Blob files were not reclaimed after deleting all the SST files about rocksdb

[jonahgao]

@jonahgao Would you be up for contributing your fix in the form of a PR?

Sure. I am adding a unit test for this case.

[jonahgao]

@ltamasi Should we discard all the blob files if none of them are linked to any SST file?
This means oldest_blob_file_with_linked_ssts == kInvalidBlobFileNumber.

Current：

rocksdb/db/version_builder.cc

Lines 1066 to 1074 in ed46981

	void SaveBlobFilesTo(VersionStorageInfo* vstorage) const {
	assert(vstorage);
	assert(base_vstorage_);
	vstorage->ReserveBlob(base_vstorage_->GetBlobFiles().size() +
	mutable_blob_file_metas_.size());
	const uint64_t oldest_blob_file_with_linked_ssts =
	GetMinOldestBlobFileNumber();

After：

 void SaveBlobFilesTo(VersionStorageInfo* vstorage) const {
    assert(vstorage);

    assert(base_vstorage_);
    vstorage->ReserveBlob(base_vstorage_->GetBlobFiles().size() +
                          mutable_blob_file_metas_.size());

    const uint64_t oldest_blob_file_with_linked_ssts =
        GetMinOldestBlobFileNumber();

    // If there are no blob files with linked SSTs, meaning that there are no
    // valid blob files
    if (oldest_blob_file_with_linked_ssts == kInvalidBlobFileNumber) {
      return;
    }
   ...
}

[ltamasi]

@jonahgao Thanks for reporting this issue! VersionBuilder is supposed to skip saving all blob files before the oldest referenced one but we're indeed missing some logic that would handle the case where all existing blob files become unreferenced.

[ltamasi]

@jonahgao Would you be up for contributing your fix in the form of a PR?