encoding/json/jsontext: avoid pinning application data in pools

Previously, we put a jsontext.Encoder (or Decoder)
back into a pool with minimal reset logic.
This was semantically safe since we always did a full reset
after obtaining an Encoder/Decoder back out of the pool.

However, this meant that so long as an Encoder/Decoder was
alive in the pool, any application data referenced by the coder
would be kept alive longer than necessary.
Explicitly, clear such fields so that application data
can be more aggressively garbage collected.

Performance:

	name               old time/op    new time/op    delta
	Unmarshal/Bool-32    52.0ns ± 3%    50.3ns ± 3%  -3.30%  (p=0.001 n=10+10)
	Marshal/Bool-32      55.4ns ± 3%    54.4ns ± 2%  -1.75%  (p=0.006 n=10+9)

This only impacts the performance of discrete Marshal/Unmarshal calls.
For the simplest possible call (i.e., to marsha/unmarshal a bool),
there is a 1-2ns slow down. This is an appropriate slowdown
for an improvement in memory utilization.

Change-Id: I5e7d7827473773e53a9dcb3d7fe9052a75481e9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/713640
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
TryBot-Bypass: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Auto-Submit: Damien Neil <dneil@google.com>

src/encoding/json/jsontext/pools.go

@@ -54,6 +54,10 @@ func getBufferedEncoder(opts ...Options) *Encoder {
 	return e
 }
 func putBufferedEncoder(e *Encoder) {
+	if cap(e.s.availBuffer) > 64<<10 {
+		e.s.availBuffer = nil // avoid pinning arbitrarily large amounts of memory
+	}
+
 	// Recycle large buffers only if sufficiently utilized.
 	// If a buffer is under-utilized enough times sequentially,
 	// then it is discarded, ensuring that a single large buffer
@@ -95,9 +99,14 @@ func getStreamingEncoder(w io.Writer, opts ...Options) *Encoder {
 	}
 }
 func putStreamingEncoder(e *Encoder) {
+	if cap(e.s.availBuffer) > 64<<10 {
+		e.s.availBuffer = nil // avoid pinning arbitrarily large amounts of memory
+	}
 	if _, ok := e.s.wr.(*bytes.Buffer); ok {
+		e.s.wr, e.s.Buf = nil, nil // avoid pinning the provided bytes.Buffer
 		bytesBufferEncoderPool.Put(e)
 	} else {
+		e.s.wr = nil // avoid pinning the provided io.Writer
 		if cap(e.s.Buf) > 64<<10 {
 			e.s.Buf = nil // avoid pinning arbitrarily large amounts of memory
 		}
@@ -126,6 +135,7 @@ func getBufferedDecoder(b []byte, opts ...Options) *Decoder {
 	return d
 }
 func putBufferedDecoder(d *Decoder) {
+	d.s.buf = nil // avoid pinning the provided buffer
 	bufferedDecoderPool.Put(d)
 }
 
@@ -142,8 +152,10 @@ func getStreamingDecoder(r io.Reader, opts ...Options) *Decoder {
 }
 func putStreamingDecoder(d *Decoder) {
 	if _, ok := d.s.rd.(*bytes.Buffer); ok {
+		d.s.rd, d.s.buf = nil, nil // avoid pinning the provided bytes.Buffer
 		bytesBufferDecoderPool.Put(d)
 	} else {
+		d.s.rd = nil // avoid pinning the provided io.Reader
 		if cap(d.s.buf) > 64<<10 {
 			d.s.buf = nil // avoid pinning arbitrarily large amounts of memory
 		}

src/encoding/json/v2/arshal.go

@@ -571,5 +571,6 @@ func putStrings(s *stringSlice) {
 	if cap(*s) > 1<<10 {
 		*s = nil // avoid pinning arbitrarily large amounts of memory
 	}
+	clear(*s) // avoid pinning a reference to each string
 	stringsPools.Put(s)
 }