Avoid race-conditions while executing sub-commands (#1201)

## Changes
`executor.Exec` now uses `cmd.CombinedOutput`. Previous implementation
was hanging on my windows VM during `bundle deploy` on the
`ReadAll(MultiReader(stdout, stderr))` line.

The problem is related to the fact the MultiReader reads sequentially,
and the `stdout` is the first in line. Even simple `io.ReadAll(stdout)`
hangs on me, as it seems like the command that we spawn (python wheel
build) waits for the error stream to be finished before closing stdout
on its own side? Reading `stderr` (or `out`) in a separate go-routine
fixes the deadlock, but `cmd.CombinedOutput` feels like a simpler
solution.

Also noticed that Exec was not removing `scriptFile` after itself, fixed
that too.

## Tests
Unit tests and manually

libs/exec/exec.go

@@ -90,18 +90,25 @@ func NewCommandExecutorWithExecutable(dir string, execType ExecutableType) (*Exe
 	}, nil
 }
 
-func (e *Executor) StartCommand(ctx context.Context, command string) (Command, error) {
+func (e *Executor) prepareCommand(ctx context.Context, command string) (*osexec.Cmd, *execContext, error) {
 	ec, err := e.shell.prepare(command)
+	if err != nil {
+		return nil, nil, err
+	}
+	cmd := osexec.CommandContext(ctx, ec.executable, ec.args...)
+	cmd.Dir = e.dir
+	return cmd, ec, nil
+}
+
+func (e *Executor) StartCommand(ctx context.Context, command string) (Command, error) {
+	cmd, ec, err := e.prepareCommand(ctx, command)
 	if err != nil {
 		return nil, err
 	}
-	return e.start(ctx, ec)
+	return e.start(ctx, cmd, ec)
 }
 
-func (e *Executor) start(ctx context.Context, ec *execContext) (Command, error) {
-	cmd := osexec.CommandContext(ctx, ec.executable, ec.args...)
-	cmd.Dir = e.dir
-
+func (e *Executor) start(ctx context.Context, cmd *osexec.Cmd, ec *execContext) (Command, error) {
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
 		return nil, err
@@ -116,17 +123,12 @@ func (e *Executor) start(ctx context.Context, ec *execContext) (Command, error)
 }
 
 func (e *Executor) Exec(ctx context.Context, command string) ([]byte, error) {
-	cmd, err := e.StartCommand(ctx, command)
+	cmd, ec, err := e.prepareCommand(ctx, command)
 	if err != nil {
 		return nil, err
 	}
-
-	res, err := io.ReadAll(io.MultiReader(cmd.Stdout(), cmd.Stderr()))
-	if err != nil {
-		return nil, err
-	}
-
-	return res, cmd.Wait()
+	defer os.Remove(ec.scriptFile)
+	return cmd.CombinedOutput()
 }
 
 func (e *Executor) ShellType() ExecutableType {

libs/exec/exec_test.go

@@ -32,6 +32,15 @@ func TestExecutorWithComplexInput(t *testing.T) {
 	assert.Equal(t, "Hello\nWorld\n", string(out))
 }
 
+func TestExecutorWithStderr(t *testing.T) {
+	executor, err := NewCommandExecutor(".")
+	assert.NoError(t, err)
+	out, err := executor.Exec(context.Background(), "echo 'Hello' && >&2 echo 'Error'")
+	assert.NoError(t, err)
+	assert.NotNil(t, out)
+	assert.Equal(t, "Hello\nError\n", string(out))
+}
+
 func TestExecutorWithInvalidCommand(t *testing.T) {
 	executor, err := NewCommandExecutor(".")
 	assert.NoError(t, err)
@@ -108,16 +117,16 @@ func TestExecutorCleanupsTempFiles(t *testing.T) {
 	executor, err := NewCommandExecutor(".")
 	assert.NoError(t, err)
 
-	ec, err := executor.shell.prepare("echo 'Hello'")
+	cmd, ec, err := executor.prepareCommand(context.Background(), "echo 'Hello'")
 	assert.NoError(t, err)
 
-	cmd, err := executor.start(context.Background(), ec)
+	command, err := executor.start(context.Background(), cmd, ec)
 	assert.NoError(t, err)
 
 	fileName := ec.args[1]
 	assert.FileExists(t, fileName)
 
-	err = cmd.Wait()
+	err = command.Wait()
 	assert.NoError(t, err)
 	assert.NoFileExists(t, fileName)
 }