fix(windows-agent): Truncate log files (#672)

This PR renames any existing log to `log.old` when the program starts,
overwriting the file if it already exists. It also tests that if a log
exists a log.old is successfully created.

---

UDENG-2355

windows-agent/cmd/ubuntu-pro-agent/main.go

@@ -3,6 +3,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -66,6 +67,13 @@ func setLoggerOutput(a app) (func(), error) {
 
 	logFile := filepath.Join(publicDir, "log")
 
+	// Move old log file
+	oldLogFile := filepath.Join(publicDir, "log.old")
+	err = os.Rename(logFile, oldLogFile)
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
+		log.Warnf("Could not archive previous log file: %v", err)
+	}
+
 	f, err := os.OpenFile(logFile, os.O_APPEND|os.O_CREATE, 0600)
 	if err != nil {
 		return nil, fmt.Errorf("could not open log file: %v", err)

windows-agent/cmd/ubuntu-pro-agent/main_linux_test.go

@@ -26,7 +26,8 @@ func TestRunSignal(t *testing.T) {
 			// Signal handlers tests: can’t be parallel
 
 			a := myApp{
-				done: make(chan struct{}),
+				done:   make(chan struct{}),
+				tmpDir: t.TempDir(),
 			}
 
 			var rc int

windows-agent/cmd/ubuntu-pro-agent/main_test.go

@@ -2,6 +2,8 @@ package main
 
 import (
 	"errors"
+	"os"
+	"path/filepath"
 	"testing"
 	"time"
 
@@ -42,16 +44,28 @@ func (a *myApp) PublicDir() (string, error) {
 func TestRun(t *testing.T) {
 	t.Parallel()
 
+	fooContent := "foo"
+	emptyContent := ""
+
 	tests := map[string]struct {
+		existingLogContent string
+
 		runError         bool
 		usageErrorReturn bool
 		logDirError      bool
 
-		wantReturnCode int
+		wantReturnCode        int
+		wantOldLogFileContent *string
 	}{
 		"Run and exit successfully":                                {},
 		"Run and exit successfully despite logs not being written": {logDirError: true},
 
+		// Log file handling
+		"Existing log file has been renamed to old":       {existingLogContent: "foo", wantOldLogFileContent: &fooContent},
+		"Existing empty log file has been renamed to old": {existingLogContent: "-", wantOldLogFileContent: &emptyContent},
+		"Ignore when failing to archive log file":         {existingLogContent: "OLD_IS_DIRECTORY", wantReturnCode: 0},
+
+		// Error cases
 		"Run and return error":                   {runError: true, wantReturnCode: 1},
 		"Run and return usage error":             {usageErrorReturn: true, runError: true, wantReturnCode: 2},
 		"Run and usage error only does not fail": {usageErrorReturn: true, runError: false, wantReturnCode: 0},
@@ -71,6 +85,27 @@ func TestRun(t *testing.T) {
 				a.tmpDir = "PUBLIC_DIR_ERROR"
 			}
 
+			var logFile, oldLogFile string
+			publicDir, err := a.PublicDir()
+			if err == nil {
+				logFile = filepath.Join(publicDir, "log")
+				oldLogFile = logFile + ".old"
+				switch tc.existingLogContent {
+				case "":
+				case "OLD_IS_DIRECTORY":
+					err := os.Mkdir(oldLogFile, 0700)
+					require.NoError(t, err, "Setup: create invalid log.old file")
+					err = os.WriteFile(logFile, []byte("Old log content"), 0600)
+					require.NoError(t, err, "Setup: creating pre-existing log file")
+				case "-":
+					tc.existingLogContent = ""
+					fallthrough
+				default:
+					err := os.WriteFile(logFile, []byte(tc.existingLogContent), 0600)
+					require.NoError(t, err, "Setup: creating pre-existing log file")
+				}
+			}
+
 			var rc int
 			wait := make(chan struct{})
 			go func() {
@@ -84,6 +119,19 @@ func TestRun(t *testing.T) {
 			<-wait
 
 			require.Equal(t, tc.wantReturnCode, rc, "Return expected code")
+
+			// Don't check for log files if the directory was not writable
+			if logFile == "" {
+				return
+			}
+			if tc.wantOldLogFileContent != nil {
+				require.FileExists(t, oldLogFile, "Old log file should exist")
+				content, err := os.ReadFile(oldLogFile)
+				require.NoError(t, err, "Should be able to read old log file")
+				require.Equal(t, tc.existingLogContent, string(content), "Old log file content should be log's content")
+			} else {
+				require.NoFileExists(t, oldLogFile, "Old log file should not exist")
+			}
 		})
 	}
 }