cmd/syncthing: Implement log rotation (fixes #6104) (#6198)

Since we've taken upon ourselves to create a log file by default on
Windows, this adds proper management of that log file. There are two new
options:

  -log-max-old-files="3"    Number of old files to keep (zero to keep only current).
  -log-max-size="10485760"  Maximum size of any file (zero to disable log rotation).

The default values result in four files (syncthing.log, synchting.0.log,
..., syncthing.3.log) each up to 10 MiB in size. To not use log rotation
at all, the user can say --log-max-size=0.

cmd/syncthing/monitor_test.go

@@ -7,6 +7,7 @@
 package main
 
 import (
+	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -14,6 +15,123 @@ import (
 	"time"
 )
 
+func TestRotatedFile(t *testing.T) {
+	// Verify that log rotation happens.
+
+	dir, err := ioutil.TempDir("", "syncthing")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	open := func(name string) (io.WriteCloser, error) {
+		return os.Create(name)
+	}
+
+	logName := filepath.Join(dir, "log.txt")
+	testData := []byte("12345678\n")
+	maxSize := int64(len(testData) + len(testData)/2)
+
+	// We allow the log file plus two rotated copies.
+	rf := newRotatedFile(logName, open, maxSize, 2)
+
+	// Write some bytes.
+	if _, err := rf.Write(testData); err != nil {
+		t.Fatal(err)
+	}
+	// They should be in the log.
+	checkSize(t, logName, len(testData))
+	checkNotExist(t, logName+".0")
+
+	// Write some more bytes. We should rotate and write into a new file as the
+	// new bytes don't fit.
+	if _, err := rf.Write(testData); err != nil {
+		t.Fatal(err)
+	}
+	checkSize(t, logName, len(testData))
+	checkSize(t, numberedFile(logName, 0), len(testData))
+	checkNotExist(t, logName+".1")
+
+	// Write another byte. That should fit without causing an extra rotate.
+	_, _ = rf.Write([]byte{42})
+	checkSize(t, logName, len(testData)+1)
+	checkSize(t, numberedFile(logName, 0), len(testData))
+	checkNotExist(t, numberedFile(logName, 1))
+
+	// Write some more bytes. We should rotate and write into a new file as the
+	// new bytes don't fit.
+	if _, err := rf.Write(testData); err != nil {
+		t.Fatal(err)
+	}
+	checkSize(t, logName, len(testData))
+	checkSize(t, numberedFile(logName, 0), len(testData)+1) // the one we wrote extra to, now rotated
+	checkSize(t, numberedFile(logName, 1), len(testData))
+	checkNotExist(t, numberedFile(logName, 2))
+
+	// Write some more bytes. We should rotate and write into a new file as the
+	// new bytes don't fit.
+	if _, err := rf.Write(testData); err != nil {
+		t.Fatal(err)
+	}
+	checkSize(t, logName, len(testData))
+	checkSize(t, numberedFile(logName, 0), len(testData))
+	checkSize(t, numberedFile(logName, 1), len(testData)+1)
+	checkNotExist(t, numberedFile(logName, 2)) // exceeds maxFiles so deleted
+}
+
+func TestNumberedFile(t *testing.T) {
+	// Mostly just illustrates where the number ends up and makes sure it
+	// doesn't crash without an extension.
+
+	cases := []struct {
+		in  string
+		num int
+		out string
+	}{
+		{
+			in:  "syncthing.log",
+			num: 42,
+			out: "syncthing.42.log",
+		},
+		{
+			in:  filepath.Join("asdfasdf", "syncthing.log.txt"),
+			num: 42,
+			out: filepath.Join("asdfasdf", "syncthing.log.42.txt"),
+		},
+		{
+			in:  "syncthing-log",
+			num: 42,
+			out: "syncthing-log.42",
+		},
+	}
+
+	for _, tc := range cases {
+		res := numberedFile(tc.in, tc.num)
+		if res != tc.out {
+			t.Errorf("numberedFile(%q, %d) => %q, expected %q", tc.in, tc.num, res, tc.out)
+		}
+	}
+}
+
+func checkSize(t *testing.T, name string, size int) {
+	t.Helper()
+	info, err := os.Lstat(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if info.Size() != int64(size) {
+		t.Errorf("%s wrong size: %d != expected %d", name, info.Size(), size)
+	}
+}
+
+func checkNotExist(t *testing.T, name string) {
+	t.Helper()
+	_, err := os.Lstat(name)
+	if !os.IsNotExist(err) {
+		t.Errorf("%s should not exist", name)
+	}
+}
+
 func TestAutoClosedFile(t *testing.T) {
 	os.RemoveAll("_autoclose")
 	defer os.RemoveAll("_autoclose")