fix for checkpoint directory with trailing slash(es)

Summary:
previously if `checkpoint_dir` contained a trailing slash, we'd attempt to create the `.tmp` directory under `checkpoint_dir` due to simply concatenating `checkpoint_dir + ".tmp"`. This failed because `checkpoint_dir` hadn't been created yet and our directory creation is non-recursive. This PR fixes the issue by always creating the `.tmp` directory in the same parent as `checkpoint_dir` by stripping trailing slashes before concatenating.
Closes https://github.com/facebook/rocksdb/pull/3275

Differential Revision: D6574952

Pulled By: ajkr

fbshipit-source-id: a6daa6777a901eac2460cd0140c9515f7241aefc

utilities/checkpoint/checkpoint_impl.cc

@@ -59,7 +59,21 @@ Status CheckpointImpl::CreateCheckpoint(const std::string& checkpoint_dir,
       db_options.info_log,
       "Started the snapshot process -- creating snapshot in directory %s",
       checkpoint_dir.c_str());
-  std::string full_private_path = checkpoint_dir + ".tmp";
+
+  size_t final_nonslash_idx = checkpoint_dir.find_last_not_of('/');
+  if (final_nonslash_idx == std::string::npos) {
+    // npos means it's only slashes, which means it's the root directory, but it
+    // shouldn't be because we verified above the directory doesn't exist.
+    assert(false);
+    return Status::InvalidArgument("invalid checkpoint directory name");
+  }
+
+  std::string full_private_path =
+      checkpoint_dir.substr(0, final_nonslash_idx + 1) + ".tmp";
+  ROCKS_LOG_INFO(
+      db_options.info_log,
+      "Snapshot process -- using temporary directory %s",
+      full_private_path.c_str());
   // create snapshot directory
   s = db_->GetEnv()->CreateDir(full_private_path);
   uint64_t sequence_number = 0;