Re: pgsql: Fix race condition in pg_ctl reading postmaster.pid.

On 15.10.2012 17:05, Tom Lane wrote:
> Heikki Linnakangas<heikki(dot)linnakangas(at)iki(dot)fi> writes:
>> Fix race condition in pg_ctl reading postmaster.pid.
>> If postmaster changed postmaster.pid while pg_ctl was reading it, pg_ctl
>> could overrun the buffer it allocated for the file. Fix by reading the
>> whole file to memory with one read() call.
>
> Maybe I'm just not awake enough, but that code doesn't look to me like
> it does the right thing with a non-newline-terminated file. Doesn't it
> drop the last character of the last line?

No. It loops until the next-to-last character when it searches for
newlines. When it hits that, it creates the final string to return,
which includes the final character, whether it's a newline or not.

BTW, an easy way to test this is to edit postmaster.opts on a running
server, and do "pg_ctl status". It prints out the contents of
postmaster.opts, as parsed by the function.

> Given the way that pg_ctl uses the file, I think that the old logic of
> "pretend the file ends with a newline" is wrong anyway. If we do manage
> to see an intermediate state of the file, it would be better to not
> return the last line at all than to return a truncated (a/k/a wrong)
> value for that line. So I'd vote to rejigger the logic to ignore any
> data after the last newline.

Good point. I'll rejigger..

- Heikki