IOBuf: Fix Bug in Cloning with Headroom and Tailroom

Summary:
If the source IOBuf is not chained, we short-circuit the cloning logic and
instead share the existing buffer. This works when we implicitly require the
clone to have the same headroom and tailroom requirements as the source
IOBuf. However, the API allows setting other values. We need to test that we
meet those requirements in order to use the shortcut. This diff adds those
checks.

Reviewed By: yfeldblum

Differential Revision: D18180574

fbshipit-source-id: 31bfaabd7663cf83faf919d8cb242df0a7a2969d

folly/io/IOBuf.cpp

@@ -611,7 +611,7 @@ IOBuf IOBuf::cloneCoalescedAsValue() const {
 IOBuf IOBuf::cloneCoalescedAsValueWithHeadroomTailroom(
     std::size_t newHeadroom,
     std::size_t newTailroom) const {
-  if (!isChained()) {
+  if (!isChained() && newHeadroom <= headroom() && newTailroom <= tailroom()) {
     return cloneOneAsValue();
   }
   // Coalesce into newBuf

folly/io/test/IOBufTest.cpp

@@ -1536,6 +1536,24 @@ TEST(IOBuf, CloneCoalescedChain) {
   EXPECT_EQ(b->computeChainDataLength(), c.length()); // Same length
   gen.seed(fillSeed);
   checkBuf(&c, gen); // Same contents
+
+  auto newHeadroom = b->headroom() + 10;
+  auto newTailroom = b->tailroom();
+  c = b->cloneCoalescedAsValueWithHeadroomTailroom(newHeadroom, newTailroom);
+  EXPECT_FALSE(c.isChained()); // Not chained
+  EXPECT_GE(c.headroom(), newHeadroom);
+  EXPECT_GE(c.tailroom(), newTailroom);
+  gen.seed(fillSeed);
+  checkBuf(&c, gen); // Same contents
+
+  newHeadroom = b->headroom();
+  newTailroom = b->tailroom() + 10;
+  c = b->cloneCoalescedAsValueWithHeadroomTailroom(newHeadroom, newTailroom);
+  EXPECT_FALSE(c.isChained()); // Not chained
+  EXPECT_GE(c.headroom(), newHeadroom);
+  EXPECT_GE(c.tailroom(), newTailroom);
+  gen.seed(fillSeed);
+  checkBuf(&c, gen); // Same contents
 }
 
 TEST(IOBuf, CloneCoalescedSingle) {
@@ -1553,6 +1571,24 @@ TEST(IOBuf, CloneCoalescedSingle) {
   EXPECT_EQ(b->capacity(), c->capacity());
   EXPECT_EQ(b->data(), c->data());
   EXPECT_EQ(b->length(), c->length());
+
+  auto newHeadroom = b->headroom() + 10;
+  auto newTailroom = b->tailroom();
+  c = b->cloneCoalescedWithHeadroomTailroom(newHeadroom, newTailroom);
+  EXPECT_FALSE(c->isChained()); // Not chained
+  EXPECT_EQ(
+      ByteRange(c->data(), c->length()), ByteRange(b->data(), b->length()));
+  EXPECT_GE(c->headroom(), newHeadroom);
+  EXPECT_GE(c->tailroom(), newTailroom);
+
+  newHeadroom = b->headroom();
+  newTailroom = b->tailroom() + 10;
+  c = b->cloneCoalescedWithHeadroomTailroom(newHeadroom, newTailroom);
+  EXPECT_FALSE(c->isChained()); // Not chained
+  EXPECT_EQ(
+      ByteRange(c->data(), c->length()), ByteRange(b->data(), b->length()));
+  EXPECT_GE(c->headroom(), newHeadroom);
+  EXPECT_GE(c->tailroom(), newTailroom);
 }
 
 TEST(IOBuf, fillIov) {