Add a FS flag to detect and correct corruption (#12408)

Summary:
Add a flag in `IOOptions` to request the file system to make best efforts to detect data corruption and reconstruct the data if possible. This will be used by RocksDB to retry a read if the previous read returns corrupt data (checksum mismatch). Add a new op to `FSSupportedOps` that, if supported, will trigger this behavior in RocksDB.

Pull Request resolved: #12408

Reviewed By: akankshamahajan15

Differential Revision: D54564218

Pulled By: anand1976

fbshipit-source-id: bc401dcd22a7d320bf63b5183c41714acdce39f5

include/rocksdb/file_system.h

@@ -81,7 +81,14 @@ enum class IOType : uint8_t {
 
 // enum representing various operations supported by underlying FileSystem.
 // These need to be set in SupportedOps API for RocksDB to use them.
-enum FSSupportedOps { kAsyncIO, kFSBuffer };
+enum FSSupportedOps {
+  kAsyncIO,   // Supports async reads
+  kFSBuffer,  // Supports handing off the file system allocated read buffer
+              // to the caller of Read/MultiRead
+  kVerifyAndReconstructRead,  // Supports a higher level of data integrity. See
+                              // the verify_and_reconstruct_read flag in
+                              // IOOptions.
+};
 
 // Per-request options that can be passed down to the FileSystem
 // implementation. These are hints and are not necessarily guaranteed to be
@@ -120,6 +127,18 @@ struct IOOptions {
   // directories and list only files in GetChildren API.
   bool do_not_recurse;
 
+  // Setting this flag indicates a corruption was detected by a previous read,
+  // so the caller wants to re-read the data with much stronger data integrity
+  // checking and correction, i.e requests the file system to reconstruct the
+  // data from redundant copies and verify checksums, if available, in order
+  // to have a better chance of success. It is expected that this will have a
+  // much higher overhead than a normal read.
+  // This is a hint. At a minimum, the file system should implement this flag in
+  // FSRandomAccessFile::Read and FSSequentialFile::Read
+  // NOTE: The file system must support kVerifyAndReconstructRead in
+  // FSSupportedOps, otherwise this feature will not be used.
+  bool verify_and_reconstruct_read;
+
   // EXPERIMENTAL
   Env::IOActivity io_activity = Env::IOActivity::kUnknown;