internal/imports: stop leaking listeners

(*dirInfoCache).ScanAndListen saves a listener when it's called. That
listener is a chain of callbacks that leads all the way back up to the
original caller, i.e. the gopls completion code. It needs to unregister
that listener, but in cases where the context was cancelled before it
finished its initial walk of the cache, it failed to. In gopls' case,
that means retaining the entire state of the workspace as of completion
triggering.

Return a real stop function on all paths.

Change-Id: Iff3046e627b1fa89f576371c4742fee6fdca7589
Reviewed-on: https://go-review.googlesource.com/c/tools/+/217677
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit cf5ba95)
Reviewed-on: https://go-review.googlesource.com/c/tools/+/217678
Reviewed-by: Robert Findley <rfindley@google.com>

Author: heschi

internal/imports/mod_cache.go

@@ -132,28 +132,29 @@ func (d *dirInfoCache) ScanAndListen(ctx context.Context, listener cacheListener
 	}
 	d.mu.Unlock()
 
+	stop := func() {
+		cancel()
+		d.mu.Lock()
+		delete(d.listeners, cookie)
+		d.mu.Unlock()
+		for i := 0; i < maxInFlight; i++ {
+			<-sema
+		}
+	}
+
 	// Process the pre-existing keys.
 	for _, k := range keys {
 		select {
 		case <-ctx.Done():
-			cancel()
-			return func() {}
+			return stop
 		default:
 		}
 		if v, ok := d.Load(k); ok {
 			listener(v)
 		}
 	}
 
-	return func() {
-		cancel()
-		d.mu.Lock()
-		delete(d.listeners, cookie)
-		d.mu.Unlock()
-		for i := 0; i < maxInFlight; i++ {
-			<-sema
-		}
-	}
+	return stop
 }
 
 // Store stores the package info for dir.