exec_command segfault partial fix (#280)

* Mostly undo 73a2be9 to fix _other_ excec_command segfaults

* Replace % with format where touched

* Update test_exec_command to test repeated calls to ensure the object remains usable

Author: Qalthos

docs/changelog-fragments/280.bugfix.rst

@@ -0,0 +1,2 @@
+Improved ``channel.exec_command`` to always use a newly created ``ssh_channel`` to avoid
+segfaults on repeated calls -- by :user:`Qalthos`

src/pylibsshext/channel.pyx

@@ -89,12 +89,10 @@ cdef class Channel:
     def poll(self, timeout=-1, stderr=0):
         if timeout < 0:
             rc = libssh.ssh_channel_poll(self._libssh_channel, stderr)
-            if rc == libssh.SSH_ERROR:
-                raise LibsshChannelException("Failed to poll channel: [%d]" % rc)
         else:
             rc = libssh.ssh_channel_poll_timeout(self._libssh_channel, timeout, stderr)
-            if rc == libssh.SSH_ERROR:
-                raise LibsshChannelException("Failed to poll channel: [%d]" % rc)
+        if rc == libssh.SSH_ERROR:
+            raise LibsshChannelException("Failed to poll channel: [{0}]".format(rc))
         return rc
 
     def read_nonblocking(self, size=1024, stderr=0):
@@ -113,7 +111,10 @@ cdef class Channel:
         return self.read_nonblocking(size=size, stderr=stderr)
 
     def write(self, data):
-        return libssh.ssh_channel_write(self._libssh_channel, PyBytes_AS_STRING(data), len(data))
+        written = libssh.ssh_channel_write(self._libssh_channel, PyBytes_AS_STRING(data), len(data))
+        if written == libssh.SSH_ERROR:
+            raise LibsshChannelException("Failed to write to ssh channel")
+        return written
 
     def sendall(self, data):
         return self.write(data)
@@ -139,23 +140,35 @@ cdef class Channel:
         return response
 
     def exec_command(self, command):
-        rc = libssh.ssh_channel_request_exec(self._libssh_channel, command.encode("utf-8"))
+        # request_exec requires a fresh channel each run, so do not use the existing channel
+        cdef libssh.ssh_channel channel = libssh.ssh_channel_new(self._libssh_session)
+        if channel is NULL:
+            raise MemoryError
+
+        rc = libssh.ssh_channel_open_session(channel)
+        if rc != libssh.SSH_OK:
+            libssh.ssh_channel_free(channel)
+            raise LibsshChannelException("Failed to open_session: [{0}]".format(rc))
 
+        rc = libssh.ssh_channel_request_exec(channel, command.encode("utf-8"))
         if rc != libssh.SSH_OK:
-            self.close()
-            raise CalledProcessError()
+            libssh.ssh_channel_close(channel)
+            libssh.ssh_channel_free(channel)
+            raise LibsshChannelException("Failed to execute command [{0}]: [{1}]".format(command, rc))
         result = CompletedProcess(args=command, returncode=-1, stdout=b'', stderr=b'')
 
         cdef callbacks.ssh_channel_callbacks_struct cb
         memset(&cb, 0, sizeof(cb))
         cb.channel_data_function = <callbacks.ssh_channel_data_callback>&_process_outputs
         cb.userdata = <void *>result
         callbacks.ssh_callbacks_init(&cb)
-        callbacks.ssh_set_channel_callbacks(self._libssh_channel, &cb)
-
-        libssh.ssh_channel_send_eof(self._libssh_channel)
+        callbacks.ssh_set_channel_callbacks(channel, &cb)
 
-        result.returncode = self.get_channel_exit_status()
+        libssh.ssh_channel_send_eof(channel)
+        result.returncode = libssh.ssh_channel_get_exit_status(channel)
+        if channel is not NULL:
+            libssh.ssh_channel_close(channel)
+            libssh.ssh_channel_free(channel)
 
         return result
 

src/pylibsshext/includes/libssh.pxd

@@ -21,7 +21,7 @@ from libc.stdint cimport uint32_t
 
 cdef extern from "libssh/libssh.h" nogil:
 
-    cpdef const char * libssh_version "SSH_STRINGIFY(LIBSSH_VERSION)"
+    cdef const char * libssh_version "SSH_STRINGIFY(LIBSSH_VERSION)"
 
     cdef struct ssh_session_struct:
         pass

tests/unit/channel_test.py

@@ -32,11 +32,14 @@ def ssh_channel(ssh_client_session):
     'Ref: https://github.com/ansible/pylibssh/issues/57',  # noqa: WPS326
     strict=False,
 )
-@pytest.mark.forked()  # noqa: PT023 -- it's unclear if braces are needed here
+@pytest.mark.forked
 def test_exec_command(ssh_channel):
     """Test getting the output of a remotely executed command."""
     u_cmd_out = ssh_channel.exec_command('echo -n Hello World').stdout.decode()
     assert u_cmd_out == u'Hello World'  # noqa: WPS302
+    # Test that repeated calls to exec_command do not segfault.
+    u_cmd_out = ssh_channel.exec_command('echo -n Hello Again').stdout.decode()
+    assert u_cmd_out == u'Hello Again'  # noqa: WPS302
 
 
 def test_double_close(ssh_channel):