Fix cursor insert inconsistency

Summary:
The invariant after Cursor::insert is that the cursor points to the buffer
after the insert. That invariant was not followed in the branch where the
new buffer was just prepended. This change fixes the bug.

Test Plan:
Unit test modified, all tests run with
fbconfig -r folly && fbmake runtests_opt

Reviewed By: davejwatson@fb.com

FB internal diff: D1114749

folly/io/Cursor.h

 /*
- * Copyright 2013 Facebook, Inc.
+ * Copyright 2014 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -501,7 +501,7 @@ class RWCursor
     folly::IOBuf* nextBuf;
     if (this->offset_ == 0) {
       // Can just prepend
-      nextBuf = buf.get();
+      nextBuf = this->crtBuf_;
       this->crtBuf_->prependChain(std::move(buf));
     } else {
       std::unique_ptr<folly::IOBuf> remaining;

folly/io/test/IOBufCursorTest.cpp

 /*
- * Copyright 2013 Facebook, Inc.
+ * Copyright 2014 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -315,9 +315,11 @@ TEST(IOBuf, cloneAndInsert) {
     cursor.skip(1);
 
     cursor.insert(std::move(cloned));
-    EXPECT_EQ(6, iobuf1->countChainElements());
+    cursor.insert(folly::IOBuf::create(0));
+    EXPECT_EQ(7, iobuf1->countChainElements());
     EXPECT_EQ(14, iobuf1->computeChainDataLength());
-    // Check that nextBuf got set correctly
+    // Check that nextBuf got set correctly to the buffer with 1 byte left
+    EXPECT_EQ(1, cursor.peek().second);
     cursor.read<uint8_t>();
   }
 
@@ -331,7 +333,7 @@ TEST(IOBuf, cloneAndInsert) {
     cursor.skip(1);
 
     cursor.insert(std::move(cloned));
-    EXPECT_EQ(7, iobuf1->countChainElements());
+    EXPECT_EQ(8, iobuf1->countChainElements());
     EXPECT_EQ(15, iobuf1->computeChainDataLength());
     // Check that nextBuf got set correctly
     cursor.read<uint8_t>();
@@ -344,7 +346,7 @@ TEST(IOBuf, cloneAndInsert) {
     EXPECT_EQ(1, cloned->computeChainDataLength());
 
     cursor.insert(std::move(cloned));
-    EXPECT_EQ(8, iobuf1->countChainElements());
+    EXPECT_EQ(9, iobuf1->countChainElements());
     EXPECT_EQ(16, iobuf1->computeChainDataLength());
     // Check that nextBuf got set correctly
     cursor.read<uint8_t>();