log: fix wrong source file:line in JSON logs from release builds

JSON logs reported "source":"slog/logger.go:256" instead of the real
caller. getCaller skips logging-machinery frames by file path, but
release builds use -trimpath which rewrites the standard library slog
frame's path to "log/slog/logger.go" - matching neither the "/log/" nor
the "log.go" check, so it was reported as the source. Also skip frames
whose function belongs to the log/slog package, which is immune to
-trimpath.

fs/log/slog.go

@@ -79,6 +79,19 @@ func mapLogLevelNames(groups []string, a slog.Attr) slog.Attr {
 	return a
 }
 
+// isLogFrame reports whether frame belongs to the logging machinery
+// and so should be skipped when finding the real caller.
+//
+// rclone's own log packages are matched by file path, but the standard
+// library log/slog package is matched by function name: under -trimpath
+// its path is rewritten to "log/slog/logger.go", which neither file
+// check catches.
+func isLogFrame(frame runtime.Frame) bool {
+	file := frame.File
+	return strings.HasPrefix(frame.Function, "log/slog.") ||
+		strings.Contains(file, "/log/") || strings.HasSuffix(file, "log.go")
+}
+
 // get the file and line number of the caller skipping skip levels
 func getCaller(skip int) string {
 	var pc [64]uintptr
@@ -92,10 +105,10 @@ func getCaller(skip int) string {
 	for more {
 		frame, more = frames.Next()
 
-		file := frame.File
-		if strings.Contains(file, "/log/") || strings.HasSuffix(file, "log.go") {
+		if isLogFrame(frame) {
 			continue
 		}
+		file := frame.File
 		line := frame.Line
 
 		// shorten file name

fs/log/slog_test.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"os"
 	"regexp"
+	"runtime"
 	"strings"
 	"sync"
 	"testing"
@@ -71,6 +72,33 @@ func TestGetCaller(t *testing.T) {
 	// assert.Equal(t, "slog_test.go", match[1])
 }
 
+// Test isLogFrame skips logging-machinery frames, including the
+// standard library log/slog frames whose path is trimmed by -trimpath.
+func TestIsLogFrame(t *testing.T) {
+	tests := []struct {
+		name string
+		file string
+		fn   string
+		want bool
+	}{
+		// rclone's own log packages
+		{"fsLog", "github.com/rclone/rclone/fs/log.go", "github.com/rclone/rclone/fs.Debugf", true},
+		{"fsLogSlog", "github.com/rclone/rclone/fs/log/slog.go", "github.com/rclone/rclone/fs/log.(*OutputHandler).Handle", true},
+		// standard library slog with -trimpath: path is "log/slog/logger.go"
+		{"slogTrimpath", "log/slog/logger.go", "log/slog.(*Logger).log", true},
+		// standard library slog without -trimpath: full path contains "/log/"
+		{"slogFullPath", "/usr/local/go/src/log/slog/logger.go", "log/slog.(*Logger).Log", true},
+		// a real caller must not be skipped
+		{"realCaller", "github.com/rclone/rclone/backend/drive/drive.go", "github.com/rclone/rclone/backend/drive.(*Fs).newObjectWithInfo", false},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := isLogFrame(runtime.Frame{File: tc.file, Function: tc.fn})
+			assert.Equal(t, tc.want, got)
+		})
+	}
+}
+
 // Test formatStdLogHeader for various flag combinations.
 func TestFormatStdLogHeader(t *testing.T) {
 	cases := []struct {