Summary:
Without this diff, this is what happens to compaction output file if it's a subclass of WritableFileWrapper:
- during compaction, all `PrepareWrite()` calls update `last_preallocated_block_` of the `WritableFileWrapper` itself, not of `target_`, since `PrepareWrite()` is not virtual,
- `PrepareWrite()` calls `Allocate()`, which is virtual; it does `fallocate()` on `target_`,
- after writing data, `target_->Close()` calls `GetPreallocationStatus()` of `target_`; it returns `last_preallocated_block_` of `target_`, which is zero because it was never touched before,
- `target_->Close()` doesn't call `ftruncate()`; file remains big.
This diff fixes it in a straightforward way, by making the methods virtual. `WritableFileWrapper` ends up having the useless fields `last_preallocated_block_` and `preallocation_block_size_`. I think ideally the preallocation logic should be outside `WritableFile`, the same way as `log_writer.h` and `file_reader_writer.h` moved some non-platform-specific logic out of Env, but that's probably not worth the effort now.
Test Plan: `make -j check`; I'm going to deploy it on our test tier and see if it fixes space reclamation problem there
Reviewers: yhchiang, sdong
Reviewed By: sdong
Subscribers: andrewkr, dhruba, sdong
Differential Revision: https://reviews.facebook.net/D54681
4e85b74790