When Drush Make fails to apply patches...

The issue

These last few days, I had noticed a problem with Drush Make and patches: some patches, be they rolled by our team or from elsewhere, would apply without a glitch, but some others, which worked normally according to the test bot on Drupal.org, would fail to apply without any obvious reason.

I had mostly put it out of my list of pressing issues when I really had to use an old version of OpenLayers, 7.x-2.0-alpha2 to be specific, AND apply a patch fixing one of the bugs in that module: behaviors plugin not being located correctly (http://drupal.org/node/1898662 if you want details). So I rolled the patch, tested it locally, the qa.d.o bot applied it and did not report more errors than expected for that old version.... and my Drush Make install refused to apply it.

Here was the relevant excerpt:

projects[domain] = 3.7
projects[domain][patch][] = "http://drupal.org/files/domain-foreach_argument-1879502-1.patch"
; ...snip...
projects[openlayers] = 2.0-alpha2
projects[openlayers][patch][] = "http://drupal.org/files/0001-Fix-the-path-file-declaration-for-behaviors.patch"
The Domain patch applied normally, but the OpenLayers patch would't apply. What could be wrong ?

The diagnostic

After tracing my way into DrushMakeProject::applyPatches(), I got a more explicit message: neither git patch nor trusted old patch could locate the patched file, includes/openlayers.behaviors.inc. Why ?

Comparing a standalone checkout of OpenLayers 7.x-2.0-alpha2 and the one in /tmp/make_tmp_(some id)__build__/sites/all/modules/contrib/openlayers, the problem became more obvious: that file was missing, as well as a good number of others. What ? Download failure ?

Not in the least: checking openlayers.info, I noticed the version downloaded by OpenLayers was no longer the chosen 7.x-2.0-alpha2 specified in the Makefile and which it previously downloaded normally, but the currently latest 7.x-2.0-beta3. Ahah...

The fix

After digging a bit more into Drush, it appeared that whenever you specify an extra info bit about a project download, lines in the short format like projects[openlayers] = 2.0-alpha2 are ignored, so Drush downloads the latest published version. The fix became obvious: use the "extended syntax", like this, for the same excerpt:

projects[domain][type] = module
projects[domain][version] = 3.7
projects[domain][patch][] = "http://drupal.org/files/domain-foreach_argument-1879502-1.patch"
; ... snip ...
projects[openlayers][type] = module
projects[openlayers][version] = 2.0-alpha2
projects[openlayers][patch][] = "http://drupal.org/files/0001-Fix-the-path-file-declaration-for-behaviors.patch"

This also explained why the other patches applied normally: each of them had been rolled against the latest module version, so the specified version was ignored, but the version actually being downloaded ended up being the same, and the patch applied normally.