ARROW-15036: [C++] Automatically configure S3 SDK configuration param…

…eter "maxConnections"

This should allow uses cases where the user wants more than 25 threads in the I/O thread pool because they are doing highly concurrent S3 work.  See ARROW-14965

Closes #11929 from westonpace/feature/ARROW-15036--expose-max-connections-param

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/filesystem/s3fs.cc

@@ -690,7 +690,8 @@ class ClientBuilder {
 
   Aws::Client::ClientConfiguration* mutable_config() { return &client_config_; }
 
-  Result<std::shared_ptr<S3Client>> BuildClient() {
+  Result<std::shared_ptr<S3Client>> BuildClient(
+      util::optional<io::IOContext> io_context = util::nullopt) {
     credentials_provider_ = options_.credentials_provider;
     if (!options_.region.empty()) {
       client_config_.region = ToAwsString(options_.region);
@@ -742,6 +743,11 @@ class ClientBuilder {
       client_config_.proxyPassword = ToAwsString(options_.proxy_options.password);
     }
 
+    if (io_context) {
+      // TODO: Once ARROW-15035 is done we can get rid of the "at least 25" fallback
+      client_config_.maxConnections = std::max(io_context->executor()->GetCapacity(), 25);
+    }
+
     auto client = std::make_shared<S3Client>(
         credentials_provider_, client_config_,
         Aws::Client::AWSAuthV4Signer::PayloadSigningPolicy::Never,
@@ -1617,7 +1623,7 @@ class S3FileSystem::Impl : public std::enable_shared_from_this<S3FileSystem::Imp
   explicit Impl(S3Options options, io::IOContext io_context)
       : builder_(std::move(options)), io_context_(io_context) {}
 
-  Status Init() { return builder_.BuildClient().Value(&client_); }
+  Status Init() { return builder_.BuildClient(io_context_).Value(&client_); }
 
   const S3Options& options() const { return builder_.options(); }