Add support for streaming delimited messages (#529)

* Add support for streaming delimited messages

This allows developers to easily dump and load multiple messages
from a stream in a way that is compatible with official
protobuf implementations (such as Java's
`MessageLite#writeDelimitedTo(...)`).

* Add Java compatibility tests for streaming

These tests stream data such as messages to output files, have a
Java binary read them and then write them back using the
`protobuf-java` functions, and then read them back in on the Python
side to check that the returned data is as expected. This checks
that the official Java implementation (and so any other matching
implementations) can properly parse outputs from Betterproto, and
vice-versa, ensuring compatibility in these functions between the
two.

* Replace `xxxxableBuffer` with `SupportsXxxx`

src/betterproto/__init__.py

@@ -50,7 +50,10 @@ from .grpc.grpclib_client import ServiceStub as ServiceStub
 
 
 if TYPE_CHECKING:
-    from _typeshed import ReadableBuffer
+    from _typeshed import (
+        SupportsRead,
+        SupportsWrite,
+    )
 
 
 # Proto 3 data types
@@ -127,6 +130,9 @@ WIRE_FIXED_32_TYPES = [TYPE_FLOAT, TYPE_FIXED32, TYPE_SFIXED32]
 WIRE_FIXED_64_TYPES = [TYPE_DOUBLE, TYPE_FIXED64, TYPE_SFIXED64]
 WIRE_LEN_DELIM_TYPES = [TYPE_STRING, TYPE_BYTES, TYPE_MESSAGE, TYPE_MAP]
 
+# Indicator of message delimitation in streams
+SIZE_DELIMITED = -1
+
 
 # Protobuf datetimes start at the Unix Epoch in 1970 in UTC.
 def datetime_default_gen() -> datetime:
@@ -322,7 +328,7 @@ def _pack_fmt(proto_type: str) -> str:
     }[proto_type]
 
 
-def dump_varint(value: int, stream: BinaryIO) -> None:
+def dump_varint(value: int, stream: "SupportsWrite[bytes]") -> None:
     """Encodes a single varint and dumps it into the provided stream."""
     if value < -(1 << 63):
         raise ValueError(
@@ -531,7 +537,7 @@ def _dump_float(value: float) -> Union[float, str]:
     return value
 
 
-def load_varint(stream: BinaryIO) -> Tuple[int, bytes]:
+def load_varint(stream: "SupportsRead[bytes]") -> Tuple[int, bytes]:
     """
     Load a single varint value from a stream. Returns the value and the raw bytes read.
     """
@@ -569,7 +575,7 @@ class ParsedField:
     raw: bytes
 
 
-def load_fields(stream: BinaryIO) -> Generator[ParsedField, None, None]:
+def load_fields(stream: "SupportsRead[bytes]") -> Generator[ParsedField, None, None]:
     while True:
         try:
             num_wire, raw = load_varint(stream)
@@ -881,7 +887,7 @@ class Message(ABC):
             self.__class__._betterproto_meta = meta  # type: ignore
         return meta
 
-    def dump(self, stream: BinaryIO) -> None:
+    def dump(self, stream: "SupportsWrite[bytes]", delimit: bool = False) -> None:
         """
         Dumps the binary encoded Protobuf message to the stream.
 
@@ -889,7 +895,11 @@ class Message(ABC):
         -----------
         stream: :class:`BinaryIO`
             The stream to dump the message to.
+        delimit:
+            Whether to prefix the message with a varint declaring its size.
         """
+        if delimit == SIZE_DELIMITED:
+            dump_varint(len(self), stream)
 
         for field_name, meta in self._betterproto.meta_by_field_name.items():
             try:
@@ -1207,7 +1217,11 @@ class Message(ABC):
             meta.group is not None and self._group_current.get(meta.group) == field_name
         )
 
-    def load(self: T, stream: BinaryIO, size: Optional[int] = None) -> T:
+    def load(
+        self: T,
+        stream: "SupportsRead[bytes]",
+        size: Optional[int] = None,
+    ) -> T:
         """
         Load the binary encoded Protobuf from a stream into this message instance. This
         returns the instance itself and is therefore assignable and chainable.
@@ -1219,12 +1233,17 @@ class Message(ABC):
         size: :class:`Optional[int]`
             The size of the message in the stream.
             Reads stream until EOF if ``None`` is given.
+            Reads based on a size delimiter prefix varint if SIZE_DELIMITED is given.
 
         Returns
         --------
         :class:`Message`
             The initialized message.
         """
+        # If the message is delimited, parse the message delimiter
+        if size == SIZE_DELIMITED:
+            size, _ = load_varint(stream)
+
         # Got some data over the wire
         self._serialized_on_wire = True
         proto_meta = self._betterproto
@@ -1297,7 +1316,7 @@ class Message(ABC):
 
         return self
 
-    def parse(self: T, data: "ReadableBuffer") -> T:
+    def parse(self: T, data: bytes) -> T:
         """
         Parse the binary encoded Protobuf into this message instance. This
         returns the instance itself and is therefore assignable and chainable.