playground: prevent caching memory-related runtime errors

The existing implementation only prevents code caching if the code has
certain compile-time errors. Code with deterministic runtime errors, such as
out of memory errors, are being cached. This causes errors to be
returned in the Go Tour if a different user's run caused an error.
The cache had to be manually cleared in order to clean out the bad run.
This change prevents code that threw an "out of memory" or a "cannot
allocate memory" error from being cached.

Fixes golang/go#26926

Change-Id: Ib281f3076bc674e7c2f08bf9b5c4be36da22d28e
Reviewed-on: https://go-review.googlesource.com/130035
Reviewed-by: Andrew Bonventre <[email protected]>

sandbox.go

@@ -41,6 +41,9 @@ const (
 	progName = "prog.go"
 )
 
+// Runtime error strings that will not be cached if found in an Event message.
+var nonCachingRuntimeErrors = []string{"out of memory", "cannot allocate memory"}
+
 type request struct {
 	Body string
 }
@@ -87,6 +90,18 @@ func (s *server) commandHandler(cachePrefix string, cmdFunc func(*request) (*res
 				http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 				return
 			}
+			for _, el := range resp.Events {
+				if el.Kind != "stderr" {
+					continue
+				}
+				for _, e := range nonCachingRuntimeErrors {
+					if strings.Contains(el.Message, e) {
+						s.log.Errorf("cmdFunc runtime error: %q", el.Message)
+						http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+						return
+					}
+				}
+			}
 			if err := s.cache.Set(key, resp); err != nil {
 				s.log.Errorf("cache.Set(%q, resp): %v", key, err)
 			}

server_test.go

@@ -166,6 +166,15 @@ func TestCommandHandler(t *testing.T) {
 		if r.Body == "error" {
 			return &response{Errors: "errors"}, nil
 		}
+		if r.Body == "oom-error" {
+			// To throw an oom in a local playground instance, increase the server timeout
+			// to 20 seconds (within sandbox.go), spin up the Docker instance and run
+			// this code: https://play.golang.org/p/aaCv86m0P14.
+			return &response{Events: []Event{{"out of memory", "stderr", 0}}}, nil
+		}
+		if r.Body == "allocate-memory-error" {
+			return &response{Events: []Event{{"cannot allocate memory", "stderr", 0}}}, nil
+		}
 		resp := &response{Events: []Event{{r.Body, "stdout", 0}}}
 		return resp, nil
 	})
@@ -191,6 +200,10 @@ func TestCommandHandler(t *testing.T) {
 			[]byte(`{"Errors":"errors","Events":null}
 `),
 		},
+		{"Out of memory error in response body event message", http.MethodPost, http.StatusInternalServerError,
+			[]byte(`{"Body":"oom-error"}`), nil},
+		{"Cannot allocate memory error in response body event message", http.MethodPost, http.StatusInternalServerError,
+			[]byte(`{"Body":"allocate-memory-error"}`), nil},
 	}
 
 	for _, tc := range testCases {