move wait for block to fault injector

Summary:
We have some code in the SaplingBackingStore tests to poll the fault injector
until a fault is blocked. This is used to ensure that a fault is really blocked
before we continue. It prevents tests from being flakey.

I am going to need this in another test to prevent it from being flakey. So I
wanna move it to a shared place. Seems like putting it on the fault injector
directly makes sense.

This diff is moving code, no changes to the logic. Adding a test and some
changes in the next diffs.

Reviewed By: jdelliot

Differential Revision: D60931459

fbshipit-source-id: cfbb124edbb0645787a3c670fc0b8cc4c953dc9b

eden/common/utils/FaultInjector.cpp

@@ -7,15 +7,19 @@
 
 #include "eden/common/utils/FaultInjector.h"
 
+#include <chrono>
+#include <string_view>
+
 #include <folly/Overload.h>
 #include <folly/logging/xlog.h>
-#include <string_view>
 
 using folly::SemiFuture;
 using folly::Unit;
 
 namespace facebook::eden {
 
+using namespace std::chrono_literals;
+
 FaultInjector::Fault::Fault(
     std::string_view regex,
     FaultBehavior&& b,
@@ -365,4 +369,16 @@ std::vector<std::string> FaultInjector::getBlockedFaults(
   return results;
 }
 
+bool FaultInjector::waitUntilBlocked(
+    std::string_view keyClass,
+    std::chrono::milliseconds timeout) {
+  while (getBlockedFaults(keyClass).size() == 0 && timeout > 0s) {
+    timeout -= 1s;
+    /* sleep override */
+    sleep(1);
+  }
+
+  return getBlockedFaults(keyClass).size() != 0;
+}
+
 } // namespace facebook::eden

eden/common/utils/FaultInjector.h

@@ -234,6 +234,15 @@ class FaultInjector {
 
   std::vector<std::string> getBlockedFaults(std::string_view keyClass);
 
+  /**
+   * Blocks up to timeout milliseconds waiting for a client to be blocked on
+   * the specified fault (keyClass). Returns true if a client is blocked, and
+   * false if the timeout was exceeded before a client was blocked.
+   */
+  bool waitUntilBlocked(
+      std::string_view keyClass,
+      std::chrono::milliseconds timeout);
+
  private:
   struct Block {};
   struct Delay {