plugin android: workaround to seek() bug, fixing file upload:

seek() is bugged on current version of P4A, arguments are inverted.
More details can be found at https://github.com/kivy/python-for-android/issues/1768

sat/plugins/plugin_misc_android.py

@@ -20,6 +20,7 @@
 import sys
 import os
 import os.path
+import tempfile
 from sat.core.i18n import _, D_
 from sat.core.constants import Const as C
 from sat.core.log import getLogger
@@ -27,6 +28,7 @@ from sat.core import exceptions
 from twisted.internet import reactor
 from twisted.internet import protocol
 from twisted.internet import error as int_error
+from twisted.web import client as web_client
 
 log = getLogger(__name__)
 
@@ -57,6 +59,38 @@ STATE_STOPPED = "stopped"
 STATES = (STATE_RUNNING, STATE_PAUSED, STATE_STOPPED)
 
 
+def determineLength_workaround(self, fObj):
+    """Method working around seek() bug on Android"""
+    try:
+        seek = fObj.seek
+        tell = fObj.tell
+    except AttributeError:
+        return web_client.UNKNOWN_LENGTH
+    originalPosition = tell()
+    seek(os.SEEK_END)
+    end = tell()
+    seek(os.SEEK_SET, originalPosition)
+    return end - originalPosition
+
+
+def patch_seek_bug():
+    """Check seek bug and apply a workaround if still here
+
+    cf. https://github.com/kivy/python-for-android/issues/1768
+    """
+    with tempfile.TemporaryFile() as f:
+        f.write(b'1234567890')
+        f.seek(0, os.SEEK_END)
+        size = f.tell()
+    if size == 10:
+        log.info(u"seek() bug not present anymore, workaround code can be removed")
+    else:
+        log.warning(u"seek() bug detected, applying a workaround")
+        web_client.FileBodyProducer._determineLength = determineLength_workaround
+
+patch_seek_bug()
+
+
 class FrontendStateProtocol(protocol.Protocol):
 
     def __init__(self, android_plugin):