[dev.fuzz] testing: add support for testing.F.Add of []byte

Change-Id: I183693fec6a643b2f27cc379a422e2b42d8eca90
Reviewed-on: https://go-review.googlesource.com/c/go/+/255339
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Trust: Katie Hockman <katie@golang.org>

src/testing/fuzz.go

@@ -39,11 +39,24 @@ type corpusEntry struct {
 	b []byte
 }
 
-// Add will add the arguments to the seed corpus for the fuzz target. This
-// cannot be invoked after or within the Fuzz function. The args must match
+// Add will add the arguments to the seed corpus for the fuzz target. This will
+// be a no-op if called after or within the Fuzz function. The args must match
 // those in the Fuzz function.
 func (f *F) Add(args ...interface{}) {
-	return
+	if len(args) == 0 {
+		panic("testing: Add must have at least one argument")
+	}
+	if len(args) != 1 {
+		// TODO: support more than one argument
+		panic("testing: Add only supports one argument currently")
+	}
+	switch v := args[0].(type) {
+	case []byte:
+		f.corpus = append(f.corpus, corpusEntry{v})
+	// TODO: support other types
+	default:
+		panic("testing: Add only supports []byte currently")
+	}
 }
 
 // Fuzz runs the fuzz function, ff, for fuzz testing. It runs ff in a separate

src/testing/fuzz_test.go

@@ -0,0 +1,42 @@
+package testing_test
+
+import (
+	"testing"
+)
+
+func TestFuzzAdd(t *testing.T) {
+	matchFunc := func(a, b string) (bool, error) { return true, nil }
+	tests := []struct {
+		name string
+		fn   func(f *testing.F)
+		ok   bool
+	}{
+		{
+			"empty",
+			func(f *testing.F) { f.Add() },
+			false,
+		},
+		{
+			"multiple arguments",
+			func(f *testing.F) { f.Add([]byte("hello"), []byte("bye")) },
+			false,
+		},
+		{
+			"string",
+			func(f *testing.F) { f.Add("hello") },
+			false,
+		},
+		{
+			"bytes",
+			func(f *testing.F) { f.Add([]byte("hello")) },
+			true,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got, want := testing.RunFuzzTargets(matchFunc, []testing.InternalFuzzTarget{{Fn: tc.fn}}), tc.ok; got != want {
+				t.Errorf("testing.Add: ok %t, want %t", got, want)
+			}
+		})
+	}
+}