Restructure the shutdown procedure for the archiver process to allow it to
finish archiving everything (when there's no error), and to eliminate various hazards as best we can. This fixes a previous 8.3 patch that caused the postmaster to kill and then restart the archiver during shutdown (!?). The new behavior is that the archiver is allowed to run unmolested until the bgwriter has exited; then it is sent SIGUSR2 to tell it to do a final archiving cycle and quit. We only SIGQUIT the archiver if we want a panic stop; this is important since SIGQUIT will also be sent to any active archive_command. The postmaster also now doesn't SIGQUIT the stats collector until the bgwriter is done, since the bgwriter can send stats messages in 8.3. The postmaster will not exit until both the archiver and stats collector are gone; this provides some defense (not too bulletproof) against conflicting archiver or stats collector processes being started by a new postmaster instance. We continue the prior practice that the archiver will check for postmaster death immediately before issuing any archive_command; that gives some additional protection against conflicting archivers. Also, modify the archiver process to notice SIGTERM and refuse to issue any more archive commands if it gets it. The postmaster doesn't ever send it SIGTERM; we assume that any such signal came from init and is a notice of impending whole-system shutdown. In this situation it seems imprudent to try to start new archive commands --- if they aren't extremely quick they're likely to get SIGKILL'd by init. All per discussion.
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