buffer: Add support for length-prefixed bytes/text

Add read_len_prefixed_bytes(), read_len_prefixed_utf8(), and
write_len_prefixed_bytes() methods.

Author: 1st1

buffer.pxd

@@ -36,6 +36,7 @@ cdef class WriteBuffer:
     cdef write_buffer(self, WriteBuffer buf)
     cdef write_byte(self, char b)
     cdef write_bytes(self, bytes data)
+    cdef write_len_prefixed_bytes(self, bytes data)
     cdef write_bytestring(self, bytes string)
     cdef write_str(self, str string, str encoding)
     cdef write_utf8(self, str string)
@@ -103,6 +104,9 @@ cdef class ReadBuffer:
     cdef inline read_utf8(self):
         return self.read_null_str().decode('utf-8')
 
+    cdef inline read_len_prefixed_utf8(self):
+        return self.read_len_prefixed_bytes().decode('utf-8')
+
     cdef feed_data(self, data)
     cdef inline _ensure_first_buf(self)
     cdef _switch_to_next_buf(self)
@@ -111,6 +115,7 @@ cdef class ReadBuffer:
     cdef inline _read_into(self, char *buf, ssize_t nbytes)
     cdef inline _read_and_discard(self, ssize_t nbytes)
     cdef bytes read_bytes(self, ssize_t nbytes)
+    cdef bytes read_len_prefixed_bytes(self)
     cdef inline int32_t read_int32(self) except? -1
     cdef inline int16_t read_int16(self) except? -1
     cdef inline read_null_str(self)

buffer.pyx

@@ -154,6 +154,19 @@ cdef class WriteBuffer:
     cdef write_utf8(self, str string):
         self.write_bytestring(string.encode('utf-8'))
 
+    cdef write_len_prefixed_bytes(self, bytes data):
+        # Write a length-prefixed (not NULL-terminated) UTF-8 string.
+        cdef:
+            char *buf
+            ssize_t size
+
+        cpython.PyBytes_AsStringAndSize(data, &buf, &size)
+        if size > _MAXINT32:
+            raise exceptions.BufferError('string is too large')
+        # `size` does not account for the NULL at the end.
+        self.write_int32(<int32_t>size)
+        self.write_cstr(buf, size)
+
     cdef write_cstr(self, const char *data, ssize_t len):
         self._check_readonly()
         self._ensure_alloced(len)
@@ -381,6 +394,13 @@ cdef class ReadBuffer:
         self._read_into(buf, nbytes)
         return result
 
+    cdef bytes read_len_prefixed_bytes(self):
+        cdef int32_t size = self.read_int32()
+        if size < 0:
+            raise exceptions.BufferError(
+                'negative length for a len-prefixed bytes value')
+        return self.read_bytes(size)
+
     cdef inline char read_byte(self) except? -1:
         cdef const char *first_byte