add ldb unsafe_remove_sst_file subcommand (facebook#7335)

Summary:
This is adapted from facebook#6678 but takes a different approach, avoiding opening a read-write DB and avoiding the `DeleteFile()` API.

First, this PR refactors how options variables are initialized in `ldb` so it can be reused in a subcommand that doesn't open a DB:

- Separated remaining option initialization logic out of `OpenDB()`. The new `PrepareOptions()` function initializes the full options state.
- Fixed an old TODO about applying the subcommand CF option overrides to the proper `ColumnFamilyOptions` object.

Second, this PR adds the `ldb unsafe_remove_sst_file` subcommand. It uses the `VersionSet`-level APIs to remove the file with the specified number.

Pull Request resolved: facebook#7335

Test Plan: played with interactive python and this file removal command. Verified openability/correct results in case of multiple column families, multiple levels, etc.

Reviewed By: pdillinger

Differential Revision: D23454575

Pulled By: ajkr

fbshipit-source-id: 039b7a8cbfc42fd123dcb25821eef51d61148afe

HISTORY.md

@@ -14,6 +14,7 @@
 ### New Features
 * A new option `std::shared_ptr<FileChecksumGenFactory> file_checksum_gen_factory` is added to `BackupableDBOptions`. The default value for this option is `nullptr`. If this option is null, the default backup engine checksum function (crc32c) will be used for creating, verifying, or restoring backups. If it is not null and is set to the DB custom checksum factory, the custom checksum function used in DB will also be used for creating, verifying, or restoring backups, in addition to the default checksum function (crc32c). If it is not null and is set to a custom checksum factory different than the DB custom checksum factory (which may be null), BackupEngine will return `Status::InvalidArgument()`.
 * A new field `std::string requested_checksum_func_name` is added to `FileChecksumGenContext`, which enables the checksum factory to create generators for a suite of different functions.
+* Added a new subcommand, `ldb unsafe_remove_sst_file`, which removes a lost or corrupt SST file from a DB's metadata. This command involves data loss and must not be used on a live DB.
 
 ### Performance Improvements
 * Reduce thread number for multiple DB instances by re-using one global thread for statistics dumping and persisting.

include/rocksdb/utilities/ldb_cmd.h

@@ -84,7 +84,9 @@ class LDBCommand {
 
  bool ValidateCmdLineOptions();
 
- virtual Options PrepareOptionsForOpenDB();
+ virtual void PrepareOptions();
+
+ virtual void OverrideBaseOptions();
 
  virtual void SetDBOptions(Options options) { options_ = options; }