encoding/json/jsontext: preserve buffer capacity in Encoder.Reset

This does the equivalent of CL 681177 for the Encoder.
It preserves the internal buffer between resets.

Change-Id: I5e9353b6d7755e067d4f9a4d1ea3d8f056253027
Reviewed-on: https://go-review.googlesource.com/c/go/+/690375
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/encoding/json/jsontext/decode.go

@@ -138,12 +138,10 @@ func (d *Decoder) Reset(r io.Reader, opts ...Options) {
 	case d.s.Flags.Get(jsonflags.WithinArshalCall):
 		panic("jsontext: cannot reset Decoder passed to json.UnmarshalerFrom")
 	}
-	// If the decoder was previously aliasing a bytes.Buffer,
+	// Reuse the buffer if it does not alias a previous [bytes.Buffer].
-	// invalidate the alias to avoid writing into the bytes.Buffer's
-	// internal buffer.
 	b := d.s.buf[:0]
 	if _, ok := d.s.rd.(*bytes.Buffer); ok {
-		b = nil // avoid reusing b since it aliases the previous bytes.Buffer.
+		b = nil
 	}
 	d.s.reset(b, r, opts...)
 }

src/encoding/json/jsontext/decode_test.go

@@ -1269,14 +1269,14 @@ func TestPeekableDecoder(t *testing.T) {
 // TestDecoderReset tests that the decoder preserves its internal
 // buffer between Reset calls to avoid frequent allocations when reusing the decoder.
 // It ensures that the buffer capacity is maintained while avoiding aliasing
-// issues with bytes.Buffer.
+// issues with [bytes.Buffer].
 func TestDecoderReset(t *testing.T) {
-	// Create a decoder with a reasonably large JSON input to ensure buffer growth
+	// Create a decoder with a reasonably large JSON input to ensure buffer growth.
 	largeJSON := `{"key1":"value1","key2":"value2","key3":"value3","key4":"value4","key5":"value5"}`
 	dec := NewDecoder(strings.NewReader(largeJSON))
 
 	t.Run("Test capacity preservation", func(t *testing.T) {
-		// Read the first JSON value to grow the internal buffer
+		// Read the first JSON value to grow the internal buffer.
 		val1, err := dec.ReadValue()
 		if err != nil {
 			t.Fatalf("first ReadValue failed: %v", err)
@@ -1285,22 +1285,22 @@ func TestDecoderReset(t *testing.T) {
 			t.Fatalf("first ReadValue = %q, want %q", val1, largeJSON)
 		}
 
-		// Get the buffer capacity after first use
+		// Get the buffer capacity after first use.
 		initialCapacity := cap(dec.s.buf)
 		if initialCapacity == 0 {
 			t.Fatalf("expected non-zero buffer capacity after first use")
 		}
 
-		// Reset with a new reader - this should preserve the buffer capacity
+		// Reset with a new reader - this should preserve the buffer capacity.
 		dec.Reset(strings.NewReader(largeJSON))
 
-		// Verify the buffer capacity is preserved (or at least not smaller)
+		// Verify the buffer capacity is preserved (or at least not smaller).
 		preservedCapacity := cap(dec.s.buf)
 		if preservedCapacity < initialCapacity {
 			t.Fatalf("buffer capacity reduced after Reset: got %d, want at least %d", preservedCapacity, initialCapacity)
 		}
 
-		// Read the second JSON value to ensure the decoder still works correctly
+		// Read the second JSON value to ensure the decoder still works correctly.
 		val2, err := dec.ReadValue()
 		if err != nil {
 			t.Fatalf("second ReadValue failed: %v", err)
@@ -1317,7 +1317,7 @@ func TestDecoderReset(t *testing.T) {
 		dec.Reset(bb)
 		bbBuf = bb.Bytes()
 
-		// Read the third JSON value to ensure functionality with bytes.Buffer
+		// Read the third JSON value to ensure functionality with bytes.Buffer.
 		val3, err := dec.ReadValue()
 		if err != nil {
 			t.Fatalf("fourth ReadValue failed: %v", err)

src/encoding/json/jsontext/encode.go

@@ -107,12 +107,17 @@ func (e *Encoder) Reset(w io.Writer, opts ...Options) {
 	case e.s.Flags.Get(jsonflags.WithinArshalCall):
 		panic("jsontext: cannot reset Encoder passed to json.MarshalerTo")
 	}
-	e.s.reset(nil, w, opts...)
+	// Reuse the buffer if it does not alias a previous [bytes.Buffer].
+	b := e.s.Buf[:0]
+	if _, ok := e.s.wr.(*bytes.Buffer); ok {
+		b = nil
+	}
+	e.s.reset(b, w, opts...)
 }
 
 func (e *encoderState) reset(b []byte, w io.Writer, opts ...Options) {
 	e.state.reset()
-	e.encodeBuffer = encodeBuffer{Buf: b, wr: w, bufStats: e.bufStats}
+	e.encodeBuffer = encodeBuffer{Buf: b, wr: w, availBuffer: e.availBuffer, bufStats: e.bufStats}
 	if bb, ok := w.(*bytes.Buffer); ok && bb != nil {
 		e.Buf = bb.AvailableBuffer() // alias the unused buffer of bb
 	}

