Just after the last set of minor versions of Postgres has been released, the following commit has popped up on all the stable branches to improve a long-time weird behavior of pg_ctl start.
commit: 6bcce25801c3fcb219e0d92198889ec88c74e2ff author: Tom Lane date: Mon, 12 Oct 2015 18:30:36 -0400 Fix "pg_ctl start -w" to test child process status directly. pg_ctl start with -w previously relied on a heuristic that the postmaster would surely always manage to create postmaster.pid within five seconds. Unfortunately, that fails much more often than we would like on some of the slower, more heavily loaded buildfarm members. [...] Back-patch to all supported versions, both because the current behavior is buggy and because we must do that if we want the buildfarm failures to go away. Tom Lane and Michael Paquier
Before this commit pg_ctl used the assumption that a postmaster would create its PID file within 5 seconds after starting, hence running successively this command produced rather inconsistent exit codes. Take for example this sequence of commands:
$ pg_ctl -w start && echo "## 1st command run" && \ pg_ctl -w start && echo "## 2nd command run" && \ sleep 5 && \ pg_ctl -w start && echo "## 3rd command run" done server started ## 1st command run done server started ## 2nd command run pg_ctl: another server might be running; trying to start server anyway waiting for server to start.... pg_ctl: this data directory appears to be running a pre-existing postmaster stopped waiting pg_ctl: could not start server Examine the log output.
When using pg_ctl in wait mode (with -w), the first and second commands actually succeed, and that is only after a couple of seconds that pg_ctl reports an actual failure, symbolized by postmaster.pid being detected. Now, with the previous patch applied, things get more consistent. When running two times in a row “pg_ctl start” the second one will properly fail:
$ pg_ctl -w start && echo "## 1st command run" && \ pg_ctl -w start && echo "## 2nd command run" done server started ## 1st command run pg_ctl: another server might be running; trying to start server anyway waiting for server to start.... stopped waiting pg_ctl: could not start server Examine the log output.
The old logic based on system() is replaced by a combination of fork() and exec() which has the advantage to allow fetching the PID of the postmaster, that pg_ctl can reliably wait for using waitpid(). One limitation is on Windows, where what is waited for is not the postmaster PID but the PID of the shell process that launched the postmaster, so there is a need to wait for two or three seconds to have a reliable result. Still the new behavior is better than the old one that caused random failures in a handful of slow buildfarm machines, like Raspberry PIs.
This will be available in the next set of minor versions, aka 9.4.6, 9.3.11, 9.2.14 and 9.1.20. Production environments usually do not rely on that, still that’s something to be careful about, now things get right.