CDRIVER-6053 do not buffer pooled streams (#2064)

* disable buffering for streams

The stream buffer is tied to the lifetime of a stream. A large response may grow the buffer and result in higher memory usage until the stream is destroyed. A stream may live as long as the client/pool.

The buffer appears unnecessary, since a separate buffer is allocated, and read directly into, for the server responses.

* remove unnecessary `buffered` parameter from `mongoc_client_connect`

* warn about allocation strategy of buffered stream

Co-authored-by: Ezra Chung <[email protected]>

Author: kevinAlbs

src/libmongoc/doc/mongoc_stream_buffered_new.rst

@@ -9,8 +9,7 @@ Synopsis
 .. code-block:: c
 
   mongoc_stream_t *
-  mongoc_stream_buffered_new (mongoc_stream_t *base_stream,
-                          size_t buffer_size);
+  mongoc_stream_buffered_new (mongoc_stream_t *base_stream, size_t buffer_size);
 
 Parameters
 ----------
@@ -22,6 +21,10 @@ This function shall create a new :symbol:`mongoc_stream_t` that buffers bytes to
 
 ``buffer_size`` will be used as the initial buffer size. It may grow past this size.
 
+.. warning::
+  
+  The internal buffer does not reduce in size once grown. Receiving a large message may result in a large allocation that persists until the returned :symbol:`mongoc_stream_t` is freed with :symbol:`mongoc_stream_destroy()`.
+
 Returns
 -------
 

src/libmongoc/src/mongoc/mongoc-client-private.h

@@ -209,8 +209,7 @@ mongoc_stream_t *
 mongoc_client_connect_tcp (int32_t connecttimeoutms, const mongoc_host_list_t *host, bson_error_t *error);
 
 mongoc_stream_t *
-mongoc_client_connect (bool buffered,
-                       bool use_ssl,
+mongoc_client_connect (bool use_ssl,
                        void *ssl_opts_void,
                        const mongoc_uri_t *uri,
                        const mongoc_host_list_t *host,

src/libmongoc/src/mongoc/mongoc-client.c

@@ -761,8 +761,7 @@ mongoc_client_connect_unix (const mongoc_host_list_t *host, bson_error_t *error)
 }
 
 mongoc_stream_t *
-mongoc_client_connect (bool buffered,
-                       bool use_ssl,
+mongoc_client_connect (bool use_ssl,
                        void *ssl_opts_void,
                        const mongoc_uri_t *uri,
                        const mongoc_host_list_t *host,
@@ -851,9 +850,6 @@ mongoc_client_connect (bool buffered,
    if (!base_stream) {
       return NULL;
    }
-   if (buffered) {
-      return mongoc_stream_buffered_new (base_stream, 1024);
-   }
    return base_stream;
 }
 
@@ -897,13 +893,12 @@ mongoc_client_default_stream_initiator (const mongoc_uri_t *uri,
 
 #if defined(MONGOC_ENABLE_SSL_OPENSSL) && OPENSSL_VERSION_NUMBER >= 0x10100000L
    SSL_CTX *ssl_ctx = client->topology->scanner->openssl_ctx;
-   return mongoc_client_connect (
-      true, use_ssl, ssl_opts_void, uri, host, (void *) ssl_ctx, MONGOC_SHARED_PTR_NULL, error);
+   return mongoc_client_connect (use_ssl, ssl_opts_void, uri, host, (void *) ssl_ctx, MONGOC_SHARED_PTR_NULL, error);
 #elif defined(MONGOC_ENABLE_SSL_SECURE_CHANNEL)
    return mongoc_client_connect (
-      true, use_ssl, ssl_opts_void, uri, host, NULL, client->topology->scanner->secure_channel_cred_ptr, error);
+      use_ssl, ssl_opts_void, uri, host, NULL, client->topology->scanner->secure_channel_cred_ptr, error);
 #else
-   return mongoc_client_connect (true, use_ssl, ssl_opts_void, uri, host, NULL, MONGOC_SHARED_PTR_NULL, error);
+   return mongoc_client_connect (use_ssl, ssl_opts_void, uri, host, NULL, MONGOC_SHARED_PTR_NULL, error);
 #endif
 }
 

src/libmongoc/src/mongoc/mongoc-server-monitor.c

@@ -948,8 +948,7 @@ _server_monitor_setup_connection (mongoc_server_monitor_t *server_monitor,
       secure_channel_cred_ptr = server_monitor->topology->scanner->secure_channel_cred_ptr;
 #endif
 
-      server_monitor->stream = mongoc_client_connect (false,
-                                                      ssl_opts_void != NULL,
+      server_monitor->stream = mongoc_client_connect (ssl_opts_void != NULL,
                                                       ssl_opts_void,
                                                       server_monitor->uri,
                                                       &server_monitor->description->host,