Revert "Revert "fix: Use SyscallConn for isTTY which is safe during f…

internal/entryhuman/entry.go

@@ -12,6 +12,7 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
+	"syscall"
 	"time"
 	"unicode"
 
@@ -224,9 +225,25 @@ func isTTY(w io.Writer) bool {
 	if w == forceColorWriter {
 		return true
 	}
-	f, ok := w.(interface {
-		Fd() uintptr
-	})
+	// SyscallConn is safe during file close.
+	if sc, ok := w.(interface {
+		SyscallConn() (syscall.RawConn, error)
+	}); ok {
+		conn, err := sc.SyscallConn()
+		if err != nil {
+			return false
+		}
+		var isTerm bool
+		err = conn.Control(func(fd uintptr) {
+			isTerm = term.IsTerminal(int(fd))
+		})
+		if err != nil {
+			return false
+		}
+		return isTerm
+	}
+	// Fallback to unsafe Fd.
+	f, ok := w.(interface{ Fd() uintptr })
 	return ok && term.IsTerminal(int(f.Fd()))
 }
 

internal/entryhuman/entry_test.go

@@ -178,6 +178,34 @@ func TestEntry(t *testing.T) {
 			assert.Equal(t, "entry matches", string(wantByt), gotBuf.String())
 		})
 	}
+
+	t.Run("isTTY during file close", func(t *testing.T) {
+		t.Parallel()
+
+		tmpdir := t.TempDir()
+		f, err := os.CreateTemp(tmpdir, "slog")
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer f.Close()
+
+		done := make(chan struct{}, 2)
+		go func() {
+			entryhuman.Fmt(new(bytes.Buffer), f, slog.SinkEntry{
+				Level: slog.LevelCritical,
+				Fields: slog.M(
+					slog.F("hey", "hi"),
+				),
+			})
+			done <- struct{}{}
+		}()
+		go func() {
+			_ = f.Close()
+			done <- struct{}{}
+		}()
+		<-done
+		<-done
+	})
 }
 
 func BenchmarkFmt(b *testing.B) {