Increase SFTP chunk size to increase the SFTP throughput in both directions

docs/changelog-fragments/664.bugfix.rst

@@ -0,0 +1 @@
+Improved performance of SFTP transfers by using larger transfer chunks -- by :user:`Jakuje`.

src/pylibsshext/includes/libssh.pxd

@@ -27,6 +27,10 @@ cdef extern from "libssh/libssh.h" nogil:
         pass
     ctypedef ssh_session_struct* ssh_session
 
+    cdef struct ssh_string_struct:
+        pass
+    ctypedef ssh_string_struct* ssh_string
+
     cdef struct ssh_key_struct:
         pass
     ctypedef ssh_key_struct* ssh_key

src/pylibsshext/includes/sftp.pxd

@@ -17,7 +17,9 @@
 #
 from posix.types cimport mode_t
 
-from pylibsshext.includes.libssh cimport ssh_channel, ssh_session
+from libc cimport stdint
+
+from pylibsshext.includes.libssh cimport ssh_channel, ssh_session, ssh_string
 
 
 cdef extern from "libssh/sftp.h" nogil:
@@ -30,6 +32,31 @@ cdef extern from "libssh/sftp.h" nogil:
         pass
     ctypedef sftp_file_struct * sftp_file
 
+    struct sftp_attributes_struct:
+        char *name
+        char *longname
+        stdint.uint32_t flags
+        stdint.uint8_t type
+        stdint.uint64_t size
+        stdint.uint32_t uid
+        stdint.uint32_t gid
+        char *owner
+        char *group
+        stdint.uint32_t permissions
+        stdint.uint64_t atime64
+        stdint.uint32_t atime
+        stdint.uint32_t atime_nseconds
+        stdint.uint64_t createtime
+        stdint.uint32_t createtime_nseconds
+        stdint.uint64_t mtime64
+        stdint.uint32_t mtime
+        stdint.uint32_t mtime_nseconds
+        ssh_string acl
+        stdint.uint32_t extended_count
+        ssh_string extended_type
+        ssh_string extended_data
+    ctypedef sftp_attributes_struct * sftp_attributes
+
     cdef int SSH_FX_OK
     cdef int SSH_FX_EOF
     cdef int SSH_FX_NO_SUCH_FILE
@@ -55,5 +82,8 @@ cdef extern from "libssh/sftp.h" nogil:
     ssize_t sftp_read(sftp_file file, const void *buf, size_t count)
     int sftp_get_error(sftp_session sftp)
 
+    sftp_attributes sftp_stat(sftp_session session, const char *path)
+
+
 cdef extern from "sys/stat.h" nogil:
     cdef int S_IRWXU

src/pylibsshext/sftp.pyx

@@ -18,11 +18,15 @@
 from posix.fcntl cimport O_CREAT, O_RDONLY, O_TRUNC, O_WRONLY
 
 from cpython.bytes cimport PyBytes_AS_STRING
+from cpython.mem cimport PyMem_Free, PyMem_Malloc
 
 from pylibsshext.errors cimport LibsshSFTPException
 from pylibsshext.session cimport get_libssh_session
 
 
+SFTP_MAX_CHUNK = 32_768  # 32kB
+
+
 MSG_MAP = {
     sftp.SSH_FX_OK: "No error",
     sftp.SSH_FX_EOF: "End-of-file encountered",
@@ -63,7 +67,7 @@ cdef class SFTP:
             rf = sftp.sftp_open(self._libssh_sftp_session, remote_file_b, O_WRONLY | O_CREAT | O_TRUNC, sftp.S_IRWXU)
             if rf is NULL:
                 raise LibsshSFTPException("Opening remote file [%s] for write failed with error [%s]" % (remote_file, self._get_sftp_error_str()))
-            buffer = f.read(1024)
+            buffer = f.read(SFTP_MAX_CHUNK)
 
             while buffer != b"":
                 length = len(buffer)
@@ -76,38 +80,54 @@ cdef class SFTP:
                             self._get_sftp_error_str(),
                         )
                     )
-                buffer = f.read(1024)
+                buffer = f.read(SFTP_MAX_CHUNK)
             sftp.sftp_close(rf)
 
     def get(self, remote_file, local_file):
         cdef sftp.sftp_file rf
-        cdef char read_buffer[1024]
+        cdef char *read_buffer = NULL
+        cdef sftp.sftp_attributes attrs
 
         remote_file_b = remote_file
         if isinstance(remote_file_b, unicode):
             remote_file_b = remote_file.encode("utf-8")
 
