New internal bounded string append APIs (#1816)

To efficiently support JSON serialization with bounded length, this is a new internal string API that can express bounded UTF-8-preserving append operations directly.

This is a prerequisite for work on CDRIVER-3775 (structured logs) and CDRIVER-4814 (performance of large document serialization).

Now a single buffer can be shared by all nesting levels of a complex document. Previously, all nesting levels of a BSON document would allocate their own string append buffer, and truncation happened as a separate step on the way out of these nested levels. Several other temporary string buffers were eliminated, including the temporary buffer that was needed for every printf. Now mcommon_string_append_printf writes directly to the destination, growing it as needed.

* test the utf8-oblivious behavior of bson_string_truncate
* document the utf8-oblivious behavior of bson_string_truncate
* new internal mcommon_string, mcommon_string_append, mcommon_json_append APIs
* some tests for UTF-8 preserving string truncation
* unit test for printf truncation between utf-8 code points
* fix for missing TXT string length validation

Author: mdbmes

src/common/src/common-bits-private.h

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2009-present MongoDB, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "common-prelude.h"
+
+#ifndef MONGO_C_DRIVER_COMMON_BITS_PRIVATE_H
+#define MONGO_C_DRIVER_COMMON_BITS_PRIVATE_H
+
+#include <bson/bson.h>
+
+
+// Round up to the next power of two uint32_t value. Saturates on overflow.
+static BSON_INLINE uint32_t
+mcommon_next_power_of_two_u32 (uint32_t v)
+{
+   if (v == 0) {
+      return 1;
+   }
+
+   // https://graphics.stanford.edu/%7Eseander/bithacks.html#RoundUpPowerOf2
+   v--;
+   v |= v >> 1;
+   v |= v >> 2;
+   v |= v >> 4;
+   v |= v >> 8;
+   v |= v >> 16;
+   v++;
+
+   if (v == 0) {
+      return UINT32_MAX;
+   } else {
+      return v;
+   }
+}
+
+
+#endif /* MONGO_C_DRIVER_COMMON_BITS_PRIVATE_H */