[dev.fuzz] internal/fuzz: fix two bugs in BenchmarkWorkerFuzzOverhead

* The exec count must be set to 0 before calling
  workerServer.fuzz. This was causing fuzz to run indefinitely after
  the first benchmark iteration, since it wouldn't hit the termination
  condition of being equal to fuzzArgs.Limit.
* Added an assertion that the count must be lower than fuzzArgs.Limit
  at the beginning of workerServer.fuzz.
* Also closed and deleted shared memory at the end of each benchmark run.

Change-Id: Iab465f8a4997ebd652aec04d0ff9bb60b802829e
Reviewed-on: https://go-review.googlesource.com/c/go/+/334129
Trust: Jay Conrod <jayconrod@google.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

fuzz/worker.go

@@ -632,6 +632,9 @@ func (ws *workerServer) fuzz(ctx context.Context, args fuzzArgs) (resp fuzzRespo
 		resp.Count = mem.header().count
 		ws.memMu <- mem
 	}()
+	if args.Limit > 0 && mem.header().count >= args.Limit {
+		panic(fmt.Sprintf("mem.header().count %d already exceeds args.Limit %d", mem.header().count, args.Limit))
+	}
 
 	vals, err := unmarshalCorpusFile(mem.valueCopy())
 	if err != nil {

fuzz/worker_test.go

@@ -25,6 +25,11 @@ func BenchmarkWorkerFuzzOverhead(b *testing.B) {
 	if err != nil {
 		b.Fatalf("failed to create temporary shared memory file: %s", err)
 	}
+	defer func() {
+		if err := mem.Close(); err != nil {
+			b.Error(err)
+		}
+	}()
 
 	initialVal := []interface{}{make([]byte, 32)}
 	encodedVals := marshalCorpusFile(initialVal...)
@@ -36,6 +41,7 @@ func BenchmarkWorkerFuzzOverhead(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		ws.m = newMutator()
 		mem.setValue(encodedVals)
+		mem.header().count = 0
 
 		ws.fuzz(context.Background(), fuzzArgs{Limit: 1})
 	}