src/server/worker/api_server.go
func (a *APIServer) runUserErrorHandlingCode(ctx context.Context, logger *taggedLogger, environ []string, stats *pps.ProcessStats, rawDatumTimeout *types.Duration) (retErr error) {
	logger.Logf("beginning to run user error handling code")
	defer func(start time.Time) {
		if retErr != nil {
			logger.Logf("errored running user error handling code after %v: %v", time.Since(start), retErr)
		} else {
			logger.Logf("finished running user error handling code after %v", time.Since(start))
		}
	}(time.Now())

	cmd := exec.CommandContext(ctx, a.pipelineInfo.Transform.ErrCmd[0], a.pipelineInfo.Transform.ErrCmd[1:]...)
	if a.pipelineInfo.Transform.ErrStdin != nil {
		cmd.Stdin = strings.NewReader(strings.Join(a.pipelineInfo.Transform.ErrStdin, "\n") + "\n")
	}
	cmd.Stdout = logger.userLogger()
	cmd.Stderr = logger.userLogger()
	cmd.Env = environ
	if a.uid != nil && a.gid != nil {
		cmd.SysProcAttr = &syscall.SysProcAttr{
			Credential: &syscall.Credential{
				Uid: *a.uid,
				Gid: *a.gid,
			},
		}
	}
	cmd.Dir = a.pipelineInfo.Transform.WorkingDir
	err := cmd.Start()
	if err != nil {
		return fmt.Errorf("error cmd.Start: %v", err)
	}
	// A context w a deadline will successfully cancel/kill
	// the running process (minus zombies)
	state, err := cmd.Process.Wait()
	if err != nil {
		return fmt.Errorf("error cmd.Wait: %v", err)
	}
	if isDone(ctx) {
		if err = ctx.Err(); err != nil {
			return err
		}
	}
	// Because of this issue: https://github.com/golang/go/issues/18874
	// We forked os/exec so that we can call just the part of cmd.Wait() that
	// happens after blocking on the process. Unfortunately calling
	// cmd.Process.Wait() then cmd.Wait() will produce an error. So instead we
	// close the IO using this helper
	err = cmd.WaitIO(state, err)
	// We ignore broken pipe errors, these occur very occasionally if a user
	// specifies Stdin but their process doesn't actually read everything from
	// Stdin. This is a fairly common thing to do, bash by default ignores
	// broken pipe errors.
	if err != nil && !strings.Contains(err.Error(), "broken pipe") {
		// (if err is an acceptable return code, don't return err)
		if exiterr, ok := err.(*exec.ExitError); ok {
			if status, ok := exiterr.Sys().(syscall.WaitStatus); ok {
				for _, returnCode := range a.pipelineInfo.Transform.AcceptReturnCode {
					if int(returnCode) == status.ExitStatus() {
						return nil
					}
				}
			}
		}
		return fmt.Errorf("error cmd.WaitIO: %v", err)
	}
	return nil
}
func (a *APIServer) runUserErrorHandlingCode(ctx context.Context, logger *taggedLogger, environ []string, stats *pps.ProcessStats, rawDatumTimeout *types.Duration) (retErr error) {
	logger.Logf("beginning to run user error handling code")
	defer func(start time.Time) {
		if retErr != nil {
			logger.Logf("errored running user error handling code after %v: %v", time.Since(start), retErr)
		} else {
			logger.Logf("finished running user error handling code after %v", time.Since(start))
		}
	}(time.Now())

	cmd := exec.CommandContext(ctx, a.pipelineInfo.Transform.ErrCmd[0], a.pipelineInfo.Transform.ErrCmd[1:]...)
	if a.pipelineInfo.Transform.ErrStdin != nil {
		cmd.Stdin = strings.NewReader(strings.Join(a.pipelineInfo.Transform.ErrStdin, "\n") + "\n")
	}
	cmd.Stdout = logger.userLogger()
	cmd.Stderr = logger.userLogger()
	cmd.Env = environ
	if a.uid != nil && a.gid != nil {
		cmd.SysProcAttr = &syscall.SysProcAttr{
			Credential: &syscall.Credential{
				Uid: *a.uid,
				Gid: *a.gid,
			},
		}
	}
	cmd.Dir = a.pipelineInfo.Transform.WorkingDir
	err := cmd.Start()
	if err != nil {
		return fmt.Errorf("error cmd.Start: %v", err)
	}
	// A context w a deadline will successfully cancel/kill
	// the running process (minus zombies)
	state, err := cmd.Process.Wait()
	if err != nil {
		return fmt.Errorf("error cmd.Wait: %v", err)
	}
	if isDone(ctx) {
		if err = ctx.Err(); err != nil {
			return err
		}
	}
	// Because of this issue: https://github.com/golang/go/issues/18874
	// We forked os/exec so that we can call just the part of cmd.Wait() that
	// happens after blocking on the process. Unfortunately calling
	// cmd.Process.Wait() then cmd.Wait() will produce an error. So instead we
	// close the IO using this helper
	err = cmd.WaitIO(state, err)
	// We ignore broken pipe errors, these occur very occasionally if a user
	// specifies Stdin but their process doesn't actually read everything from
	// Stdin. This is a fairly common thing to do, bash by default ignores
	// broken pipe errors.
	if err != nil && !strings.Contains(err.Error(), "broken pipe") {
		// (if err is an acceptable return code, don't return err)
		if exiterr, ok := err.(*exec.ExitError); ok {
			if status, ok := exiterr.Sys().(syscall.WaitStatus); ok {
				for _, returnCode := range a.pipelineInfo.Transform.AcceptReturnCode {
					if int(returnCode) == status.ExitStatus() {
						return nil
					}
				}
			}
		}
		return fmt.Errorf("error cmd.WaitIO: %v", err)
	}
	return nil
}
// Run user error code and return the combined output of stdout and stderr.