobs and drop the directories rather than stopping their
+// reading a directory instead of going through the rest of its
-// read: the range over jobs has to run out for the pool to tear down
+// entries. A walk that kept going would return the subdirectory below
-// and close its event stream.
+// to descend into. Unlike a file event, that return is not a send the
-func TestWalkWorkersDropQueuedDirs(t *testing.T) {
+// cancellation can abandon, so the test catches the regression every
+// time.
+func TestWalkOneDirStopsWhenCancelled(t *testing.T) {
 	t.Parallel()
 
 	dir := t.TempDir()
-	writeEmptyFiles(t, dir, walkCancelFilesPerDir)
+
+	err := os.Mkdir(filepath.Join(dir, "sub"), 0o750)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Unbuffered and unread: on a cancelled scan every send gives up.
+	events := make(chan walkEvent)
+
+	subs := walkOneDir(cancelledContext(t), dirJob{path: dir}, false, events)
+	if len(subs) != 0 {
+		t.Errorf("cancelled walkOneDir returned %+v to descend into, "+
+			"want none", subs)
+	}
+}
+
+// TestWalkWorkersDropQueuedDirs checks that cancelled walk workers keep
+// reading jobs and drop the directories rather than stopping their
+// read: the range over jobs has to run out for the pool to tear down
+// and close its event stream. The queued directory does not exist, so
+// a worker that walked it anyway would send a warning before
+// walkOneDir's own cancellation check could stop it. On a cancelled
+// scan that send delivers or gives up at random, so with 64 jobs
+// queued the regression has a one in 2^64 chance of passing.
+func TestWalkWorkersDropQueuedDirs(t *testing.T) {
+	t.Parallel()
+
+	missing := filepath.Join(t.TempDir(), "missing")
 
 	jobs, _, events := startWalkWorkers(cancelledContext(t), 2, false)
 
-	for range 4 {
+	for range 64 {
-		jobs <- dirJob{path: dir}
+		jobs <- dirJob{path: missing}
 	}
 
 	close(jobs)