Invalidate header caches when spam rules change. closes #3473
User 'exg' on #mutt reported a problem when using header cache with IMAP and spam rules. Spam tags are retrieved from cache and not updated when spam rules change. 1. Enable header caching in muttrc 2. Set spam rules in muttrc 3. Use IMAP (or POP or Maildir). Your spam rules store spam tags into your message headers, and these get cached. 4. Quit mutt 5. Change your spam rules in muttrc 6. Relaunch mutt 7. Because of header caching, the rfc822 headers are not reparsed, and spam rules are not reapplied. User has outdated spam tags. Spam rules are applied during rfc822 header parsing because it's the only time we get a full and unadulterated view of all headers. Once header parse is done we have only processed headers and 'user headers' (X-headers). We might do spam rule matching against the struct HEADER copy of headers, except that: a. the header might not be in HEADER if it's non-standard and not an X-header; b. spam rules are regular expressions matching any arbitrary header line, and we would need to map these regexps to the header used to create the HEADER fields. This would change the syntax and behavior of spam rules dramatically. One solution, perhaps stopgap, is to invalidate the header cache when spam rules change. That's what this patch does. It alters the hcache versioning algorithm so that in addition to using the compiled-in checksum of mutt's structures definitions, we also mix in the user's runtime spam rules. Any time spam rules change the hcachever hash value changes, causing the hcache to be rewritten.
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