restic-from-command: abort snapshot on non-zero exit codes

internal/fs/fs_reader_command.go

@@ -1,6 +1,7 @@
 package fs
 
 import (
+	"github.com/restic/restic/internal/errors"
 	"io"
 	"os/exec"
 )
@@ -11,13 +12,43 @@ import (
 type ReadCloserCommand struct {
 	Cmd    *exec.Cmd
 	Stdout io.ReadCloser
+
+	bytesRead bool
 }
 
-func (fp *ReadCloserCommand) Read(p []byte) (n int, err error) {
-	return fp.Stdout.Read(p)
+// Read populate the array with data from the process stdout.
+func (fp *ReadCloserCommand) Read(p []byte) (int, error) {
+	// We may encounter two different error conditions here:
+	// - EOF with no bytes read: the program terminated prematurely, so we send
+	//   a fatal error to cancel the snapshot;
+	// - an error that is not EOF: something bad happened, we need to abort the
+	//   snapshot.
+	b, err := fp.Stdout.Read(p)
+	if b == 0 && errors.Is(err, io.EOF) && !fp.bytesRead {
+		// The command terminated with no output at all. Raise a fatal error.
+		return 0, errors.Fatalf("command terminated with no output")
+	} else if err != nil && !errors.Is(err, io.EOF) {
+		// The command terminated with an error that is not EOF. Raise a fatal
+		// error.
+		return 0, errors.Fatal(err.Error())
+	} else if b > 0 {
+		fp.bytesRead = true
+	}
+	return b, err
 }
 
 func (fp *ReadCloserCommand) Close() error {
 	// No need to close fp.Stdout as Wait() closes all pipes.
-	return fp.Cmd.Wait()
+	err := fp.Cmd.Wait()
+	if err != nil {
+		// If we have information about the exit code, let's use it in the
+		// error message. Otherwise, send the error message along.
+		// In any case, use a fatal error to abort the snapshot.
+		var err2 *exec.ExitError
+		if errors.As(err, &err2) {
+			return errors.Fatalf("command terminated with exit code %d", err2.ExitCode())
+		}
+		return errors.Fatal(err.Error())
+	}
+	return nil
 }