Use singleton per-region for S3Client (#384)

Summary:
Pull Request resolved: #384

# What
* Use a singleton for each region when constructing our S3Client instead of a _single_ singleton
* This is kind of a follow-up to D36727729 (47182c6)
# Why
* Follow-up to S281873 - S3Client singleton breaks multi-region support in PCF IO

NOTE: PCF should be owning this code in the long-term since it's out of PSI's scope, but since we owned the initial SEV, I took ownership of this follow-up.

Differential Revision: D39174441

fbshipit-source-id: 73dd6d6c88e216da3f99573689ef4c4eaa7d16ed

Author: Logan Gore

fbpcf/io/cloud_util/S3Client.cpp

@@ -7,12 +7,46 @@
 
 #include "fbpcf/io/cloud_util/S3Client.h"
 
+#include <string>
+
 #include <aws/core/Aws.h>
 #include <aws/s3/S3Client.h>
 
+#include <folly/container/F14Map.h>
+#include <folly/Synchronized.h>
+
 namespace fbpcf::cloudio {
 S3Client& S3Client::getInstance(const fbpcf::aws::S3ClientOption& option) {
-  static S3Client s3Client(option);
-  return s3Client;
+  /* Due to previous problems, we create a Singleton instance of the S3Client,
+   * but there's a catch: we need a distinct S3Client for each region, or we
+   * run into other issues. For that reason, we store this map from string to
+   * S3Client with the assumption that the keys are region names. Since region
+   * is optional, we also allow for a default empty string region.
+   * ***************************** NOT THREAD SAFE ****************************
+   * NOTE: Significant refactoring is required to make this thread safe
+   * Downstream usage wants a mutable reference, but a folly::Synchronized
+   * RWLock will return a const ref to a reader, meaning it's hard to refactor.
+   * Simply trying to use folly::Synchronized around the map isn't sufficient,
+   * because we'll leak a reference to an object in the map which is unsafe.
+   * ***************************** NOT THREAD SAFE ****************************
+   */
+  static folly::Synchronized<folly::F14FastMap<std::string, S3Client>> m;
+
+  std::string defaultStr{};
+  auto region = option.region.value_or(defaultStr);
+
+  m.withWLock([&](auto& clientMap) {
+    if (clientMap.find(region) == clientMap.end()) {
+      clientMap.at(region) = S3Client{option};
+    }
+  });
+
+  /* You may see this and think, "Hey, the NOT THREAD SAFE warning above is 
+   * outdated, it looks like we fixed it!", but you're wrong. This still does
+   * not fully solve the problem. Because the downstream consumer takes a
+   * mutable reference, there's no guarantee that this is thread safe. It's
+   * better than nothing, but you still shouldn't fully trust this code.
+   */
+  return m.wlock()->at(region);
 }
 } // namespace fbpcf::cloudio