Make lottie_convert work on platforms without /dev/stdout (#1424)

Fixes #1423.

bridge/helper/helper.go

@@ -248,35 +248,52 @@ func CanConvertTgsToX() error {
 // This relies on an external command, which is ugly, but works.
 func ConvertTgsToX(data *[]byte, outputFormat string, logger *logrus.Entry) error {
 	// lottie can't handle input from a pipe, so write to a temporary file:
-	tmpFile, err := ioutil.TempFile(os.TempDir(), "matterbridge-lottie-*.tgs")
+	tmpInFile, err := ioutil.TempFile(os.TempDir(), "matterbridge-lottie-input-*.tgs")
 	if err != nil {
 		return err
 	}
-	tmpFileName := tmpFile.Name()
+	tmpInFileName := tmpInFile.Name()
 	defer func() {
-		if removeErr := os.Remove(tmpFileName); removeErr != nil {
-			logger.Errorf("Could not delete temporary file %s: %v", tmpFileName, removeErr)
+		if removeErr := os.Remove(tmpInFileName); removeErr != nil {
+			logger.Errorf("Could not delete temporary (input) file %s: %v", tmpInFileName, removeErr)
+		}
+	}()
+	// lottie can handle writing to a pipe, but there is no way to do that platform-independently.
+	// "/dev/stdout" won't work on Windows, and "-" upsets Cairo for some reason. So we need another file:
+	tmpOutFile, err := ioutil.TempFile(os.TempDir(), "matterbridge-lottie-output-*.data")
+	if err != nil {
+		return err
+	}
+	tmpOutFileName := tmpOutFile.Name()
+	defer func() {
+		if removeErr := os.Remove(tmpOutFileName); removeErr != nil {
+			logger.Errorf("Could not delete temporary (output) file %s: %v", tmpOutFileName, removeErr)
 		}
 	}()
 
-	if _, writeErr := tmpFile.Write(*data); writeErr != nil {
+	if _, writeErr := tmpInFile.Write(*data); writeErr != nil {
 		return writeErr
 	}
 	// Must close before calling lottie to avoid data races:
-	if closeErr := tmpFile.Close(); closeErr != nil {
+	if closeErr := tmpInFile.Close(); closeErr != nil {
 		return closeErr
 	}
 
 	// Call lottie to transform:
-	cmd := exec.Command("lottie_convert.py", "--input-format", "lottie", "--output-format", outputFormat, tmpFileName, "/dev/stdout")
+	cmd := exec.Command("lottie_convert.py", "--input-format", "lottie", "--output-format", outputFormat, tmpInFileName, tmpOutFileName)
+	cmd.Stdout = nil
 	cmd.Stderr = nil
 	// NB: lottie writes progress into to stderr in all cases.
-	stdout, stderr := cmd.Output()
+	_, stderr := cmd.Output()
 	if stderr != nil {
 		// 'stderr' already contains some parts of Stderr, because it was set to 'nil'.
 		return stderr
 	}
+	dataContents, err := ioutil.ReadFile(tmpOutFileName)
+	if err != nil {
+		return err
+	}
 
-	*data = stdout
+	*data = dataContents
 	return nil
 }