Subject: Re: [PATCH 1/3] t7610-mergetool: do not place pipelines headed by `yes` in subshells

From: SZEDER Gábor <[email-masked]>
Date: Mon, 10 Jun 2019 20:57:08 +0200

On Mon, Jun 10, 2019 at 08:29:56PM +0200, Johannes Schindelin wrote:
> Hi,
> 
> On Mon, 10 Jun 2019, SZEDER Gábor wrote:
> 
> > > diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
> > > index 5b61c10a9c..b67440882b 100755
> > > --- a/t/t7610-mergetool.sh
> > > +++ b/t/t7610-mergetool.sh
> > > @@ -131,13 +131,13 @@ test_expect_success 'custom mergetool' '
> > >  	git checkout -b test$test_count branch1 &&
> > >  	git submodule update -N &&
> > >  	test_must_fail git merge master &&
> > > -	( yes "" | git mergetool both ) &&
> > > -	( yes "" | git mergetool file1 file1 ) &&
> > > -	( yes "" | git mergetool file2 "spaced name" ) &&
> > > -	( yes "" | git mergetool subdir/file3 ) &&
> > > -	( yes "d" | git mergetool file11 ) &&
> > > -	( yes "d" | git mergetool file12 ) &&
> > > -	( yes "l" | git mergetool submod ) &&
> > > +	yes "" | git mergetool both &&
> > > +	yes "" | git mergetool file1 file1 &&
> > > +	yes "" | git mergetool file2 "spaced name" &&
> > > +	yes "" | git mergetool subdir/file3 &&
> > > +	yes "d" | git mergetool file11 &&
> > > +	yes "d" | git mergetool file12 &&
> > > +	yes "l" | git mergetool submod &&
> > >  	test "$(cat file1)" = "master updated" &&
> > >  	test "$(cat file2)" = "master new" &&
> > >  	test "$(cat subdir/file3)" = "master new sub" &&
> >
> > Another possibility for eliminating a few more subshells might be to
> > turn these
> >
> >   test "$(cat file1)" = "that"'
> >
> > checks into
> >
> >   echo that >expect &&
> >   test_cmp expect file1
> >
> > because 'test_cmp' on Windows first compares the two files in shell
> > and runs 'diff' only when there is a difference to report.
> 
> When you remember that spawning processes is much more expensive on
> Windows, still, than I/O, you will realize that this adds even more
> expense. Instead of a spawn & read, you are suggesting essentially a
> write, spawn, read & read, and that is only the best case.

No, instead of a spawn (the subshell of the command substitution), spawn
('cat'), read, I suggest a write, read, read (no subshell or external
process in 'mingw_test_cmp's main code path).

> In the worst case, it would be a write, spawn, read & read, spawn, read &
> read.

It would be a write, read, read, only one spawn (diff), read read, but
I assume that the tests succeed, so I ignore this worst case.

> (Even if the first spawn is an MSYS2 spawn on Windows, which is more
> expensive than the MINGW spawn for the `git diff`, if that is a `git diff`
> rather than `diff`, didn't check...)
> 
> So I am rather negative about this suggestion ;-)
> 
> Ciao,
> Dscho