log/slog: catch panics during formatting

Fixes #61648

Change-Id: I6b7f4948ca89142a358d74624607daf42ea8b304
Reviewed-on: https://go-review.googlesource.com/c/go/+/514135
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>

2 files changed, 85 insertions, 0 deletions

diff --git a/src/log/slog/handler.go b/src/log/slog/handler.go
index a73983cda3..d18321fc6f 100644
--- a/src/log/slog/handler.go
+++ b/src/log/slog/handler.go
@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog/internal/buffer"
+	"reflect"
 	"slices"
 	"strconv"
 	"sync"
@@ -512,6 +513,23 @@ func (s *handleState) appendString(str string) {
 }
 
 func (s *handleState) appendValue(v Value) {
+	defer func() {
+		if r := recover(); r != nil {
+			// If it panics with a nil pointer, the most likely cases are
+			// an encoding.TextMarshaler or error fails to guard against nil,
+			// in which case "<nil>" seems to be the feasible choice.
+			//
+			// Adapted from the code in fmt/print.go.
+			if v := reflect.ValueOf(v.any); v.Kind() == reflect.Pointer && v.IsNil() {
+				s.appendString("<nil>")
+				return
+			}
+
+			// Otherwise just print the original panic message.
+			s.appendString(fmt.Sprintf("!PANIC: %v", r))
+		}
+	}()
+
 	var err error
 	if s.h.json {
 		err = appendJSONValue(s, v)
diff --git a/src/log/slog/logger_test.go b/src/log/slog/logger_test.go
index 2f5b31939c..559b9d66b4 100644
--- a/src/log/slog/logger_test.go
+++ b/src/log/slog/logger_test.go
@@ -12,6 +12,7 @@ import (
 	"io"
 	"log"
 	loginternal "log/internal"
+	"os"
 	"path/filepath"
 	"regexp"
 	"runtime"
@@ -83,6 +84,13 @@ func TestConnections(t *testing.T) {
 	Info("msg", "a", 1)
 	checkLogOutput(t, logbuf.String(), `logger_test.go:\d+: INFO msg a=1`)
 	logbuf.Reset()
+	Info("msg", "p", nil)
+	checkLogOutput(t, logbuf.String(), `logger_test.go:\d+: INFO msg p=<nil>`)
+	logbuf.Reset()
+	var r *regexp.Regexp
+	Info("msg", "r", r)
+	checkLogOutput(t, logbuf.String(), `logger_test.go:\d+: INFO msg r=<nil>`)
+	logbuf.Reset()
 	Warn("msg", "b", 2)
 	checkLogOutput(t, logbuf.String(), `logger_test.go:\d+: WARN msg b=2`)
 	logbuf.Reset()
@@ -571,3 +579,62 @@ func wantAllocs(t *testing.T, want int, f func()) {
 		t.Errorf("got %d allocs, want %d", got, want)
 	}
 }
+
+// panicTextAndJsonMarshaler is a type that panics in MarshalText and MarshalJSON.
+type panicTextAndJsonMarshaler struct {
+	msg any
+}
+
+func (p panicTextAndJsonMarshaler) MarshalText() ([]byte, error) {
+	panic(p.msg)
+}
+
+func (p panicTextAndJsonMarshaler) MarshalJSON() ([]byte, error) {
+	panic(p.msg)
+}
+
+func TestPanics(t *testing.T) {
+	// Revert any changes to the default logger. This is important because other
+	// tests might change the default logger using SetDefault. Also ensure we
+	// restore the default logger at the end of the test.
+	currentLogger := Default()
+	t.Cleanup(func() {
+		SetDefault(currentLogger)
+		log.SetOutput(os.Stderr)
+		log.SetFlags(log.LstdFlags)
+	})
+
+	var logBuf bytes.Buffer
+	log.SetOutput(&logBuf)
+	log.SetFlags(log.Lshortfile &^ log.LstdFlags)
+
+	SetDefault(New(newDefaultHandler(loginternal.DefaultOutput)))
+	for _, pt := range []struct {
+		in  any
+		out string
+	}{
+		{(*panicTextAndJsonMarshaler)(nil), `logger_test.go:\d+: INFO msg p=<nil>`},
+		{panicTextAndJsonMarshaler{io.ErrUnexpectedEOF}, `logger_test.go:\d+: INFO msg p="!PANIC: unexpected EOF"`},
+		{panicTextAndJsonMarshaler{"panicking"}, `logger_test.go:\d+: INFO msg p="!PANIC: panicking"`},
+		{panicTextAndJsonMarshaler{42}, `logger_test.go:\d+: INFO msg p="!PANIC: 42"`},
+	} {
+		Info("msg", "p", pt.in)
+		checkLogOutput(t, logBuf.String(), pt.out)
+		logBuf.Reset()
+	}
+
+	SetDefault(New(NewJSONHandler(&logBuf, nil)))
+	for _, pt := range []struct {
+		in  any
+		out string
+	}{
+		{(*panicTextAndJsonMarshaler)(nil), `{"time":".*?","level":"INFO","msg":"msg","p":null}`},
+		{panicTextAndJsonMarshaler{io.ErrUnexpectedEOF}, `{"time":".*?","level":"INFO","msg":"msg","p":"!PANIC: unexpected EOF"}`},
+		{panicTextAndJsonMarshaler{"panicking"}, `{"time":".*?","level":"INFO","msg":"msg","p":"!PANIC: panicking"}`},
+		{panicTextAndJsonMarshaler{42}, `{"time":".*?","level":"INFO","msg":"msg","p":"!PANIC: 42"}`},
+	} {
+		Info("msg", "p", pt.in)
+		checkLogOutput(t, logBuf.String(), pt.out)
+		logBuf.Reset()
+	}
+}