src/encoding/json/jsontext/encode_test.go

@@ -735,3 +735,95 @@ func testEncoderErrors(t *testing.T, where jsontest.CasePos, opts []Options, cal
 		t.Fatalf("%s: Encoder.OutputOffset = %v, want %v", where, gotOffset, wantOffset)
 	}
 }
+
+// TestEncoderReset tests that the encoder preserves its internal
+// buffer between Reset calls to avoid frequent allocations when reusing the encoder.
+// It ensures that the buffer capacity is maintained while avoiding aliasing
+// issues with [bytes.Buffer].
+func TestEncoderReset(t *testing.T) {
+	// Create an encoder with a reasonably large JSON input to ensure buffer growth.
+	largeJSON := `{"key1":"value1","key2":"value2","key3":"value3","key4":"value4","key5":"value5"}` + "\n"
+	bb := new(bytes.Buffer)
+	enc := NewEncoder(struct{ io.Writer }{bb}) // mask out underlying [bytes.Buffer]
+
+	t.Run("Test capacity preservation", func(t *testing.T) {
+		// Write the first JSON value to grow the internal buffer.
+		err := enc.WriteValue(append(enc.AvailableBuffer(), largeJSON...))
+		if err != nil {
+			t.Fatalf("first WriteValue failed: %v", err)
+		}
+		if bb.String() != largeJSON {
+			t.Fatalf("first WriteValue = %q, want %q", bb.String(), largeJSON)
+		}
+
+		// Get the buffer capacity after first use.
+		initialCapacity := cap(enc.s.Buf)
+		initialCacheCapacity := cap(enc.s.availBuffer)
+		if initialCapacity == 0 {
+			t.Fatalf("expected non-zero buffer capacity after first use")
+		}
+		if initialCacheCapacity == 0 {
+			t.Fatalf("expected non-zero cache capacity after first use")
+		}
+
+		// Reset with a new writer - this should preserve the buffer capacity.
+		bb.Reset()
+		enc.Reset(struct{ io.Writer }{bb})
+
+		// Verify the buffer capacity is preserved (or at least not smaller).
+		preservedCapacity := cap(enc.s.Buf)
+		if preservedCapacity < initialCapacity {
+			t.Fatalf("buffer capacity reduced after Reset: got %d, want at least %d", preservedCapacity, initialCapacity)
+		}
+		preservedCacheCapacity := cap(enc.s.availBuffer)
+		if preservedCacheCapacity < initialCacheCapacity {
+			t.Fatalf("cache capacity reduced after Reset: got %d, want at least %d", preservedCapacity, initialCapacity)
+		}
+
+		// Write the second JSON value to ensure the encoder still works correctly.
+		err = enc.WriteValue(append(enc.AvailableBuffer(), largeJSON...))
+		if err != nil {
+			t.Fatalf("second WriteValue failed: %v", err)
+		}
+		if bb.String() != largeJSON {
+			t.Fatalf("second WriteValue = %q, want %q", bb.String(), largeJSON)
+		}
+	})
+
+	t.Run("Test aliasing with bytes.Buffer", func(t *testing.T) {
+		// Test with bytes.Buffer to verify proper aliasing behavior.
+		bb.Reset()
+		enc.Reset(bb)
+
+		// Write the third JSON value to ensure functionality with bytes.Buffer.
+		err := enc.WriteValue([]byte(largeJSON))
+		if err != nil {
+			t.Fatalf("fourth WriteValue failed: %v", err)
+		}
+		if bb.String() != largeJSON {
+			t.Fatalf("fourth WriteValue = %q, want %q", bb.String(), largeJSON)
+		}
+		// The encoder buffer should alias bytes.Buffer's internal buffer.
+		if cap(enc.s.Buf) == 0 || cap(bb.AvailableBuffer()) == 0 || &enc.s.Buf[:1][0] != &bb.AvailableBuffer()[:1][0] {
+			t.Fatalf("encoder buffer does not alias bytes.Buffer")
+		}
+	})
+
+	t.Run("Test aliasing removed after Reset", func(t *testing.T) {
+		// Reset with a new reader and verify the buffer is not aliased.
+		bb.Reset()
+		enc.Reset(struct{ io.Writer }{bb})
+		err := enc.WriteValue([]byte(largeJSON))
+		if err != nil {
+			t.Fatalf("fifth WriteValue failed: %v", err)
+		}
+		if bb.String() != largeJSON {
+			t.Fatalf("fourth WriteValue = %q, want %q", bb.String(), largeJSON)
+		}
+
+		// The encoder buffer should not alias the bytes.Buffer's internal buffer.
+		if cap(enc.s.Buf) == 0 || cap(bb.AvailableBuffer()) == 0 || &enc.s.Buf[:1][0] == &bb.AvailableBuffer()[:1][0] {
+			t.Fatalf("encoder buffer aliases bytes.Buffer")
+		}
+	})
+}