+        attrs = sftp.sftp_stat(self._libssh_sftp_session, remote_file_b)
+        if attrs is NULL:
+            raise LibsshSFTPException("Failed to stat the remote file [%s]. Error: [%s]"
+                                      % (remote_file, self._get_sftp_error_str()))
+        file_size = attrs.size
+
         rf = sftp.sftp_open(self._libssh_sftp_session, remote_file_b, O_RDONLY, sftp.S_IRWXU)
         if rf is NULL:
             raise LibsshSFTPException("Opening remote file [%s] for read failed with error [%s]" % (remote_file, self._get_sftp_error_str()))
 
-        with open(local_file, 'wb') as f:
-            while True:
-                file_data = sftp.sftp_read(rf, <void *>read_buffer, sizeof(char) * 1024)
-                if file_data == 0:
-                    break
-                elif file_data < 0:
-                    sftp.sftp_close(rf)
-                    raise LibsshSFTPException("Reading data from remote file [%s] failed with error [%s]"
-                                              % (remote_file, self._get_sftp_error_str()))
-
-                bytes_written = f.write(read_buffer[:file_data])
-                if bytes_written and file_data != bytes_written:
-                    sftp.sftp_close(rf)
-                    raise LibsshSFTPException("Number of bytes [%s] read from remote file [%s]"
-                                              " does not match number of bytes [%s] written to local file [%s]"
-                                              " due to error [%s]"
-                                              % (file_data, remote_file, bytes_written, local_file, self._get_sftp_error_str()))
+        try:
+            with open(local_file, 'wb') as f:
+                buffer_size = min(SFTP_MAX_CHUNK, file_size)
+                read_buffer = <char *>PyMem_Malloc(buffer_size)
+                if read_buffer is NULL:
+                    raise LibsshSFTPException("Memory allocation error")
+
+                while True:
+                    file_data = sftp.sftp_read(rf, <void *>read_buffer, sizeof(char) * buffer_size)
+                    if file_data == 0:
+                        break
+                    elif file_data < 0:
+                        sftp.sftp_close(rf)
+                        raise LibsshSFTPException("Reading data from remote file [%s] failed with error [%s]"
+                                                  % (remote_file, self._get_sftp_error_str()))
+
+                    bytes_written = f.write(read_buffer[:file_data])
+                    if bytes_written and file_data != bytes_written:
+                        sftp.sftp_close(rf)
+                        raise LibsshSFTPException("Number of bytes [%s] read from remote file [%s]"
+                                                  " does not match number of bytes [%s] written to local file [%s]"
+                                                  " due to error [%s]"
+                                                  % (file_data, remote_file, bytes_written, local_file, self._get_sftp_error_str()))
+        finally:
+            if read_buffer is not NULL:
+                PyMem_Free(read_buffer)
         sftp.sftp_close(rf)
 
     def close(self):

tests/unit/sftp_test.py

@@ -8,6 +8,8 @@
 
 import pytest
 
+from pylibsshext.sftp import SFTP_MAX_CHUNK
+
 
 @pytest.fixture
 def sftp_session(ssh_client_session):
@@ -21,16 +23,17 @@ def sftp_session(ssh_client_session):
 
 
 @pytest.fixture(
-    params=(32, 1024 + 1),
+    params=(32, SFTP_MAX_CHUNK + 1),
     ids=('small-payload', 'large-payload'),
 )
 def transmit_payload(request: pytest.FixtureRequest) -> bytes:
     """Generate binary test payloads of assorted sizes.
 
     The choice 32 is arbitrary small value.
 
-    The choice 1024 + 1 is meant to be 1B larger than the chunk size used in
-    :file:`sftp.pyx` to make sure we excercise at least two rounds of reading/writing.
+    The choice SFTP_MAX_CHUNK + 1 (32kB + 1B) is meant to be 1B larger than the chunk
+    size used in :file:`sftp.pyx` to make sure we excercise at least two rounds of
+    reading/writing.
     """
     payload_len = request.param
     random_bytes = [ord(random.choice(string.printable)) for _ in range(payload_len)]