sandbox: timeout runsc commands

The current mechanism for forcing a process to die ater a timeout is not
sufficient. This change fixes issues that were causing processes to run
forever on the sandbox.

- Gracefully terminate processes before we kill them inside of our
gVisor process. This helps capture valuable debug output for the user.

- Return a friendlier error when our run context times out on the
playground.

- Add a test that timeouts are handled gracefully.

- Reduce concurrent goroutines in our sandbox run handler by replacing
goroutine copy functions with a custom writer (limitedWriter) that
returns an error if too much output is returned, halting the program.

- Custom writers (limitedWriter, switchWriter) also fix timing errors
when calling Wait() too soon on a Command, before we have read all of
the data. It also fixes a different error from trying to read data after
a program has terminated.

- Remove goroutine from startContainer, and use a ticker + context
timeout for synchronization.

Updates golang/go#25224
Updates golang/go#38343

Change-Id: Ie9d65220e5c4f39272ea70b45c4b472bcd7069bb
Reviewed-on: https://go-review.googlesource.com/c/playground/+/227652
Run-TryBot: Alexander Rakoczy <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: toothrot

Makefile

@@ -16,7 +16,7 @@ runlocal:
 
 test_go:
 	# Run fast tests first: (and tests whether, say, things compile)
-	GO111MODULE=on go test -v
+	GO111MODULE=on go test -v ./...
 
 test_gvisor: docker
 	docker kill sandbox_front_test || true

sandbox.go

@@ -526,6 +526,10 @@ func sandboxRun(ctx context.Context, exePath string, testParam string) (sandboxt
 	sreq.GetBody = func() (io.ReadCloser, error) { return ioutil.NopCloser(bytes.NewReader(exeBytes)), nil }
 	res, err := sandboxBackendClient().Do(sreq)
 	if err != nil {
+		if ctx.Err() == context.DeadlineExceeded {
+			execRes.Error = "timeout running program"
+			return execRes, nil
+		}
 		return execRes, fmt.Errorf("POST %q: %w", sandboxBackendURL(), err)
 	}
 	defer res.Body.Close()