RFC: [hg] backingstore: use IOBuf::TakeOwnershipOp to achieve zero-copy

Summary: Uses `folly::IOBuf`'s `TakeOwnership` instead of `CopyBuffer` to achieve zero-copy of file content we get from Rust.

Reviewed By: chadaustin

Differential Revision: D18264900

fbshipit-source-id: 45baa70cb771a2bff0b31feaa1b85569625218eb

eden/scm/lib/backingstore/c_api/HgNativeBackingStore.cpp

@@ -7,7 +7,6 @@
 
 #include "eden/scm/lib/backingstore/c_api/HgNativeBackingStore.h"
 
-#include <folly/Optional.h>
 #include <folly/Range.h>
 #include <folly/String.h>
 #include <folly/io/IOBuf.h>
@@ -17,6 +16,21 @@
 
 namespace facebook {
 namespace eden {
+namespace {
+/**
+ * Convert a `RustCBytes` into `folly::IOBuf` without copying the underlying
+ * data.
+ */
+std::unique_ptr<folly::IOBuf> bytesToIOBuf(RustCBytes* bytes) {
+  return folly::IOBuf::takeOwnership(
+      reinterpret_cast<void*>(bytes->ptr),
+      bytes->len,
+      [](void* /* buf */, void* userData) {
+        rust_cbytes_free(reinterpret_cast<RustCBytes*>(userData));
+      },
+      reinterpret_cast<void*>(bytes));
+}
+} // namespace
 
 HgNativeBackingStore::HgNativeBackingStore(folly::StringPiece repository) {
   RustCFallible<RustBackingStore> store(
@@ -30,7 +44,7 @@ HgNativeBackingStore::HgNativeBackingStore(folly::StringPiece repository) {
   store_ = store.unwrap();
 }
 
-folly::Optional<folly::IOBuf> HgNativeBackingStore::getBlob(
+std::unique_ptr<folly::IOBuf> HgNativeBackingStore::getBlob(
     folly::ByteRange name,
     folly::ByteRange node) {
   RustCFallible<RustCBytes> result(
@@ -42,13 +56,10 @@ folly::Optional<folly::IOBuf> HgNativeBackingStore::getBlob(
     XLOG(DBG5) << "Error while getting blob name=" << name.data()
                << " node=" << folly::hexlify(node)
                << " from backingstore: " << result.getError();
-    return folly::none;
+    return nullptr;
   }
-  auto buffer = result.get();
-  auto iobuf =
-      folly::IOBuf{folly::IOBuf::COPY_BUFFER, buffer->ptr, buffer->len};
 
-  return iobuf;
+  return bytesToIOBuf(result.unwrap().release());
 }
 
 std::shared_ptr<RustTree> HgNativeBackingStore::getTree(folly::ByteRange node) {

eden/scm/lib/backingstore/c_api/HgNativeBackingStore.h

@@ -13,8 +13,6 @@
 
 namespace folly {
 class IOBuf;
-template <typename T>
-class Optional;
 } // namespace folly
 
 namespace facebook {
@@ -24,7 +22,7 @@ class HgNativeBackingStore {
  public:
   explicit HgNativeBackingStore(folly::StringPiece repository);
 
-  folly::Optional<folly::IOBuf> getBlob(
+  std::unique_ptr<folly::IOBuf> getBlob(
       folly::ByteRange name,
       folly::ByteRange node);
 

eden/scm/lib/backingstore/c_api/RustBackingStore.h

@@ -7,7 +7,7 @@
  * This file is generated with cbindgen. Please run `./tools/cbindgen.sh` to
  * update this file.
  *
- * @generated SignedSource<<85582de8715286965cfc5e5472a8dab0>>
+ * @generated SignedSource<<d4d2d4d612a8215f35cd4ef138904bab>>
  *
  */
 
@@ -89,7 +89,7 @@ template<typename T>
 struct RustVec;
 
 struct RustCBytes {
-  const uint8_t *ptr;
+  uint8_t *ptr;
   size_t len;
   RustVec<uint8_t> *vec;
 folly::ByteRange asByteRange() const {
@@ -112,7 +112,7 @@ struct RustTreeEntry {
 struct RustTree {
   const RustTreeEntry *entries;
   /// This makes sure `entries` above is pointing to a valid memory.
-  RustVec<RustTreeEntry> *_entries;
+  RustVec<RustTreeEntry> *entries_ptr;
   uintptr_t length;
   RustCBytes hash;
 };

eden/scm/lib/backingstore/src/raw/cbytes.rs

@@ -13,15 +13,15 @@ use libc::size_t;
 
 #[repr(C)]
 pub struct CBytes {
-    ptr: *const u8,
+    ptr: *mut u8,
     len: size_t,
     vec: *mut Vec<u8>,
 }
 
 impl CBytes {
     pub fn from_vec(vec: Vec<u8>) -> Self {
-        let vec = Box::new(vec);
-        let ptr = vec.as_ptr();
+        let mut vec = Box::new(vec);
+        let ptr = vec.as_mut_ptr();
 
         Self {
             ptr,