devices/loopback: enable partition scanning as an option

We can now add an entire device and then get the partitions added
to our environment for use, rather than to have to map each partition
in to a separate loopback device.

This is a prep patch for https://github.com/osbuild/osbuild/issues/1495

devices/org.osbuild.loopback

@@ -19,7 +19,9 @@ documentation for `osbuil.util.udev.UdevInhibitor`.
 
 import argparse
 import errno
+import glob
 import os
+import stat
 import sys
 from typing import Dict
 
@@ -35,6 +37,10 @@ SCHEMA = """
     "type": "string",
     "description": "File to associate with the loopback device"
   },
+  "partscan": {
+    "type": "boolean",
+    "description": "Perform partition scanning after device creation"
+  },
   "start": {
     "type": "number",
     "description": "Start of the data segment (in sectors)",
@@ -71,10 +77,9 @@ class LoopbackService(devices.DeviceService):
         lock = UdevInhibitor.for_device(lo.LOOP_MAJOR, lo.minor)
         lo.on_close = lambda _l: lock.release()
 
-    def make_loop(self, fd: int, offset, sizelimit, lock):
+    def make_loop(self, fd: int, offset, sizelimit, lock, partscan):
         if not sizelimit:
-            stat = os.fstat(fd)
-            sizelimit = stat.st_size - offset
+            sizelimit = os.fstat(fd).st_size - offset
         else:
             sizelimit *= self.sector_size
 
@@ -83,7 +88,7 @@ class LoopbackService(devices.DeviceService):
                                   offset=offset,
                                   sizelimit=sizelimit,
                                   blocksize=self.sector_size,
-                                  partscan=False,
+                                  partscan=partscan,
                                   autoclear=True)
 
         return lo
@@ -94,12 +99,13 @@ class LoopbackService(devices.DeviceService):
         start = options.get("start", 0) * self.sector_size
         size = options.get("size")
         lock = options.get("lock", False)
+        partscan = options.get("partscan", False)
 
         path = os.path.join(tree, filename.lstrip("/"))
 
         self.fd = os.open(path, os.O_RDWR | os.O_CLOEXEC)
         try:
-            self.lo = self.make_loop(self.fd, start, size, lock)
+            self.lo = self.make_loop(self.fd, start, size, lock, partscan)
         except Exception as error:  # pylint: disable: broad-except
             self.close()
             raise error from None
@@ -111,6 +117,15 @@ class LoopbackService(devices.DeviceService):
             dir_fd = os.open(devpath, os.O_CLOEXEC | os.O_PATH)
             with ctx.suppress_oserror(errno.EEXIST):
                 self.lo.mknod(dir_fd)
+            # If partscan was enabled let's find any partition
+            # based devices that were added (i.e. loop0p1) and
+            # copy them into our custom /dev/ directory:
+            if partscan:
+                for device in glob.glob(os.path.join("/dev/", f"{self.lo.devname}p*")):
+                    os.mknod(os.path.basename(device),
+                             mode=(0o600 | stat.S_IFBLK),
+                             device=os.stat(device).st_rdev,
+                             dir_fd=dir_fd)
         finally:
             if dir_fd > -1:
                 os.close(dir_fd)