Allow ldb to open DB as secondary (#5537)

Summary:
Right now ldb can open running DB through read-only DB. However, it might leave info logs files to the read-only DB directory. Add an option to open the DB as secondary to avoid it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5537

Test Plan:
Run
./ldb scan  --max_keys=10 --db=/tmp/rocksdbtest-2491/dbbench --secondary_path=/tmp --no_value --hex
and
./ldb get 0x00000000000000103030303030303030 --hex --db=/tmp/rocksdbtest-2491/dbbench --secondary_path=/tmp
against a normal db_bench run and observe the output changes. Also observe that no new info logs files are created under /tmp/rocksdbtest-2491/dbbench.
Run without --secondary_path and observe that new info logs created under /tmp/rocksdbtest-2491/dbbench.

Differential Revision: D16113886

fbshipit-source-id: 4e09dec47c2528f6ca08a9e7a7894ba2d9daebbb

HISTORY.md

@@ -22,6 +22,7 @@
 * Add an option `unordered_write` which trades snapshot guarantees with higher write throughput. When used with WRITE_PREPARED transactions with two_write_queues=true, it offers higher throughput with however no compromise on guarantees.
 * Allow DBImplSecondary to remove memtables with obsolete data after replaying MANIFEST and WAL.
 * Add an option `failed_move_fall_back_to_copy` (default is true) for external SST ingestion. When `move_files` is true and hard link fails, ingestion falls back to copy if `failed_move_fall_back_to_copy` is true. Otherwise, ingestion reports an error.
+* Add argument `--secondary_path` to ldb to open the database as the secondary instance. This would keep the original DB intact.
 
 ### Performance Improvements
 * Reduce binary search when iterator reseek into the same data block.

include/rocksdb/utilities/ldb_cmd.h

@@ -31,6 +31,7 @@ class LDBCommand {
   // Command-line arguments
   static const std::string ARG_DB;
   static const std::string ARG_PATH;
+  static const std::string ARG_SECONDARY_PATH;
   static const std::string ARG_HEX;
   static const std::string ARG_KEY_HEX;
   static const std::string ARG_VALUE_HEX;
@@ -128,6 +129,10 @@ class LDBCommand {
  protected:
   LDBCommandExecuteResult exec_state_;
   std::string db_path_;
+  // If empty, open DB as primary. If non-empty, open the DB as secondary
+  // with this secondary path. When running against a database opened by
+  // another process, ldb wll leave the source directory completely intact. 
+  std::string secondary_path_;
   std::string column_family_name_;
   DB* db_;
   DBWithTTL* db_ttl_;

tools/ldb_cmd.cc

@@ -47,6 +47,7 @@ namespace rocksdb {
 
 const std::string LDBCommand::ARG_DB = "db";
 const std::string LDBCommand::ARG_PATH = "path";
+const std::string LDBCommand::ARG_SECONDARY_PATH = "secondary_path";
 const std::string LDBCommand::ARG_HEX = "hex";
 const std::string LDBCommand::ARG_KEY_HEX = "key_hex";
 const std::string LDBCommand::ARG_VALUE_HEX = "value_hex";
@@ -321,6 +322,12 @@ LDBCommand::LDBCommand(const std::map<std::string, std::string>& options,
     column_family_name_ = kDefaultColumnFamilyName;
   }
 
+  itr = options.find(ARG_SECONDARY_PATH);
+  secondary_path_ = "";
+  if (itr != options.end()) {
+    secondary_path_ = itr->second;
+  }
+
   is_key_hex_ = IsKeyHex(options, flags);
   is_value_hex_ = IsValueHex(options, flags);
   is_db_ttl_ = IsFlagPresent(flags, ARG_TTL);
@@ -360,6 +367,10 @@ void LDBCommand::OpenDB() {
       exec_state_ = LDBCommandExecuteResult::Failed(
           "ldb doesn't support TTL DB with multiple column families");
     }
+    if (!secondary_path_.empty()) {
+      exec_state_ = LDBCommandExecuteResult::Failed(
+          "Open as secondary is not supported for TTL DB yet.");
+    }
     if (is_read_only_) {
       st = DBWithTTL::Open(options_, db_path_, &db_ttl_, 0, true);
     } else {
@@ -382,7 +393,7 @@ void LDBCommand::OpenDB() {
         }
       }
     }
-    if (is_read_only_) {
+    if (is_read_only_ && secondary_path_.empty()) {
       if (column_families_.empty()) {
         st = DB::OpenForReadOnly(options_, db_path_, &db_);
       } else {
@@ -391,10 +402,19 @@ void LDBCommand::OpenDB() {
       }
     } else {
       if (column_families_.empty()) {
-        st = DB::Open(options_, db_path_, &db_);
+        if (secondary_path_.empty()) {
+          st = DB::Open(options_, db_path_, &db_);
+        } else {
+          st = DB::OpenAsSecondary(options_, db_path_, secondary_path_, &db_);
+        }
       } else {
-        st = DB::Open(options_, db_path_, column_families_, &handles_opened,
-                      &db_);
+        if (secondary_path_.empty()) {
+          st = DB::Open(options_, db_path_, column_families_, &handles_opened,
+                        &db_);
+        } else {
+          st = DB::OpenAsSecondary(options_, db_path_, secondary_path_,
+                                   column_families_, &handles_opened, &db_);
+        }
       }
     }
   }
@@ -452,6 +472,7 @@ ColumnFamilyHandle* LDBCommand::GetCfHandle() {
 std::vector<std::string> LDBCommand::BuildCmdLineOptions(
     std::vector<std::string> options) {
   std::vector<std::string> ret = {ARG_DB,
+                                  ARG_SECONDARY_PATH,
                                   ARG_BLOOM_BITS,
                                   ARG_BLOCK_SIZE,
                                   ARG_AUTO_COMPACTION,