[dev.fuzz] cmd/go: exclude additional packages from fuzzing instrumentation

Counters in these packages are incremented by background goroutines
for testing and internal/fuzz. They cause some inputs to seem
"interesting" when they don't directly provide new coverage.

Updates golang/go#46410

Change-Id: Ibe6bb3177f3b2ba23382a1693a4c6a576f94a423
Reviewed-on: https://go-review.googlesource.com/c/go/+/342993
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

2 files changed, 22 insertions, 7 deletions

diff --git a/src/cmd/go/internal/test/test.go b/src/cmd/go/internal/test/test.go
index 012a75123b..75345a8223 100644
--- a/src/cmd/go/internal/test/test.go
+++ b/src/cmd/go/internal/test/test.go
@@ -831,11 +831,18 @@ func runTest(ctx context.Context, cmd *base.Command, args []string) {
 	fuzzFlags := work.FuzzInstrumentFlags()
 	if testFuzz != "" && fuzzFlags != nil {
 		// Don't instrument packages which may affect coverage guidance but are
-		// unlikely to be useful.
+		// unlikely to be useful. Most of these are used by the testing or
+		// internal/fuzz concurrently with fuzzing.
 		var fuzzNoInstrument = map[string]bool{
-			"testing":       true,
+			"context":       true,
 			"internal/fuzz": true,
+			"reflect":       true,
 			"runtime":       true,
+			"sync":          true,
+			"sync/atomic":   true,
+			"syscall":       true,
+			"testing":       true,
+			"time":          true,
 		}
 		for _, p := range load.TestPackageList(ctx, pkgOpts, pkgs) {
 			if fuzzNoInstrument[p.ImportPath] {
diff --git a/src/cmd/go/testdata/script/test_fuzz_cache.txt b/src/cmd/go/testdata/script/test_fuzz_cache.txt
index a6c9cafada..10e4c2926f 100644
--- a/src/cmd/go/testdata/script/test_fuzz_cache.txt
+++ b/src/cmd/go/testdata/script/test_fuzz_cache.txt
@@ -35,19 +35,27 @@ go 1.16
 -- y_test.go --
 package y
 
-import "testing"
+import (
+	"io"
+	"testing"
+)
 
 func FuzzY(f *testing.F) {
 	f.Add([]byte("y"))
-	f.Fuzz(func(t *testing.T, b []byte) { Y(b) })
+	f.Fuzz(func(t *testing.T, b []byte) { Y(io.Discard, b) })
 }
 -- y.go --
 package y
 
-import "bytes"
+import (
+	"bytes"
+	"io"
+)
 
-func Y(b []byte) bool {
-	return bytes.Equal(b, []byte("y"))
+func Y(w io.Writer, b []byte) {
+	if !bytes.Equal(b, []byte("y")) {
+		w.Write([]byte("not equal"))
+	}
 }
 -- empty/empty.go --
 package empty