Return buffered data on first EOF in tube.readline() (#2376)

* Return buffered data on first EOF in tube.readline()

When there is still data available in the tube buffer when an EOFError occurs in
`tube.recvline()`, return that data even though it doesn't contain a newline.
The next time `tube.recvline()` is called afterwards will raise EOFError normally.

This behavior is in line with the GNU readline implementation and avoids
loss of data. It allows `tube.stream()` to print everything that's received before
the receiving end terminates.

A new warning is logged when data is returned due to an EOF informing
about the lack of the trailing newline character.

Fixes #2366

* Update CHANGELOG

* Add context.throw_eof_on_incomplete_line

Allow to control the behavior of `tube.recvline` and
be able to suppress the new warning.

* Cleanup docs

CHANGELOG.md

@@ -79,6 +79,7 @@ The table below shows which release corresponds to each branch, and what date th
 - [#2330][2330] Change `context.newline` when setting `context.os` to `"windows"`
 - [#2389][2389] Fix passing bytes to `context.log_file` and `crc.BitPolynom`
 - [#2391][2391] Fix error message when passing invalid kwargs to `xor`
+- [#2376][2376] Return buffered data on first EOF in tube.readline()
 
 [2360]: https://github.com/Gallopsled/pwntools/pull/2360
 [2356]: https://github.com/Gallopsled/pwntools/pull/2356
@@ -88,6 +89,7 @@ The table below shows which release corresponds to each branch, and what date th
 [2330]: https://github.com/Gallopsled/pwntools/pull/2330
 [2389]: https://github.com/Gallopsled/pwntools/pull/2389
 [2391]: https://github.com/Gallopsled/pwntools/pull/2391
+[2376]: https://github.com/Gallopsled/pwntools/pull/2376
 
 ## 4.13.0 (`beta`)
 

pwnlib/context/__init__.py

@@ -367,6 +367,7 @@ class ContextType(object):
         'randomize': False,
         'rename_corefiles': True,
         'newline': b'\n',
+        'throw_eof_on_incomplete_line': None,
         'noptrace': False,
         'os': 'linux',
         'proxy': None,
@@ -1490,6 +1491,25 @@ def newline(self, v):
         # circular imports
         from pwnlib.util.packing import _need_bytes
         return _need_bytes(v)
+
+    @_validator
+    def throw_eof_on_incomplete_line(self, v):
+        """Whether to raise an :class:`EOFError` if an EOF is received before a newline in ``tube.recvline``.
+
+        Controls if an :class:`EOFError` is treated as newline in ``tube.recvline`` and similar functions
+        and whether a warning should be logged about it.
+
+        Possible values are:
+
+        - ``True``: Raise an :class:`EOFError` if an EOF is received before a newline.
+        - ``False``: Return the data received so far if an EOF is received
+          before a newline without logging a warning.
+        - ``None``: Return the data received so far if an EOF is received
+          before a newline and log a warning.
+
+        Default value is ``None``.
+        """
+        return v if v is None else bool(v)
 
 
     @_validator

pwnlib/tubes/tube.py

@@ -467,19 +467,31 @@ def recvline(self, keepends=True, timeout=default):
         Receive a single line from the tube.
 
         A "line" is any sequence of bytes terminated by the byte sequence
-        set in :attr:`newline`, which defaults to ``'\n'``.
+        set in :attr:`newline`, which defaults to ``b'\n'``.
+
+        If the connection is closed (:class:`EOFError`) before a newline
+        is received, the buffered data is returned by default and a warning
+        is logged. If the buffer is empty, an :class:`EOFError` is raised.
+        This behavior can be changed by setting :meth:`pwnlib.context.ContextType.throw_eof_on_incomplete_line`.
 
         If the request is not satisfied before ``timeout`` seconds pass,
-        all data is buffered and an empty string (``''``) is returned.
+        all data is buffered and an empty byte string (``b''``) is returned.
 
         Arguments:
             keepends(bool): Keep the line ending (:const:`True`).
             timeout(int): Timeout
 
+        Raises:
+            :class:`EOFError`: The connection closed before the request
+                                 could be satisfied and the buffer is empty
+
         Return:
             All bytes received over the tube until the first
             newline ``'\n'`` is received.  Optionally retains
-            the ending.
+            the ending. If the connection is closed before a newline
+            is received, the remaining data received up to this point
+            is returned.
+
 
         Examples:
 
@@ -494,8 +506,31 @@ def recvline(self, keepends=True, timeout=default):
             >>> t.newline = b'\r\n'
             >>> t.recvline(keepends = False)
             b'Foo\nBar'
+            >>> t = tube()
+            >>> def _recv_eof(n):
+            ...     if not _recv_eof.throw:
+            ...         _recv_eof.throw = True
+            ...         return b'real line\ntrailing data'
+            ...     raise EOFError
+            >>> _recv_eof.throw = False
+            >>> t.recv_raw = _recv_eof
+            >>> t.recvline()
+            b'real line\n'
+            >>> t.recvline()
+            b'trailing data'
+            >>> t.recvline() # doctest: +ELLIPSIS
+            Traceback (most recent call last):
+            ...
+            EOFError
         """
-        return self.recvuntil(self.newline, drop = not keepends, timeout = timeout)
+        try:
+            return self.recvuntil(self.newline, drop = not keepends, timeout = timeout)
+        except EOFError:
+            if not context.throw_eof_on_incomplete_line and self.buffer.size > 0:
+                if context.throw_eof_on_incomplete_line is None:
+                    self.warn_once('EOFError during recvline. Returning buffered data without trailing newline.')
+                return self.buffer.get()
+            raise
 
     def recvline_pred(self, pred, keepends=False, timeout=default):
         r"""recvline_pred(pred, keepends=False) -> bytes