Merge 360edca70a into 49cdf0c42e

Fixes #74199

Change-Id: I6a15fbd59a3a3f8c496440f56d09d695e1504e4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/682576
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>

src/crypto/cipher/gcm.go

@@ -82,7 +82,7 @@ func newGCM(cipher Block, nonceSize, tagSize int) (AEAD, error) {
 
 // NewGCMWithRandomNonce returns the given cipher wrapped in Galois Counter
 // Mode, with randomly-generated nonces. The cipher must have been created by
-// [aes.NewCipher].
+// [crypto/aes.NewCipher].
 //
 // It generates a random 96-bit nonce, which is prepended to the ciphertext by Seal,
 // and is extracted from the ciphertext by Open. The NonceSize of the AEAD is zero,

src/encoding/json/jsontext/decode.go

@@ -138,7 +138,14 @@ 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.
+	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.
+	}
+	d.s.reset(b, r, opts...)
 }
 
 func (d *decoderState) reset(b []byte, r io.Reader, opts ...Options) {

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")
+		}
+	})
+}

src/go/doc/testdata/issue62640.0.golden

@@ -0,0 +1,22 @@
+// 
+PACKAGE issue62640
+
+IMPORTPATH
+	testdata/issue62640
+
+FILENAMES
+	testdata/issue62640.go
+
+TYPES
+	// 
+	type E struct{}
+
+	// F should be hidden within S because of the S.F field. 
+	func (E) F()
+
+	// 
+	type S struct {
+		E
+		F	int
+	}
+

src/go/doc/testdata/issue62640.1.golden

@@ -0,0 +1,22 @@
+// 
+PACKAGE issue62640
+
+IMPORTPATH
+	testdata/issue62640
+
+FILENAMES
+	testdata/issue62640.go
+
+TYPES
+	// 
+	type E struct{}
+
+	// F should be hidden within S because of the S.F field. 
+	func (E) F()
+
+	// 
+	type S struct {
+		E
+		F	int
+	}
+

src/go/doc/testdata/issue62640.2.golden

@@ -0,0 +1,25 @@
+// 
+PACKAGE issue62640
+
+IMPORTPATH
+	testdata/issue62640
+
+FILENAMES
+	testdata/issue62640.go
+
+TYPES
+	// 
+	type E struct{}
+
+	// F should be hidden within S because of the S.F field. 
+	func (E) F()
+
+	// 
+	type S struct {
+		E
+		F	int
+	}
+
+	// F should be hidden within S because of the S.F field. 
+	func (S) F()
+

src/go/doc/testdata/issue62640.go

@@ -0,0 +1,15 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue62640
+
+type E struct{}
+
+// F should be hidden within S because of the S.F field.
+func (E) F() {}
+
+type S struct {
+	E
+	F int
+}

src/runtime/mheap.go

@@ -312,8 +312,10 @@ type heapArena struct {
 	// during marking.
 	pageSpecials [pagesPerArena / 8]uint8
 
-	// pageUseSpanDartboard is a bitmap that indicates which spans are
-	// heap spans and also gcUsesSpanDartboard.
+	// pageUseSpanInlineMarkBits is a bitmap where each bit corresponds
+	// to a span, as only spans one page in size can have inline mark bits.
+	// The bit indicates that the span has a spanInlineMarkBits struct
+	// stored directly at the top end of the span's memory.
 	pageUseSpanInlineMarkBits [pagesPerArena / 8]uint8
 
 	// checkmarks stores the debug.gccheckmark state. It is only

src/testing/synctest/helper_test.go

@@ -0,0 +1,15 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package synctest_test
+
+import "testing"
+
+// helperLog is a t.Helper which logs.
+// Since it is a helper, the log prefix should contain
+// the caller's file, not helper_test.go.
+func helperLog(t *testing.T, s string) {
+	t.Helper()
+	t.Log(s)
+}

src/testing/synctest/synctest_test.go

@@ -140,6 +140,18 @@ func TestRun(t *testing.T) {
 	})
 }
 
+func TestHelper(t *testing.T) {
+	runTest(t, []string{"-test.v"}, func() {
+		synctest.Test(t, func(t *testing.T) {
+			helperLog(t, "log in helper")
+		})
+	}, `^=== RUN   TestHelper
+    synctest_test.go:.* log in helper
+--- PASS: TestHelper.*
+PASS
+$`)
+}
+
 func wantPanic(t *testing.T, want string) {
 	if e := recover(); e != nil {
 		if got := fmt.Sprint(e); got != want {

src/testing/testing.go

@@ -1261,6 +1261,9 @@ func (c *common) Skipped() bool {
 // When printing file and line information, that function will be skipped.
 // Helper may be called simultaneously from multiple goroutines.
 func (c *common) Helper() {
+	if c.isSynctest {
+		c = c.parent
+	}
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	if c.helperPCs == nil {