lnmsg: small speed-up: read first, check length after

this saves around ~13% wall clock time in ChannelDB.load_data

electrum/lnmsg.py

@@ -25,6 +25,8 @@ def _num_remaining_bytes_to_read(fd: io.BytesIO) -> int:
 
 
 def _assert_can_read_at_least_n_bytes(fd: io.BytesIO, n: int) -> None:
+    # note: it's faster to read n bytes and then check if we read n, than
+    #       to assert we can read at least n and then read n bytes.
     nremaining = _num_remaining_bytes_to_read(fd)
     if nremaining < n:
         raise UnexpectedEndOfStream(f"wants to read {n} bytes but only {nremaining} bytes left")
@@ -50,20 +52,26 @@ def read_bigsize_int(fd: io.BytesIO) -> Optional[int]:
     if first < 0xfd:
         return first
     elif first == 0xfd:
-        _assert_can_read_at_least_n_bytes(fd, 2)
-        val = int.from_bytes(fd.read(2), byteorder="big", signed=False)
+        buf = fd.read(2)
+        if len(buf) != 2:
+            raise UnexpectedEndOfStream()
+        val = int.from_bytes(buf, byteorder="big", signed=False)
         if not (0xfd <= val < 0x1_0000):
             raise FieldEncodingNotMinimal()
         return val
     elif first == 0xfe:
-        _assert_can_read_at_least_n_bytes(fd, 4)
-        val = int.from_bytes(fd.read(4), byteorder="big", signed=False)
+        buf = fd.read(4)
+        if len(buf) != 4:
+            raise UnexpectedEndOfStream()
+        val = int.from_bytes(buf, byteorder="big", signed=False)
         if not (0x1_0000 <= val < 0x1_0000_0000):
             raise FieldEncodingNotMinimal()
         return val
     elif first == 0xff:
-        _assert_can_read_at_least_n_bytes(fd, 8)
-        val = int.from_bytes(fd.read(8), byteorder="big", signed=False)
+        buf = fd.read(8)
+        if len(buf) != 8:
+            raise UnexpectedEndOfStream()
+        val = int.from_bytes(buf, byteorder="big", signed=False)
         if not (0x1_0000_0000 <= val):
             raise FieldEncodingNotMinimal()
         return val
@@ -96,8 +104,10 @@ def _read_field(*, fd: io.BytesIO, field_type: str, count: Union[int, str]) -> U
             assert field_type == 'u64'
             type_len = 8
         assert count == 1, count
-        _assert_can_read_at_least_n_bytes(fd, type_len)
-        return int.from_bytes(fd.read(type_len), byteorder="big", signed=False)
+        buf = fd.read(type_len)
+        if len(buf) != type_len:
+            raise UnexpectedEndOfStream()
+        return int.from_bytes(buf, byteorder="big", signed=False)
     elif field_type in ('tu16', 'tu32', 'tu64'):
         if field_type == 'tu16':
             type_len = 2
@@ -129,14 +139,18 @@ def _read_field(*, fd: io.BytesIO, field_type: str, count: Union[int, str]) -> U
         type_len = 33
     elif field_type == 'short_channel_id':
         type_len = 8
+
     if count == "...":
         total_len = -1  # read all
     else:
         if type_len is None:
             raise UnknownMsgFieldType(f"unknown field type: {field_type!r}")
         total_len = count * type_len
-        _assert_can_read_at_least_n_bytes(fd, total_len)
-    return fd.read(total_len)
+
+    buf = fd.read(total_len)
+    if total_len >= 0 and len(buf) != total_len:
+        raise UnexpectedEndOfStream()
+    return buf
 
 
 # TODO: maybe for "value" we could accept a list with len "count" of appropriate items