Ensure logs don't indefinitely append past capacity

Summary:
^

There could be cases, albeit unlikely, that logs could be appended for the current state indefintely that would ultimate fail due to not having enough memory.

This change puts a cap on that.

Reviewed By: mweststrate

Differential Revision: D42313904

fbshipit-source-id: 7fd96be822c9427720bccb41c6c32a39213c7652

xplat/Flipper/FlipperState.cpp

@@ -10,6 +10,8 @@
 #include "FlipperStateUpdateListener.h"
 #include "FlipperStep.h"
 
+#define FLIPPER_LOGS_CAPACITY 4096
+
 #if FLIPPER_DEBUG_LOG
 #include "Log.h"
 #endif
@@ -19,7 +21,8 @@ using namespace facebook::flipper;
 /* Class responsible for collecting state updates and combining them into a
  * view of the current state of the flipper client. */
 
-FlipperState::FlipperState() : logs("") {}
+FlipperState::FlipperState() {}
+
 void FlipperState::setUpdateListener(
     std::shared_ptr<FlipperStateUpdateListener> listener) {
   std::lock_guard<std::mutex> lock(mutex);
@@ -46,14 +49,25 @@ void FlipperState::started(std::string step) {
   }
 }
 
+void FlipperState::ensureLogsCapacity() {
+  if (logs.tellp() > FLIPPER_LOGS_CAPACITY) {
+    logs.str("");
+    logs.clear();
+    logs << "[Truncated]" << std::endl;
+  }
+}
+
 void FlipperState::success(std::string step) {
   std::shared_ptr<FlipperStateUpdateListener> localListener;
   {
     std::lock_guard<std::mutex> lock(mutex);
+    std::string message = "[Success] " + step;
 #if FLIPPER_DEBUG_LOG
-    log("[finished] " + step);
+    log(message);
 #endif
-    logs = logs + "[Success] " + step + "\n";
+    ensureLogsCapacity();
+    logs << message << std::endl;
+
     stateMap[step] = State::success;
     localListener = mListener;
   }
@@ -72,7 +86,8 @@ void FlipperState::failed(std::string step, std::string errorMessage) {
 #if FLIPPER_DEBUG_LOG
     log(message);
 #endif
-    logs = logs + message + "\n";
+    ensureLogsCapacity();
+    logs << message << std::endl;
     stateMap[step] = State::failed;
     localListener = mListener;
   }
@@ -88,7 +103,7 @@ void FlipperState::failed(std::string step, std::string errorMessage) {
 // way
 std::string FlipperState::getState() {
   std::lock_guard<std::mutex> lock(mutex);
-  return logs;
+  return logs.str();
 }
 
 std::vector<StateElement> FlipperState::getStateElements() {