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>

1 files changed, 7 insertions, 2 deletions

diff --git a/src/encoding/json/jsontext/encode.go b/src/encoding/json/jsontext/encode.go
index 562d217fef..e3b9c04ca6 100644
--- a/src/encoding/json/jsontext/encode.go
+++ b/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
 	}