Fix: share bufio.Scanner across ExecuteCommand calls

bufio.NewScanner wraps the stdout pipe in an internal 4096-byte
read-ahead buffer. Creating a new Scanner on each ExecuteCommand call
discards any bytes already buffered from the previous call, causing
the next command's BEGIN_MARKER to be lost and the goroutine to block
indefinitely waiting for output that was already consumed.

Fix by creating the Scanner once in StartShell and reusing it for the
lifetime of the shell process.

Closes #71

Author: mirkoboehm

pkg/shell/shell.go

@@ -30,6 +30,7 @@ type Shell struct {
 	cmd         *exec.Cmd
 	stdin       io.WriteCloser
 	stdout      io.ReadCloser
+	scanner     *bufio.Scanner
 	mergeStderr bool
 }
 
@@ -66,7 +67,7 @@ func StartShell(shell string, mergeStderr bool) (Shell, error) {
 	if err != nil {
 		return Shell{}, fmt.Errorf("Unable to start shell %s: %v", shell, err)
 	}
-	return Shell{cmd, stdin, stdout, mergeStderr}, nil
+	return Shell{cmd, stdin, stdout, bufio.NewScanner(stdout), mergeStderr}, nil
 }
 
 // commandResult holds the result of a command execution
@@ -117,9 +118,8 @@ func (shell *Shell) ExecuteCommand(ctx context.Context, command string, timeout
 		var stdout []string
 		var rc int
 		beginFound := false
-		scanner := bufio.NewScanner(shell.stdout)
-		for scanner.Scan() {
-			line := scanner.Text()
+		for shell.scanner.Scan() {
+			line := shell.scanner.Text()
 			if beginRx.MatchString(line) {
 				beginFound = true
 				continue