[PATCH v4 3/4] Makefile: add Perl runtime prefix support

[Dan Jacques <dnj@google.com>]

Add a new Makefile flag, RUNTIME_PREFIX_PERL, which, when enabled,
configures Perl scripts to locate the Git installation's Perl support
libraries by resolving against the script's path, rather than
hard-coding that path at build-time.

Enabling RUNTIME_PREFIX_PERL overrides the system-specific Perl script
installation path generated by MakeMaker to force installation into a
platform-neutral location, "<prefix>/share/perl5".

This change enables Git's Perl scripts to work when their Git installation
is relocated or moved to another system.

Signed-off-by: Dan Jacques <dnj@google.com>
Thanks-to: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Thanks-to: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 Makefile                               | 40 +++++++++++++++++++++++++++++++++-
 perl/header_runtime_prefix.pl.template | 24 ++++++++++++++++++++
 2 files changed, 63 insertions(+), 1 deletion(-)
 create mode 100644 perl/header_runtime_prefix.pl.template

diff --git a/Makefile b/Makefile
index 80904f8b0..741d1583f 100644
--- a/Makefile
+++ b/Makefile
@@ -425,6 +425,10 @@ all::
 #
 # to say "export LESS=FRX (and LV=-c) if the environment variable
 # LESS (and LV) is not set, respectively".
+#
+# Define RUNTIME_PREFIX_PERL to configure Git's PERL commands to locate Git
+# support libraries relative to their filesystem path instead of hard-coding
+# it.
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -485,6 +489,7 @@ pathsep = :
 
 mandir_relative = $(patsubst $(prefix)/%,%,$(mandir))
 infodir_relative = $(patsubst $(prefix)/%,%,$(infodir))
+sharedir_relative = $(patsubst $(prefix)/%,%,$(sharedir))
 htmldir_relative = $(patsubst $(prefix)/%,%,$(htmldir))
 
 export prefix bindir sharedir sysconfdir gitwebdir localedir
@@ -1967,7 +1972,38 @@ perl/PM.stamp: GIT-PERL-DEFINES FORCE
 PERL_HEADER_TEMPLATE = perl/header_fixed_prefix.pl.template
 
 PERL_DEFINES := $(PERL_PATH_SQ) $(PERLLIB_EXTRA_SQ)
-PERL_DEFINES += $(NO_PERL_MAKEMAKER)
+PERL_DEFINES += $(NO_PERL_MAKEMAKER) $(RUNTIME_PREFIX_PERL)
+
+# Support Perl runtime prefix. In this mode, a different header is installed
+# into Perl scripts. This header expects both the scripts and their support
+# library to be installed relative to $(prefix), and resolves the path to
+# the Perl libraries (perllibdir) from the executable's current path
+# (gitexecdir).
+#
+# This configuration requires both $(perllibdir) and $(gitexecdir) to be
+# relative paths, and will error if this is not the case.
+ifdef RUNTIME_PREFIX_PERL
+
+PERL_HEADER_TEMPLATE = perl/header_runtime_prefix.pl.template
+PERL_DEFINES += $(gitexecdir)
+
+# RUNTIME_PREFIX_PERL requires a $(perllibdir) value.
+ifeq ($(perllibdir),)
+perllibdir = $(sharedir_relative)/perl5
+endif
+
+ifneq ($(filter /%,$(firstword $(gitexecdir))),)
+$(error RUNTIME_PREFIX_PERL requires a relative gitexecdir, not: $(gitexecdir))
+endif
+gitexecdir_relative_SQ = $(gitexecdir_SQ)
+
+ifneq ($(filter /%,$(firstword $(perllibdir))),)
+$(error RUNTIME_PREFIX_PERL requires a relative perllibdir, not: $(perllibdir))
+endif
+perllibdir_relative_SQ = $(perllibdir_SQ)
+
+endif
+
 PERL_DEFINES += $(perllibdir)
 
 perl/perl.mak: GIT-CFLAGS GIT-PREFIX perl/Makefile perl/Makefile.PL
@@ -2001,6 +2037,8 @@ GIT-PERL-HEADER: $(PERL_HEADER_TEMPLATE) GIT-PERL-DEFINES perl/perl.mak Makefile
 	INSTLIBDIR="$$INSTLIBDIR$${INSTLIBDIR_EXTRA:+:$$INSTLIBDIR_EXTRA}" && \
 	sed -e 's=@@PATHSEP@@=$(pathsep)=g' \
 	    -e 's=@@INSTLIBDIR@@='$$INSTLIBDIR'=g' \
+	    -e 's=@@GITEXECDIR@@=$(gitexecdir_relative_SQ)=g' \
+	    -e 's=@@PERLLIBDIR@@=$(perllibdir_relative_SQ)=g' \
 	    $< >$@+ && \
 	mv $@+ $@
 
diff --git a/perl/header_runtime_prefix.pl.template b/perl/header_runtime_prefix.pl.template
new file mode 100644
index 000000000..fb9a9924d
--- /dev/null
+++ b/perl/header_runtime_prefix.pl.template
@@ -0,0 +1,24 @@
+# BEGIN RUNTIME_PREFIX_PERL generated code.
+#
+# This finds our Git::* libraries relative to the script's runtime path.
+BEGIN {
+	use lib split /@@PATHSEP@@/,
+	(
+		$ENV{GITPERLLIB}
+		||
+		do {
+			require FindBin;
+			require File::Spec;
+			my $gitexecdir_relative = '@@GITEXECDIR@@';
+			my $perllibdir_relative = '@@PERLLIBDIR@@';
+
+			($FindBin::Bin =~ m=${gitexecdir_relative}$=) ||
+			    die('Unrecognized runtime path.');
+			my $prefix = substr($FindBin::Bin, 0, -length($gitexecdir_relative));
+			my $perllibdir = File::Spec->catdir($prefix, $perllibdir_relative);
+			(-e $perllibdir) || die("Invalid library path: $perllibdir");
+			$perllibdir;
+		}
+	);
+}
+# END RUNTIME_PREFIX_PERL generated code.
-- 
2.15.0.chromium12