Return from Noop lex routine

In the Noop lexer we start a routine that's responsible for output
of the written frames at a consistent rate (to make certain
destinations happy i.e. youtube). We weren't however returning
from this in the case that the lexing function is returned, we'd
therefore then be left with a hanging routine (this would
eventually keep happening until stack overflow if had long enough
runtime for the service; not a big deal for systemd services, but
still crappy). So, now we create a context which is cancelled when
when the lex function returns and can detect this in the output
routine.

Approved-by: Alan Noble

codec/codecutil/lex.go

@@ -30,6 +30,7 @@ import (
 	"io"
 	"math"
 	"time"
+	"context"
 )
 
 // ByteLexer is used to lex bytes using a buffer size which is configured upon construction.
@@ -127,13 +128,20 @@ func Noop(dst io.Writer, src io.Reader, d time.Duration) error {
 	defer delay.Stop()
 
 	// This routine is responsible for frame output to rest of the pipeline.
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
 	go func() {
 		for {
 			err := rb.writeTo(dst)
 			if err != nil && !errors.Is(err, errBuffChanReceiveTimeout) {
 				errCh <- fmt.Errorf("could not write to dst: %w", err)
 			}
-			<-delay.C
+
+			select {
+			case <-delay.C:
+			case <-ctx.Done():
+				return
+			}
 
 			// Adjust delay using proportional controller.
 			adj := coef * float64(target-rb.len())