Re: [PATCH v5 2/3] Makefile: add Perl runtime prefix support

On Mon, Jan 08 2018, Dan Jacques jotted:

On Mon, 08 Jan 2018, Ævar Arnfjörð Bjarmason replied:

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+# it. This is intentionally separate from RUNTIME_PREFIX so that notably Windows
+# can hard-code Perl library paths while still enabling RUNTIME_PREFIX
+# resolution.

Maybe we covered this in previous submissions, but refresh my memory,
why is the *_PERL define still needed? Reading this explanation doesn't
make sense to me, but I'm probably missing something.

If we have a system where we have some perl library paths on the system
we want to use, then they'll still be in @INC after our 'use lib'-ing,
so we'll find libraries there.

The only reason I can think of for doing this for C and not for Perl
would be if you for some reason want to have a git in /tmp/git but then
use a different version of the Git.pm from some system install, but I
can't imagine why.

The reason is entirely due to the way Git-for-Windows is structured. In
Git-for-Windows, Git binaries are run directly from Windows, meaning that
they require RUNTIME_PREFIX resolution. However, Perl scripts are run from
a MinGW universe, within which filesystem paths are fixed. Therefore,
Windows Perl scripts don't require a runtime prefix resolution.

This makes sense because they are clearly functional right now without this
patch enabled :) However, we don't have the luxury of running Perl in a
separate universe on other OSes, so this patch is necessary for them.

I created a separate option because I wanted to ensure that I don't change
anything fundamental in Windows, which currently relies on runtime prefix
resoultion. On all other operating systems, Perl and binary runtime prefix
resolution is disabled by default, so if this patch set does end up having
bugs or edge cases in the Perl runtime prefix code, it won't inpact anybody's
current builds.

I can foresee a future where Windows maintainers decide that
PERL_RUNTIME_PREFIX is fine for Windows and merge the two options; however,
I didn't want to force that decision in the initial implementation.

Makes sense, well not really, But that's not your fault, but Windows's.

I do think you're being overly conservative here, the perl change is no
more invasive than the C changes (less so actually), and from anyone
who's not on Windows it makes sense to be able to enable this with just
RUNTIME_PREFIX=YesPlease, and have NO_RUNTIME_PREFIX_PERL=NotNeededHere
for Windows, if someone ends up needing it.

We usually hide stuff you might want in general, but isn't needed on one
special snowflake platform behind NO_*, not the other way around.

Maybe others disagre... 

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+	# GIT_EXEC_PATH is supplied by `git` or the test suite. Otherwise, resolve
+	# against the runtime path of this script.
+	require FindBin;
+	require File::Spec;
+	(my $prefix = $ENV{GIT_EXEC_PATH} || $FindBin::Bin) =~ s=${gitexecdir_relative}$==;

So why are we falling back on $FindBin::Bin? Just so you can do
e.g. /tmp/git2/libexec/git-core/git-svn like you can do
/tmp/git2/libexec/git-core/git-status, i.e. will this never be false if
invoked via "git"?

I don't mind it, just wondering if I'm missing something and we need to
use the fallback path in some "normal" codepath.

Yep, exactly. The ability to directly invoke Perl scripts is currently
functional in non-runtime-prefix builds, so enabling it in runtime-prefix
builds seemed appropriate. I have found this useful for testing.

However, since GIT_EXEC_PATH is probably going to be the common path,
I'll scoop the FindBin code (including the "require" statement) into a
conditional in v6 and use it only when GIT_EXEC_PATH is empty.

Both make sense.

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+	return File::Spec->catdir($prefix, $relpath);

I think you initially got some version of this from me (or not), so this
is probably my fault, but reading this again I think this would be
better as just:

    return $prefix . '@@PATHSEP@@' . $relpath;

I.e. right after this we split on @@PATHSEP@@, and that clearly works
(as opposed to using File::Spec->splitpath) since we've used it
forever.

Better just to use the same idiom on both ends to not leave the reader
wondering why we can split paths one way, but need to join them another
way.

PATHSEP is the path separator (":"), as opposed to the filesystem separator
("/"). We split on PATHSEP below b/c we need to "use lib" as an array, but
it may be a ":"-delimited string.

Yes, silly me. Nevermind.