Correct four inaccurate comments in cancel_test.go and rename a constant (closes #33)

Cleanup of the non-blocking findings from the re-review of #6. The tests
were never wrong; only comments that misdescribed the mechanism they
document, and one constant whose name claimed the wrong quantity.

- State the property the walkClock tests rely on (the index load's
  Done cost is bounded and independent of record count) instead of the
  wrong "three consultations" figure.
- Stop the poolUnwind framing from implying every test is bounded at
  two seconds; the three tests that catch their regression only as the
  test binary's timeout now say so.
- Record that hashWorker's results-send abandon branch is reachable
  from the scan path and is covered by TestScanHashWriteFailureUnwindsPool,
  so every cancellation branch has a test.
- Rename walkCancelInFlightDirs to walkCancelInFlightFiles: it is a
  file count (same value).
- Note the deliberate departure from the one-test-file-per-source-file
  convention at the top of the file.

Model: opus-4-8

TODO.md

@@ -29,6 +29,11 @@
 
 # Completed Steps
 
+- correct four inaccurate comments in `cancel_test.go` and rename
+  `walkCancelInFlightDirs` to `walkCancelInFlightFiles` (2026-09-21,
+  branch `issue-33-cancel-test-comments`, closes
+  https://git.eeqj.de/sneak/sfdupes/issues/33)
+
 - fix the lint-image pin comments and `FROM` form in `Dockerfile` and
   `Dockerfile.lint` (2026-08-10, branch `next`, closes
   https://git.eeqj.de/sneak/sfdupes/issues/25): dropped the false

cancel_test.go

@@ -13,11 +13,20 @@ import (
 	"time"
 )
 
-// poolUnwind bounds how long a goroutine is given to leave a pool
-// after its context is cancelled. Only a failing run ever waits this
-// long: a pool that ignored its cancellation parks forever, and this
-// is what turns that into a failed assertion instead of a suite that
-// hangs until the test binary's own timeout.
+// This file gathers the tests for scan cancellation and worker-pool
+// unwinding. Everything it exercises lives in scan.go, so by the repo's
+// convention of one test file per source file it would belong in
+// scan_test.go. It is kept separate on purpose: cancellation behaviour
+// cuts across both the walk pool and the hash pool as a single concern,
+// and scan_test.go is already 900+ lines. That is the deliberate
+// exception the convention otherwise expects to be stated.
+
+// poolUnwind bounds how long a goroutine is given to leave a pool after
+// its context is cancelled. The tests that use it turn a pool that
+// ignored its cancellation — and so parks forever — into a failed
+// assertion within this bound instead of a hang. Not every test in this
+// file has that property: a few catch their regression only as the test
+// binary's own timeout, and each of those says so.
 const poolUnwind = 2 * time.Second
 
 // walkClock is a context whose cancellation is driven by the scan's
@@ -28,9 +37,12 @@ const poolUnwind = 2 * time.Second
 //
 // The accounting behind the n chosen by each test: every blocking
 // channel operation in the walk selects on Done, so the walk spends
-// one consultation per file event plus a couple per directory, while
-// the index load that runs ahead of it spends a small fixed number
-// (three) whatever the record count.
+// one consultation per file event plus a couple per directory. The
+// index load that runs ahead of it also consults Done, but a bounded
+// number of times that does not grow with the record count. The tests
+// depend on that property, not on the bound's exact value: each picks
+// an n comfortably above it and well short of the walk's total, so the
+// cancellation lands deep inside the walk whatever the record count.
 type walkClock struct {
 	n    int64
 	seen atomic.Int64
@@ -90,7 +102,11 @@ const (
 	walkCancelFilesPerDir = 20
 	walkCancelFiles       = walkCancelDirs * walkCancelFilesPerDir
 	walkCancelWorkers     = 4
-	walkCancelInFlightDirs = walkCancelWorkers * walkCancelFilesPerDir
+	// The files carried by the walkCancelWorkers directories already in
+	// flight when the scan is cancelled: each such directory can still
+	// emit its walkCancelFilesPerDir files. This is a file count, not a
+	// directory count.
+	walkCancelInFlightFiles = walkCancelWorkers * walkCancelFilesPerDir
 )
 
