Modify feed url before issueing request, allow adding to config with form_alter

The general case is modifying the feed URL before you use it for download. I've just recently had a similar use case where I wanted to sign the URL using message signing library before using it to issue a request.

I am not sure yet whether a hook for modifying the URL is a good idea. The reason is that a hook won't negotiate which implementer modifies a URL and more likely than not more than one implementer modifying a URL will break it.

For now, I recommend extending FeedsHTTPFetcher and overriding the fetch() method.

Please refer to documentation and code to learn how to write a plugin (=extend one of the plugin classes): http://drupal.org/node/622696

I've been thinking more about this request. If we add an alter hook to the URL (= $source_config) before fetching, chances are we'd like to be able to also add settings to an importer configuration via form_alter. This is currently not possible because any setConfig() or addConfig() on a configurable object will be filtered by the default values declared in defaultConfig().

This patch:

A) adds a hook_feeds_prefetch_alter(&$source_config, $fetcher);
B) modifies FeedsConfigurable so that defaultConfig() is not mandatory anymore. This allows third party modules to add feeds importer settings via form_alter.

For an example on how to use this API, see here: http://pastebin.com/m297509d1

Ignore patches in #4.

"in configDefaults." needs to be "in configDefaults()."

I am not really sold on hook_feeds_prefetch_alter(). It's very general and thus suggests that we add such a hook invocation to all fetchers, maybe even a corresponding hook invokation to all parsers and processors. I'm considering renaming it to sth more specific that does not raise expectations on what else should be modifyable...

Reroll after recent commits.

I've committed the modification to FeedsConfigurable class (#4 B) as it is a good think either way.

Rerolled patch to reflect changes.

Commit from #8 breaks test I had to roll back : (.

Rerolling... this patch should apply, but configDefaultsMerged() can cause infinite loops.

Closed because Drupal 6 is no longer supported. If the issue verifiably applies to later versions, please reopen with details and update the version.