Drupal 7 Check for Available Updates Solution

On a Drupal 7 website's "Available updates" page (admin/reports/updates/update), if and when you request that Drupal manually check for updates (of modules and themes), it can sometimes fail to get available update data for most or all of the projects, regardless of how many times you retry the process. The problem is caused by errant records in the cache_update table in the Drupal database, which apparently are not removed by the update request. In theory, this should be fixable by requesting that the system clear all of the caches (admin/config/development/performance), but even that operation can fail to empty the table. In this case, you must empty — "truncate", in database parlance, not "drop" — that table yourself and then manually check for update data again.