chore(usbf): create a loopback device when a file is used

When a file is used as backing store for the shared storage,
create a loopback device to make sure writes are contained
within the limits of the file.

Signed-off-by: Cedric Hombourger <[email protected]>

.github/wordlist.txt

@@ -62,6 +62,7 @@ JQC
 keyrings
 kvm
 LEDs
+loopback
 LTS
 maxdepth
 microSD

docs/config.rst

@@ -396,13 +396,15 @@ configuration to expose either a file or a physical device or partition to
 the DUT as a Mass Storage device. The following settings are supported:
 
 * ``device``: string [optional]
-  Block device for the shared storage as seen on the host. This is the
-  same as ``file`` but a warning will be issued if the specified file
-  is not a block device. When both ``device`` and ``file`` are set,
-  ``file`` will be used.
+  Block device for the shared storage as seen on the host. A systemd
+  dependency is added to ensure the service is started only after the
+  specified block device was found. A warning is issued if the input
+  is not a block device.
 
 * ``file``: string [optional]
-  File or block device for the shared storage as seen on the host.
+  File for the shared storage: a loopback device will be created to make
+  sure that writes to the shared storage do not cause this file to
+  expand and leave the host without free space.
 
 Timeout settings
 ----------------

mtda/storage/usbf.py

@@ -12,6 +12,7 @@
 # System imports
 import os
 import stat
+import subprocess
 
 # Local imports
 import mtda.constants as CONSTS
@@ -26,9 +27,23 @@ def __init__(self, mtda):
         super().__init__(mtda)
         self.device = None
         self.file = None
+        self.loop = None
         self.mode = CONSTS.STORAGE.ON_HOST
         Composite.mtda = mtda
 
+    def cleanup(self):
+        self.mtda.debug(3, "storage.usbf.cleanup()")
+
+        result = None
+        if self.loop is not None:
+            self.mtda.debug(2, "storage.usbf.cleanup(): "
+                               "removing {}".format(self.loop))
+            cmd = ['/usr/sbin/losetup', '-d', self.loop]
+            self.loop = subprocess.check_call(cmd)
+
+        self.mtda.debug(3, "storage.usbf.cleanup(): {}".format(result))
+        return result
+
     """ Configure this storage controller from the provided configuration"""
     def configure(self, conf):
         self.mtda.debug(3, "storage.usbf.configure()")
@@ -44,8 +59,14 @@ def configure(self, conf):
                                "both 'file' ({}) and 'device' ({}) are set, "
                                "using 'file'".format(self.file, self.device))
             self.device = None
-
-        if self.file is None and self.device is not None:
+        elif self.device is None:
+            import atexit
+            cmd = ['/usr/sbin/losetup', '-f', '--show', self.file]
+            self.loop = subprocess.check_output(cmd).decode("utf-8").strip()
+            atexit.register(self.cleanup)
+            self.mtda.debug(2, "storage.usbf.configure(): created loopback "
+                               "device {} for {}".format(self.loop, self.file))
+        else:
             self.file = self.device
 
         if self.file is not None: