Bug 1567619 - Avoid double stat in preprocessed file installs; r=glandium

Followup to bug 1566044.

Differential Revision: https://phabricator.services.mozilla.com/D38763

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozpack/files.py

@@ -148,10 +148,14 @@ class BaseFile(object):
         # enough precision.
         dest_mtime = int(os.path.getmtime(dest) * 1000)
         for input in inputs:
-            if not os.path.exists(input):
+            try:
+                src_mtime = int(os.path.getmtime(input) * 1000)
+            except OSError as e:
+                if e.errno == errno.ENOENT:
                     # If an input file was removed, we should update.
                     return True
-            if dest_mtime < int(os.path.getmtime(input) * 1000):
+                raise
+            if dest_mtime < src_mtime:
                 return True
         return False