Re: Does parallel make require guards against duplicate actions?

FWIW, the old make rule was

fmgroids.h fmgrtab.c: (deps)

which is now

fmgroids.h: fmgrtab.c ;

fmgrtab.c: (deps)

I was going by this comment in parser/Makefile:

# There is no correct way to write a rule that generates two files.
# Rules with two targets don't have that meaning, they are merely
# shorthand for two otherwise separate rules. To be safe for parallel
# make, we must chain the dependencies like this.

On Mon, Jan 4, 2010 at 6:58 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> The old Gen_fmgrtab.sh script used temporary file names that included
> its process PID.  It had this comment about that:
>
> # We use the temporary files to avoid problems with concurrent runs
> # (which can happen during parallel make).
>
> The new implementation uses temp files that just have ".tmp" appended to
> the target file name.  If there is a risk that "make -j" will run the
> same action twice in parallel, this isn't good enough.  While it
> wouldn't be too tough to add the PID to the scripts, I wonder whether
> this comment is about a real problem or just a flight of fancy.  It
> doesn't seem to me that parallel make ought to be stupid enough to
> do the same action twice.  Anybody know?
>
>                        regards, tom lane
>
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