_views_fetch_data() prone to inconsistency (Especially with redis / memcache)

Problem/Motivation

The logic in _views_fetch_data() is erroneous:
Call-stack to create issue:

1. Call _views_fetch_data('table_a') using a table.
2. Call _views_fetch_data('table_b') using a table.
3. Now the static cache ($cache) in _views_fetch_data() will contain those two tables
4. Now call _views_fetch_data() without a table. And imagine the main cache entry was deleted from the cache earlier (e.g. Redis LRU / memcache cache item eviction).
   Here's what _views_fetch_data() does:
   1. Fetch the full data from cache:
      

      $data = views_cache_get('views_data', TRUE);
      

   2. If cache data where found, store them into the static cache:
      

      if (!empty($data->data)) {
        $cache = $data->data;
      }
      

   3. (ERROR) Check if $cache contains data - rebuild if empty:
      

      if (empty($cache)) {
        $cache = _views_fetch_data_build();
      }
      $fully_loaded = TRUE;
      

      Remember, we called _views_fetch_data() before this with tables, which means the static cache ($cache) wont be empty even if the full cache isn't available! So we miss a maybe crucial cache rebuild here.
      And it gets worse, $fully_loaded is set to TRUE, which means subsequent calls to _views_fetch_data() with a table won't even try to restore missing table cache entries but just mark them as missing and thus persisting the issue for every table.

This error specifically affects redis (configured as with max-memory and LRU) as well as memcache since those caching backends will evict "random" items if the memory limit is reached.

Proposed resolution

Solution to this: Use the same code to fetch / rebuild the general cache as in the table specific part of _views_fetch_data():

  // Try to load the full views cache.
  if ($data = views_cache_get('views_data', TRUE)) {
	$cache = $data->data;
  }
  else {
	// No cache entry, rebuild.
	$cache = _views_fetch_data_build();
  }

Remaining tasks

User interface changes

API changes