Revert D116335797

Summary:
This diff reverts D116335797 ("[folly] fix integer overflow in `IOBuf::coalesceAndReallocate`"). `IOBuf::coalesceAndReallocate` goes back to computing `newCapacity = newLength + newHeadroom + newTailroom` with unchecked `size_t` addition, and the `TEST(IOBuf, CoalesceCapacityOverflow)` regression test is removed.

S698837

Reviewed By: ericcfu

Differential Revision: D116862638

fbshipit-source-id: 0723175e22b0b6b1ce424469175bae5b180bbe32

Author: Nick Sum

third-party/folly/src/folly/io/IOBuf.cpp

@@ -1044,11 +1044,7 @@ void IOBuf::coalesceSlow(size_t maxLength) {
 
 void IOBuf::coalesceAndReallocate(
     size_t newHeadroom, size_t newLength, IOBuf* end, size_t newTailroom) {
-  std::size_t newCapacity = 0;
-  if (!checked_add(&newCapacity, newLength, newHeadroom, newTailroom) ||
-      newCapacity > kMaxIOBufSize) {
-    throw_exception<std::bad_alloc>();
-  }
+  std::size_t newCapacity = newLength + newHeadroom + newTailroom;
 
   // Allocate space for the coalesced buffer.
   // We always convert to an external buffer, even if we happened to be an

third-party/folly/src/folly/io/test/IOBufTest.cpp

@@ -1576,19 +1576,6 @@ TEST(IOBuf, CoalesceEmptyBuffers) {
   EXPECT_TRUE(ByteRange(StringPiece("hello")) == br);
 }
 
-// A chain whose coalesced capacity (length + newHeadroom + newTailroom)
-// overflows size_t must throw std::bad_alloc rather than wrapping around to a
-// small capacity, allocating an undersized buffer, and overflowing it while
-// copying the data.
-TEST(IOBuf, CoalesceCapacityOverflow) {
-  auto head = fromStr("hello");
-  head->insertAfterThisOne(fromStr("world"));
-  ASSERT_TRUE(head->isChained());
-
-  constexpr auto kMax = std::numeric_limits<std::size_t>::max();
-  EXPECT_THROW(head->coalesceWithHeadroomTailroom(kMax, kMax), std::bad_alloc);
-}
-
 // cloneCoalescedAsValueWithHeadroomTailroom (and the unique_ptr wrapper that
 // forwards to it) must throw std::bad_alloc when the coalesced capacity
 // overflows size_t rather than wrapping around to an undersized buffer.