1 files changed, 43 insertions, 9 deletions
diff --git a/compiler_wrapper/rusage_flag.go b/compiler_wrapper/rusage_flag.go
index 2a3768c1..690308b2 100644
--- a/compiler_wrapper/rusage_flag.go
+++ b/compiler_wrapper/rusage_flag.go
@@ -18,6 +18,26 @@ func getRusageLogFilename(env env) string {
 	return value
 }
 
+func lockFileExclusive(fd uintptr) error {
+	maxTries := 100
+	for i := 0; i < maxTries; i++ {
+		const seekSet = 0
+		err := syscall.FcntlFlock(fd, syscall.F_SETLKW, &syscall.Flock_t{
+			Type:   syscall.F_WRLCK,
+			Whence: seekSet,
+			Start:  0,
+			Len:    0,
+		})
+		if err == nil {
+			return nil
+		}
+		if err != syscall.EINTR {
+			return fmt.Errorf("locking file: %v", err)
+		}
+	}
+	return fmt.Errorf("locking file failed after %d tries", maxTries)
+}
+
 func logRusage(env env, logFileName string, compilerCmd *command) (exitCode int, err error) {
 	rusageBefore := syscall.Rusage{}
 	if err := syscall.Getrusage(syscall.RUSAGE_CHILDREN, &rusageBefore); err != nil {
@@ -49,21 +69,35 @@ func logRusage(env env, logFileName string, compilerCmd *command) (exitCode int,
 	if err := os.MkdirAll(filepath.Dir(logFileName), 0777); err != nil {
 		return 0, wrapErrorwithSourceLocf(err, "error creating rusage log directory %s", logFileName)
 	}
+
+	timeUnit := float64(time.Second)
+	data := fmt.Sprintf("%.5f : %.5f : %.5f : %d : %s : %s\n",
+		float64(elapsedRealTime)/timeUnit, float64(elapsedUserTime)/timeUnit, float64(elapsedSysTime)/timeUnit,
+		maxMemUsed, absCompilerPath,
+		strings.Join(append([]string{filepath.Base(absCompilerPath)}, compilerCmd.Args...), " "))
+
 	// Note: using file mode 0666 so that a root-created log is writable by others.
 	logFile, err := os.OpenFile(logFileName, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0666)
 	if err != nil {
-		return 0, wrapErrorwithSourceLocf(err, "error creating rusage logfile %s", logFileName)
+		return 0, wrapErrorwithSourceLocf(err, "creating rusage logfile %s", logFileName)
 	}
-	timeUnit := float64(time.Second)
-	if _, err := fmt.Fprintf(logFile, "%.5f : %.5f : %.5f : %d : %s : %s\n",
-		float64(elapsedRealTime)/timeUnit, float64(elapsedUserTime)/timeUnit, float64(elapsedSysTime)/timeUnit,
-		maxMemUsed, absCompilerPath,
-		strings.Join(append([]string{filepath.Base(absCompilerPath)}, compilerCmd.Args...), " ")); err != nil {
+
+	// O_APPEND's atomicity guarantees are only for writes up to a certain size. If we don't
+	// lock the file, we might end up with corrupted records.
+	//
+	// Note that Close()'ing the file releases all associated locks.
+	if err := lockFileExclusive(logFile.Fd()); err != nil {
 		_ = logFile.Close()
-		return 0, wrapErrorwithSourceLocf(err, "error writing rusage logfile %s", logFileName)
+		return 0, wrapErrorwithSourceLocf(err, "locking rusage logfile %s: %v", logFileName, err)
+	}
+
+	_, err = logFile.WriteString(data)
+	closeErr := logFile.Close()
+	if err != nil {
+		return 0, wrapErrorwithSourceLocf(err, "writing to rusage logfile %s: %v", logFileName, err)
 	}
-	if err := logFile.Close(); err != nil {
-		return 0, wrapErrorwithSourceLocf(err, "error closing rusage logfile %s", logFileName)
+	if closeErr != nil {
+		return 0, wrapErrorwithSourceLocf(err, "closing rusage logfile %s: %v", logFileName, closeErr)
 	}
 
 	return exitCode, nil