jsontext: fix Decoder.Reset to preserve internal buffer capacity

The Decoder.Reset method is not preserving the internal buffer between resets,
causing buffer capacity to be lost and resulting in unnecessary allocations
when reusing decoders.
This is particularly problematic when decoding many small messages.

This commit fixes the Reset method to preserve the internal buffer. It makes sure
aliasing is removed if the buffer currently points to an internal byte slice of a bytes.Buffer.
It adds a TestDecoderReset test structured into subtests to better validate the different scenarios.

Author: fpetkovski

src/encoding/json/jsontext/decode.go

@@ -103,6 +103,7 @@ type decodeBuffer struct {
 	peekPos int   // non-zero if valid offset into buf for start of next token
 	peekErr error // implies peekPos is -1
 
+	bufAlias  bool   // true if buf aliases rd
 	buf       []byte // may alias rd if it is a bytes.Buffer
 	prevStart int
 	prevEnd   int
@@ -138,7 +139,14 @@ 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")
 	}
-	d.s.reset(nil, r, opts...)
+	// If the decoder was previously aliasing a bytes.Buffer,
+	// invalidate the alias to avoid writing into the bytes.Buffer's
+	// internal buffer.
+	if d.s.bufAlias {
+		d.s.buf = nil
+		d.s.bufAlias = false
+	}
+	d.s.reset(d.s.buf[:0], r, opts...)
 }
 
 func (d *decoderState) reset(b []byte, r io.Reader, opts ...Options) {
@@ -179,6 +187,7 @@ func (d *decoderState) fetch() error {
 		case bb.Len() == 0:
 			return io.ErrUnexpectedEOF
 		case len(d.buf) == 0:
+			d.bufAlias = true
 			d.buf = bb.Next(bb.Len()) // "read" all data in the buffer
 			return nil
 		default:

src/encoding/json/jsontext/decode_test.go

@@ -1265,3 +1265,86 @@ 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.
+func TestDecoderReset(t *testing.T) {
+	// 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
+		val1, err := dec.ReadValue()
+		if err != nil {
+			t.Fatalf("first ReadValue failed: %v", err)
+		}
+		if string(val1) != largeJSON {
+			t.Fatalf("first ReadValue = %q, want %q", val1, largeJSON)
+		}
+
+		// 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
+		dec.Reset(strings.NewReader(largeJSON))
+
+		// 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
+		val2, err := dec.ReadValue()
+		if err != nil {
+			t.Fatalf("second ReadValue failed: %v", err)
+		}
+		if string(val2) != largeJSON {
+			t.Fatalf("second ReadValue = %q, want %q", val2, largeJSON)
+		}
+	})
+
+	var bbBuf []byte
+	t.Run("Test aliasing with bytes.Buffer", func(t *testing.T) {
+		// Test with bytes.Buffer to verify proper aliasing behavior.
+		bb := bytes.NewBufferString(largeJSON)
+		dec.Reset(bb)
+		bbBuf = bb.Bytes()
+
+		// 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)
+		}
+		if string(val3) != largeJSON {
+			t.Fatalf("fourth ReadValue = %q, want %q", val3, largeJSON)
+		}
+		// The decoder buffer should alias bytes.Buffer's internal buffer.
+		if len(dec.s.buf) == 0 || len(bbBuf) == 0 || &dec.s.buf[0] != &bbBuf[0] {
+			t.Fatalf("decoder 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.
+		dec.Reset(strings.NewReader(largeJSON))
+		val4, err := dec.ReadValue()
+		if err != nil {
+			t.Fatalf("fifth ReadValue failed: %v", err)
+		}
+		if string(val4) != largeJSON {
+			t.Fatalf("fourth ReadValue = %q, want %q", val4, largeJSON)
+		}
+
+		// The decoder buffer should not alias the bytes.Buffer's internal buffer.
+		if len(dec.s.buf) == 0 || len(bbBuf) == 0 || &dec.s.buf[0] == &bbBuf[0] {
+			t.Fatalf("decoder buffer aliases bytes.Buffer")
+		}
+	})
+}