log/slog: fix issue with concurrent writes

This causes instances commonHandler created by withAttrs or withGroup to
share a mutex with their parent preventing concurrent writes to their
shared writer.

Fixes #61321

Change-Id: Ieec225e88ad51c84b41bad6c409fac48c90320b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/509196
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

4 files changed, 53 insertions, 1 deletions

diff --git a/src/log/slog/handler.go b/src/log/slog/handler.go
index e479ca8a4c..8398b928c1 100644
--- a/src/log/slog/handler.go
+++ b/src/log/slog/handler.go
@@ -193,7 +193,7 @@ type commonHandler struct {
 	groupPrefix string
 	groups      []string // all groups started from WithGroup
 	nOpenGroups int      // the number of groups opened in preformattedAttrs
-	mu          sync.Mutex
+	mu          *sync.Mutex
 	w           io.Writer
 }
 
@@ -207,6 +207,7 @@ func (h *commonHandler) clone() *commonHandler {
 		groups:            slices.Clip(h.groups),
 		nOpenGroups:       h.nOpenGroups,
 		w:                 h.w,
+		mu:                h.mu, // mutex shared among all clones of this handler
 	}
 }
 
diff --git a/src/log/slog/handler_test.go b/src/log/slog/handler_test.go
index 3fb7360fc2..4f10ee5028 100644
--- a/src/log/slog/handler_test.go
+++ b/src/log/slog/handler_test.go
@@ -16,6 +16,7 @@ import (
 	"slices"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 )
@@ -106,6 +107,52 @@ func TestDefaultHandle(t *testing.T) {
 	}
 }
 
+func TestConcurrentWrites(t *testing.T) {
+	ctx := context.Background()
+	count := 1000
+	for _, handlerType := range []string{"text", "json"} {
+		t.Run(handlerType, func(t *testing.T) {
+			var buf bytes.Buffer
+			var h Handler
+			switch handlerType {
+			case "text":
+				h = NewTextHandler(&buf, nil)
+			case "json":
+				h = NewJSONHandler(&buf, nil)
+			default:
+				t.Fatalf("unexpected handlerType %q", handlerType)
+			}
+			sub1 := h.WithAttrs([]Attr{Bool("sub1", true)})
+			sub2 := h.WithAttrs([]Attr{Bool("sub2", true)})
+			var wg sync.WaitGroup
+			for i := 0; i < count; i++ {
+				sub1Record := NewRecord(time.Time{}, LevelInfo, "hello from sub1", 0)
+				sub1Record.AddAttrs(Int("i", i))
+				sub2Record := NewRecord(time.Time{}, LevelInfo, "hello from sub2", 0)
+				sub2Record.AddAttrs(Int("i", i))
+				wg.Add(1)
+				go func() {
+					defer wg.Done()
+					if err := sub1.Handle(ctx, sub1Record); err != nil {
+						t.Error(err)
+					}
+					if err := sub2.Handle(ctx, sub2Record); err != nil {
+						t.Error(err)
+					}
+				}()
+			}
+			wg.Wait()
+			for i := 1; i <= 2; i++ {
+				want := "hello from sub" + strconv.Itoa(i)
+				n := strings.Count(buf.String(), want)
+				if n != count {
+					t.Fatalf("want %d occurrences of %q, got %d", count, want, n)
+				}
+			}
+		})
+	}
+}
+
 // Verify the common parts of TextHandler and JSONHandler.
 func TestJSONAndTextHandlers(t *testing.T) {
 	// remove all Attrs
diff --git a/src/log/slog/json_handler.go b/src/log/slog/json_handler.go
index cebcfba45a..1c51ab05ff 100644
--- a/src/log/slog/json_handler.go
+++ b/src/log/slog/json_handler.go
@@ -13,6 +13,7 @@ import (
 	"io"
 	"log/slog/internal/buffer"
 	"strconv"
+	"sync"
 	"time"
 	"unicode/utf8"
 )
@@ -35,6 +36,7 @@ func NewJSONHandler(w io.Writer, opts *HandlerOptions) *JSONHandler {
 			json: true,
 			w:    w,
 			opts: *opts,
+			mu:   &sync.Mutex{},
 		},
 	}
 }
diff --git a/src/log/slog/text_handler.go b/src/log/slog/text_handler.go
index 75b66b716f..58edb2f66d 100644
--- a/src/log/slog/text_handler.go
+++ b/src/log/slog/text_handler.go
@@ -11,6 +11,7 @@ import (
 	"io"
 	"reflect"
 	"strconv"
+	"sync"
 	"unicode"
 	"unicode/utf8"
 )
@@ -33,6 +34,7 @@ func NewTextHandler(w io.Writer, opts *HandlerOptions) *TextHandler {
 			json: false,
 			w:    w,
 			opts: *opts,
+			mu:   &sync.Mutex{},
 		},
 	}
 }