Add ability to specify an offset to IOBuf::takeOwnership

Summary: Allow callers to `IOBuf::takeOwnership()` to specify valid data in the middle of the underlying buffer.

Reviewed By: yfeldblum

Differential Revision: D27583632

fbshipit-source-id: 4651dec5e53c5eea341b2b4ad3782adfbe7ff779

folly/io/IOBuf.cpp

@@ -360,13 +360,14 @@ IOBuf::IOBuf(
     TakeOwnershipOp,
     void* buf,
     std::size_t capacity,
+    std::size_t offset,
     std::size_t length,
     FreeFunction freeFn,
     void* userData,
     bool freeOnError)
     : next_(this),
       prev_(this),
-      data_(static_cast<uint8_t*>(buf)),
+      data_(static_cast<uint8_t*>(buf) + offset),
       buf_(static_cast<uint8_t*>(buf)),
       length_(length),
       capacity_(capacity),
@@ -386,6 +387,7 @@ IOBuf::IOBuf(
 unique_ptr<IOBuf> IOBuf::takeOwnership(
     void* buf,
     std::size_t capacity,
+    std::size_t offset,
     std::size_t length,
     FreeFunction freeFn,
     void* userData,
@@ -416,7 +418,7 @@ unique_ptr<IOBuf> IOBuf::takeOwnership(
       packFlagsAndSharedInfo(0, &storage->shared),
       static_cast<uint8_t*>(buf),
       capacity,
-      static_cast<uint8_t*>(buf),
+      static_cast<uint8_t*>(buf) + offset,
       length));
 
   rollback.dismiss();

folly/io/IOBuf.h

@@ -314,6 +314,9 @@ class IOBuf {
    * In the second version, the user specifies the valid length of data
    * in the buffer
    *
+   * In the third version, the user specifies the offset to the valid data
+   * and the length of valid data in the buffer.
+   *
    * On error, std::bad_alloc will be thrown.  If freeOnError is true (the
    * default) the buffer will be freed before throwing the error.
    */
@@ -324,7 +327,7 @@ class IOBuf {
       void* userData = nullptr,
       bool freeOnError = true) {
     return takeOwnership(
-        buf, capacity, capacity, freeFn, userData, freeOnError);
+        buf, capacity, 0, capacity, freeFn, userData, freeOnError);
   }
   IOBuf(
       TakeOwnershipOp op,
@@ -333,11 +336,32 @@ class IOBuf {
       FreeFunction freeFn = nullptr,
       void* userData = nullptr,
       bool freeOnError = true)
-      : IOBuf(op, buf, capacity, capacity, freeFn, userData, freeOnError) {}
+      : IOBuf(op, buf, capacity, 0, capacity, freeFn, userData, freeOnError) {}
+
+  static std::unique_ptr<IOBuf> takeOwnership(
+      void* buf,
+      std::size_t capacity,
+      std::size_t length,
+      FreeFunction freeFn = nullptr,
+      void* userData = nullptr,
+      bool freeOnError = true) {
+    return takeOwnership(
+        buf, capacity, 0, length, freeFn, userData, freeOnError);
+  }
+  IOBuf(
+      TakeOwnershipOp op,
+      void* buf,
+      std::size_t capacity,
+      std::size_t length,
+      FreeFunction freeFn = nullptr,
+      void* userData = nullptr,
+      bool freeOnError = true)
+      : IOBuf(op, buf, capacity, 0, length, freeFn, userData, freeOnError) {}
 
   static std::unique_ptr<IOBuf> takeOwnership(
       void* buf,
       std::size_t capacity,
+      std::size_t offset,
       std::size_t length,
       FreeFunction freeFn = nullptr,
       void* userData = nullptr,
@@ -346,6 +370,7 @@ class IOBuf {
       TakeOwnershipOp,
       void* buf,
       std::size_t capacity,
+      std::size_t offset,
       std::size_t length,
       FreeFunction freeFn = nullptr,
       void* userData = nullptr,

folly/io/test/IOBufTest.cpp

@@ -167,6 +167,21 @@ TEST(IOBuf, TakeOwnership) {
     fbstring str = iobuf->moveToFbString();
     EXPECT_EQ(str, "A");
   }
+
+  deleteCount = 0;
+  uint32_t size6 = 100;
+  uint8_t* buf6 = new uint8_t[size6];
+  uint32_t offset6 = 48;
+  uint32_t length6 = 48;
+  unique_ptr<IOBuf> iobuf6(IOBuf::takeOwnership(
+      buf6, size6, offset6, length6, deleteArrayBuffer, &deleteCount));
+  EXPECT_EQ(buf6 + offset6, iobuf6->data());
+  EXPECT_EQ(length6, iobuf6->length());
+  EXPECT_EQ(buf6, iobuf6->buffer());
+  EXPECT_EQ(size6, iobuf6->capacity());
+  EXPECT_EQ(0, deleteCount);
+  iobuf6.reset();
+  EXPECT_EQ(1, deleteCount);
 }
 
 TEST(IOBuf, GetUserData) {