mount tmpfs to /var/cache/apt/archives/ to prevent install failure (#18)

* fix issue with "You don't have enough free space in /var/cache/apt/archives/"

Also, don't afraid users with error messages when fail to mount / ro again

Author: lduboeuf

ubports-qa

@@ -50,18 +50,22 @@ class WritableRootFS:
         LOG.debug("Attempting to mount rootfs read-write.")
         ensure_root()
         subprocess.run(["mount", "-o", "rw,remount", "/"])
+        LOG.debug("Making sure we have enough free space for archives.")
+        subprocess.run(["mount", "-t", "tmpfs", "tmpfs", "/var/cache/apt/archives"])
 
     @classmethod
     def attempt_unmount(cls):
+        LOG.debug("Unmounting tmpfs")
+        ensure_root()
+        subprocess.run(["umount", "/var/cache/apt/archives"])
         LOG.debug("Attempting to mount rootfs read-only.")
         if not os.path.exists("/userdata/.writable_image"):
-            ensure_root()
             os.sync()
             try:
                 subprocess.run(["mount", "-o", "ro,remount", "/"], check=True)
             except subprocess.CalledProcessError:
-                LOG.error("Failed to remount root filesystem read-only.")
-                LOG.error("Please consider rebooting your device.")
+                LOG.warning("Failed to remount root filesystem read-only.")
+                LOG.warning("Please consider rebooting your device.")
 
 
 def ensure_root():