findXenSourceBackups: refactor and log reason for ignoring

Previous code structure was that of 2 nested if's, with nominal code
path being in the positive branch of each if.  Adding another
condition will bring a need for logging the reason to ignore a backup,
but doing that by converting inner `if` to an `elif` chain (arguably
the simplest modification) would bring the nominal code path to switch
to the negative/last branch of the inner `elif` chain.  At the same
time, the outer `if` would not seem special enough to deserve it
special place (and the cyclomatic complexity).

This commit leverages the existing `try:except:` block to switch to an
"error out the loop" pattern, where the nominal code path is just
linear.

Using `StopIteration` may feel like an abuse, but:
- it is the only standard `Exception` that is not a `StandardError` or
  a `Warning`, and defining a new one could seem overkill
- the closest alternative would be a `while True` loop and breaking out
  in both exceptional code paths and at the end of nominal path, where
  the `break` statements would "stop the iteration" as well

Signed-off-by: Yann Dirson <[email protected]>

product.py

@@ -534,12 +534,25 @@ def findXenSourceBackups():
         b = None
         try:
             b = util.TempMount(p, 'backup-', ['ro'], 'ext3')
-            if os.path.exists(os.path.join(b.mount_point, '.xen-backup-partition')):
-                backup = XenServerBackup(p, b.mount_point)
-                logger.log("Found a backup: %s" % (repr(backup),))
-                if backup.version >= XENSERVER_MIN_VERSION and \
-                        backup.version <= THIS_PLATFORM_VERSION:
-                    backups.append(backup)
+            if not os.path.exists(os.path.join(b.mount_point, '.xen-backup-partition')):
+                raise StopIteration()
+
+            backup = XenServerBackup(p, b.mount_point)
+            logger.log("Found a backup: %s" % (repr(backup),))
+
+            if backup.version < XENSERVER_MIN_VERSION:
+                logger.log("findXenSourceBackups: ignoring, platform too old: %s < %s" %
+                           (backup.version, XENSERVER_MIN_VERSION))
+                raise StopIteration()
+            if backup.version > THIS_PLATFORM_VERSION:
+                logger.log("findXenSourceBackups: ignoring later platform: %s > %s" %
+                           (backup.version, THIS_PLATFORM_VERSION))
+                raise StopIteration()
+
+            backups.append(backup)
+
+        except StopIteration:
+            pass
         except Exception as ex:
             logger.log("findXenSourceBackups caught exception for partition %s: %s" % (p, ex))
             pass