Update test implementations for MultiRead with fs_scratch reuse (#13195)

Summary:
This is a follow up to #13189. As mentioned in the description in the previous PR, to guard against similar bugs in the future, we should update our test implementations to reflect the real-world assumptions that we can make about `fs_scratch` when we issue reads with the filesystem buffer reuse optimization. The current test implementations reinforce the misconception that `fs_scratch` points to the same place as `result.data()` (i.e. to the start of the valid data buffer for the read result). `fs_scratch` can point to any arbitrary data structure, but for our purposes, I think we achieve what we want if we just have it point to a `Slice` which wraps the underlying result buffer inside one of its class variables.

Pull Request resolved: #13195

Test Plan: Existing unit tests test the same functionality but in an improved way with this change.

Reviewed By: hx235

Differential Revision: D66896380

Pulled By: archang19

fbshipit-source-id: 377e67ec70427716f2b7b7388d99b78003c01eb0

cache/tiered_secondary_cache_test.cc

@@ -951,12 +951,20 @@ TEST_F(DBTieredSecondaryCacheTest, FSBufferTest) {
                            IODebugContext* dbg) override {
           for (size_t i = 0; i < num_reqs; ++i) {
             FSReadRequest& req = reqs[i];
-            FSAllocationPtr buffer(new char[req.len], [](void* ptr) {
-              delete[] static_cast<char*>(ptr);
-            });
-            req.fs_scratch = std::move(buffer);
+            // See https://github.com/facebook/rocksdb/pull/13195 for why we
+            // want to set up our test implementation for FSAllocationPtr this
+            // way.
+            char* internalData = new char[req.len];
             req.status = Read(req.offset, req.len, options, &req.result,
-                              static_cast<char*>(req.fs_scratch.get()), dbg);
+                              internalData, dbg);
+
+            Slice* internalSlice = new Slice(internalData, req.len);
+            FSAllocationPtr internalPtr(internalSlice, [](void* ptr) {
+              delete[] static_cast<const char*>(
+                  static_cast<Slice*>(ptr)->data_);
+              delete static_cast<Slice*>(ptr);
+            });
+            req.fs_scratch = std::move(internalPtr);
           }
           return IOStatus::OK();
         }

db/db_io_failure_test.cc

@@ -97,12 +97,20 @@ class CorruptionFS : public FileSystemWrapper {
         for (size_t i = 0; i < num_reqs; ++i) {
           FSReadRequest& req = reqs[i];
           if (fs_.fs_buffer_) {
-            FSAllocationPtr buffer(new char[req.len], [](void* ptr) {
-              delete[] static_cast<char*>(ptr);
-            });
-            req.fs_scratch = std::move(buffer);
+            // See https://github.com/facebook/rocksdb/pull/13195 for why we
+            // want to set up our test implementation for FSAllocationPtr this
+            // way.
+            char* internalData = new char[req.len];
             req.status = Read(req.offset, req.len, options, &req.result,
-                              static_cast<char*>(req.fs_scratch.get()), dbg);
+                              internalData, dbg);
+
+            Slice* internalSlice = new Slice(internalData, req.len);
+            FSAllocationPtr internalPtr(internalSlice, [](void* ptr) {
+              delete[] static_cast<const char*>(
+                  static_cast<Slice*>(ptr)->data_);
+              delete static_cast<Slice*>(ptr);
+            });
+            req.fs_scratch = std::move(internalPtr);
           } else {
             req.status = Read(req.offset, req.len, options, &req.result,
                               req.scratch, dbg);

file/prefetch_test.cc

@@ -3316,12 +3316,24 @@ class FSBufferPrefetchTest : public testing::Test,
                            IODebugContext* dbg) override {
           for (size_t i = 0; i < num_reqs; ++i) {
             FSReadRequest& req = reqs[i];
-            FSAllocationPtr buffer(new char[req.len], [](void* ptr) {
-              delete[] static_cast<char*>(ptr);
-            });
-            req.fs_scratch = std::move(buffer);
+
+            // We cannot assume that fs_scratch points to the start of
+            // the read data. We can have the FSAllocationPtr point to a
+            // wrapper around the result buffer in our test implementation so
+            // that we can catch whenever we incorrectly make this assumption.
+            // See https://github.com/facebook/rocksdb/pull/13189 for more
+            // context.
+            char* internalData = new char[req.len];
             req.status = Read(req.offset, req.len, options, &req.result,
-                              static_cast<char*>(req.fs_scratch.get()), dbg);
+                              internalData, dbg);
+
+            Slice* internalSlice = new Slice(internalData, req.len);
+            FSAllocationPtr internalPtr(internalSlice, [](void* ptr) {
+              delete[] static_cast<const char*>(
+                  static_cast<Slice*>(ptr)->data_);
+              delete static_cast<Slice*>(ptr);
+            });
+            req.fs_scratch = std::move(internalPtr);
           }
           return IOStatus::OK();
         }