Merge lp:~robru/launchpadlib/fix-cache-creation into lp:launchpadlib

[Robert Bruce Park]

Working on a charm for porter boxes, trying to run stephane's sbuild-launchpad-chroot script a few times in parallel and discovered this race condition. Basically different processes all see the directory as not existing and then all try to create it. One wins the race and the rest lose.

[Dimitri John Ledkov (xnox)]

Looks good. Can it be checked that the error that is eaten is in fact EEXIST?

[Barry Warsaw (barry)]

On Nov 03, 2015, at 04:24 PM, Dimitri John Ledkov wrote:

>Looks good. Can it be checked that the error that is eaten is in fact EEXIST?

If that's specifically what you're trying to catch, and you have to remain
Python 2.7 compatible, the idiom is:

import errno

       try:
           os.makedirs(launchpadlib_dir, 0o700)
       except OSError as error:
           if error.errno != errno.EEXIST:
               raise

Of course, we can do better in Python 3. :)

[Dimitri John Ledkov (xnox)]

@barry launchpadlib must be bi-lingual for now, as not everything has yet ported over to python3.

[lp:~robru/launchpadlib/fix-cache-creation]

146. By Robert Bruce Park on 2015-11-03

Re-raise if errno not EEXIST.

[Robert Bruce Park (robru)]

Thanks barry, yeah I agree python3 is significantly better in this regard but I don't want to be the guy who breaks py2 support in lplib just yet ;-)

[Robert Bruce Park (robru)]

Here's the traceback this fixes btw:

https://pastebin.canonical.com/143314/

[Barry Warsaw (barry)]

On Nov 03, 2015, at 04:34 PM, Dimitri John Ledkov wrote:

>@barry launchpadlib must be bi-lingual for now, as not everything has yet
>ported over to python3.

Yep. That's why I gave the bilingual compatible idiom. :)

[Colin Watson (cjwatson)]

review: Approve

[Robert Bruce Park (robru)]

Thanks Colin!

=== modified file 'src/launchpadlib/launchpad.py'
--- src/launchpadlib/launchpad.py	2015-07-06 17:58:11 +0000
+++ src/launchpadlib/launchpad.py	2015-11-03 16:35:25 +0000
@@ -22,6 +22,7 @@
     ]
 
 import os
+import errno
 try:
     from urllib.parse import urlsplit
 except:
@@ -600,8 +601,11 @@
         if launchpadlib_dir[:1] == '~':
             raise ValueError("Must set $HOME or pass 'launchpadlib_dir' to "
                 "indicate location to store cached data")
-        if not os.path.exists(launchpadlib_dir):
+        try:
             os.makedirs(launchpadlib_dir, 0o700)
+        except OSError as err:
+            if err.errno != errno.EEXIST:
+                raise
         os.chmod(launchpadlib_dir, 0o700)
         # Determine the real service root.
         service_root = uris.lookup_service_root(service_root)
@@ -609,6 +613,9 @@
         scheme, host_name, path, query, fragment = urlsplit(service_root)
         service_root_dir = os.path.join(launchpadlib_dir, host_name)
         cache_path = os.path.join(service_root_dir, 'cache')
-        if not os.path.exists(cache_path):
+        try:
             os.makedirs(cache_path, 0o700)
+        except OSError as err:
+            if err.errno != errno.EEXIST:
+                raise
         return (service_root, launchpadlib_dir, cache_path, service_root_dir)