 // walkCancelAtDone is the consultation on which the fixture's context
@@ -154,6 +170,11 @@ func assertRecordsIntact(t *testing.T, db *sql.DB, before []string) {
 // phase and fail there instead, with a wrapped error rather than the
 // bare cancellation.
 //
+// The syncScan call here is not bounded by poolUnwind: a regression
+// that left a worker pool parked would hang it, and that regression is
+// caught only by the test binary's own timeout, not by a quick
+// assertion.
+//
 //nolint:paralleltest // counts goroutines: must not run beside others
 func TestSyncScanCancelledMidWalkKeepsRecords(t *testing.T) {
 	dir := buildWalkCancelTree(t)
@@ -211,10 +232,11 @@ func assertWalkGuardAborted(t *testing.T, st scanStats, err error) {
 	}
 
 	// The workers drop every directory still queued once the scan is
-	// cancelled, so only the directories already in flight can add to
-	// the census after the fact. A census beyond that bound would mean
-	// the cancellation was not observed where it should have been.
-	limit := walkCancelAtDone + walkCancelInFlightDirs
+	// cancelled, so only the files in the directories already in flight
+	// can add to the census after the fact. A census beyond that bound
+	// would mean the cancellation was not observed where it should have
+	// been.
+	limit := walkCancelAtDone + walkCancelInFlightFiles
 	if st.unchanged > limit {
 		t.Errorf("census covers %d files, want at most %d: the walk kept "+
 			"taking directories off the queue after cancellation",
@@ -412,7 +434,11 @@ func TestDispatchDirsClosesJobsWhenCancelled(t *testing.T) {
 
 // TestFeedHashJobsClosesJobsWhenCancelled checks that the hash feeder
 // abandons the runs it has not queued yet and still closes the job
-// channel, which is what lets the workers' range terminate.
+// channel, which is what lets the workers' range terminate. The
+// receive on jobs below is not bounded: a feeder that returned without
+// closing jobs would leave that receive with no sender and no close, so
+// this regression is caught by the test binary's timeout rather than by
+// a bounded assertion.
 func TestFeedHashJobsClosesJobsWhenCancelled(t *testing.T) {
 	t.Parallel()
 
@@ -440,6 +466,16 @@ func TestFeedHashJobsClosesJobsWhenCancelled(t *testing.T) {
 // nobody wants the hashes of — while still letting the range run out
 // so the pool tears down. The queued run names a file that does not
 // exist, so a worker that hashed it anyway would produce a result.
+//
+// hashWorker has a second cancellation exit: the send of a completed
+// result on the results channel. That branch is reachable from the
+// production scan path, not dead code — pool.stop() cancels the context
+// before it starts draining results, so a worker parked on that send
+// leaves through this case, freed by the drain rather than by an empty
+// jobs channel. It is exercised by TestScanHashWriteFailureUnwindsPool,
+// which strands every worker on a full results channel until stop()
+// unwinds the pool. With both branches covered, every cancellation
+// branch of the walk and hash pools has a test.
 func TestHashWorkerDropsQueuedRuns(t *testing.T) {
 	t.Parallel()
 
@@ -472,7 +508,10 @@ func TestHashWorkerDropsQueuedRuns(t *testing.T) {
 // TestHashPhaseCancelledReturnsContextError checks the result loop's
 // own exit: with the pool cancelled, no result will ever arrive, and
 // the loop must leave through the cancellation rather than wait for a
-// receive that cannot happen.
+// receive that cannot happen. This call is not bounded by poolUnwind: a
+// loop that dropped its cancellation case would block on that receive,
+// so the regression surfaces as the test binary's timeout rather than
+// as a bounded assertion.
 func TestHashPhaseCancelledReturnsContextError(t *testing.T) {
 	t.Parallel()