Guard is broken

Summary: Adding note and disabled test for now, will come back to fixing or killing this later.

Reviewed By: yfeldblum

Differential Revision: D14625209

fbshipit-source-id: 6e4f523c02e28fa6080da16485cba68ee78ffc80

folly/gen/Base-inl.h

@@ -1748,7 +1748,9 @@ class RangeConcat : public Operator<RangeConcat> {
  *    | eachTo<int>()
  *    | as<vector>();
  *
- *  TODO(tjackson): Rename this back to Guard.
+ *  KNOWN ISSUE: This only guards pipelines through operators which do not
+ *  retain resulting values. Exceptions thrown after operators like pmap, order,
+ *  batch, cannot be caught from here.
  **/
 template <class Exception, class ErrorHandler>
 class GuardImpl : public Operator<GuardImpl<Exception, ErrorHandler>> {

folly/gen/test/BaseTest.cpp

@@ -1201,6 +1201,22 @@ TEST(Gen, Guard) {
   // clang-format on
 }
 
+// Disabled: guard currently can't catch exceptions thrown after a buffering op.
+TEST(Gen, DISABLED_GuardThroughBuffers) {
+  using std::runtime_error;
+  // clang-format off
+  EXPECT_EQ(
+      4,
+      (from({"1", "a", "3"})
+         | guard<runtime_error>([](runtime_error&, const char*) {
+             return true;
+           })
+         | batch(1)
+         | rconcat
+         | eachTo<int>()
+         | sum));
+  // clang-format on
+}
 TEST(Gen, eachTryTo) {
   using std::runtime_error;
   // clang-format off