bridge: re-introduce the max_read_size limit

This is a regression from our port of the bridge from C to Python. The C
bridge had a limit and as fsread1 is not blocking implemented this
prevents issues of accidentally reading /dev/zero or /dev/sda which will
fill up ram endlessly.

src/cockpit/channels/filesystem.py

@@ -124,14 +124,14 @@ class FsReadChannel(GeneratorChannel):
 
     def do_yield_data(self, options: JsonObject) -> Generator[bytes, None, JsonObject]:
         path = get_str(options, 'path')
-        max_read_size = get_int(options, 'max_read_size', None)
+        max_read_size = get_int(options, 'max_read_size', 16 * 1024 * 1024)
 
         logger.debug('Opening file "%s" for reading', path)
 
         try:
             with open(path, 'rb') as filep:
                 buf = os.stat(filep.fileno())
-                if max_read_size is not None and buf.st_size > max_read_size:
+                if buf.st_size > max_read_size:
                     raise ChannelError('too-large')
 
                 if self.is_binary and stat.S_ISREG(buf.st_mode):

test/pytest/test_bridge.py

@@ -416,7 +416,7 @@ async def test_internal_metrics(transport: MockTransport) -> None:
 
 
 @pytest.mark.asyncio
-async def test_fsread1_errors(transport: MockTransport) -> None:
+async def test_fsread1_errors(transport: MockTransport, tmp_path: Path) -> None:
     await transport.check_open('fsread1', path='/etc/shadow', problem='access-denied')
     await transport.check_open('fsread1', path='/', problem='internal-error',
                                reply_keys={'message': "[Errno 21] Is a directory: '/'"})
@@ -425,6 +425,12 @@ async def test_fsread1_errors(transport: MockTransport) -> None:
                                reply_keys={'message': "attribute 'max_read_size': must have type int"})
     await transport.check_open('fsread1', path='/etc/passwd', max_read_size=1,
                                problem='too-large')
+    # default read size limit
+    big_file = tmp_path / 'bigfile.img'
+    fd = os.open(big_file, os.O_RDWR | os.O_CREAT)
+    os.posix_fallocate(fd, 0, 17 * 1024 * 1024)
+    await transport.check_open('fsread1', path=str(big_file),
+                               problem='too-large')
 
 
 @pytest.mark.asyncio