patches for PR 218, index corruption

[Dirk Bergstrom]

these patches fix PR 218, "PRs fail to show up in index".  it turns
out there were four separate race conditions that could cause index
corruption.

*) there was a race in the database locking code that allowed two
 processes to believe that they each had a full db lock.  the code
 checked to see if the lock file existed, and if it wasn't found, a
 file was created using creat().  there was a race between the check
 and the create, and the creat() call wasn't atomic.  changing
 lockfile creation to open(O_EXCL) fixed this.

*) gnats read in the index before locking the database, but didn't
 check to see if the index had been modified before writing out it's
 cached copy.  the race went like this: 

 0) gnatsd A is finishing up an edit on PR 123...
 1) gnatsd B reads index
 2) A writes new index, & unlocks DB
 3) B locks DB
 3) B edits PR 456, modifying index in memory
 4) B writes out index from memory

 thus A's changes to the index entry for PR 123 would be lost,
 overwritten by B's copy of the index.  i modified getFirstPr() so
 that it always calls checkPRChain() to check mtime of the index on
 disk before using data in memory; if cached data is stale, the index
 is re-read.  also added a final check in the writeIndex routine to
 catch problems and alert the administrator.  this adds a few stat()
 calls to an edit transaction, which is a small cost.  it has no
 effect on queries.

*) the PR locking code suffered from the same race as the database
 lock code.  i applied a similar fix.

*) the append & replace field-editing routines (pr-edit --append &
--replace options & gnatsd APPN & REPL commands) did not lock the
 database before editing.  thus, any other process could change the
 index while they were working.  added code to lock the db before
 taking either action.

the patches themselves, along with a copy of this message and appropriate 
changelog entries, are available at this URL:

http://www.otisbean.com/gnats/race-patch.txt

--
Dirk Bergstrom               address@hidden
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Juniper Networks Inc.,          Computer Geek
Tel: 707.433.0564           Fax: 707.433.0769