perl Git::LoadCPAN: emit better errors under NO_PERL_CPAN_FALLBACKS

Before my 20d2a30f8f ("Makefile: replace perl/Makefile.PL with simple
make rules", 2017-12-10) on an OS package that removed the
private-Error.pm copy we carried around manually removing the OS's
Error.pm would yield:

$ git add -p
Can't locate Error.pm in @INC (you may need to install the Error module) [...]

Now, before this change we'll instead emit this more cryptic error:

$ git add -p
BUG: '/usr/share/perl5/Git/FromCPAN' should be a directory! at /usr/share/perl5/Git/Error.pm line 36.

This is a confusing error. Now if the new NO_PERL_CPAN_FALLBACKS
option is specified and we can't find the module we'll instead emit:

$ /tmp/git/bin/git add -p
BUG: The 'Error' module is not here, but NO_PERL_CPAN_FALLBACKS was set!

[...]

Where [...] is the lengthy explanation seen in the change below, which
explains what the potential breakage is, and how to fix it.

The reason for checking @@NO_PERL_CPAN_FALLBACKS@@] against the empty
string in Perl is as opposed to checking for a boolean value is that
that's (as far as I can tell) make's idea of a string that's set, and
e.g. NO_PERL_CPAN_FALLBACKS=0 is enough to set NO_PERL_CPAN_FALLBACKS.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/INSTALL b/INSTALL
index 808e07b6599476afc8a0b3a385e73df8b7f9c63d..60e515eaf7432e77d8db8837b7bb95e105ca2126 100644 (file)
--- a/INSTALL
+++ b/INSTALL
@@ -88,9 +88,9 @@ Issues of note:
        export GIT_EXEC_PATH PATH GITPERLLIB
 
  - By default (unless NO_PERL is provided) Git will ship various perl
-   scripts & libraries it needs. However, for simplicity it doesn't
-   use the ExtUtils::MakeMaker toolchain to decide where to place the
-   perl libraries. Depending on the system this can result in the perl
+   scripts. However, for simplicity it doesn't use the
+   ExtUtils::MakeMaker toolchain to decide where to place the perl
+   libraries. Depending on the system this can result in the perl
    libraries not being where you'd like them if they're expected to be
    used by things other than Git itself.
 
@@ -102,6 +102,11 @@ Issues of note:
    Will result in e.g. perllibdir=/usr/share/perl/5.26.1 on Debian,
    perllibdir=/usr/share/perl5 (which we'd use by default) on CentOS.
 
+ - Unless NO_PERL is provided Git will ship various perl libraries it
+   needs. Distributors of Git will usually want to set
+   NO_PERL_CPAN_FALLBACKS if NO_PERL is not provided to use their own
+   copies of the CPAN modules Git needs.
+
  - Git is reasonably self-sufficient, but does depend on a few external
    programs and libraries.  Git can be used without most of them by adding
    the approriate "NO_<LIBRARY>=YesPlease" to the make command line or

diff --git a/Makefile b/Makefile
index 518c5f6be057cc7d864465b6214019e8ceabca32..4e0cdb3ca4a3c722c12b7219c72c978dddac45bd 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -2314,11 +2314,14 @@ all:: $(LIB_PERL_GEN)
 ifndef NO_PERL_CPAN_FALLBACKS
 all:: $(LIB_CPAN_GEN)
 endif
+NO_PERL_CPAN_FALLBACKS_SQ = $(subst ','\'',$(NO_PERL_CPAN_FALLBACKS))
 endif
 
 perl/build/lib/%.pm: perl/%.pm
        $(QUIET_GEN)mkdir -p $(dir $@) && \
-       sed -e 's|@@LOCALEDIR@@|$(localedir_SQ)|g' < $< > $@
+       sed -e 's|@@LOCALEDIR@@|$(localedir_SQ)|g' \
+           -e 's|@@NO_PERL_CPAN_FALLBACKS@@|$(NO_PERL_CPAN_FALLBACKS_SQ)|g' \
+       < $< > $@
 
 perl/build/man/man3/Git.3pm: perl/Git.pm
        $(QUIET_GEN)mkdir -p $(dir $@) && \

diff --git a/perl/Git/LoadCPAN.pm b/perl/Git/LoadCPAN.pm
index 229c1ee87d34387ca3f4b8dffdc7b6fe74f04f1e..e5585e75e809ef5763d3d4f19ebd185c6f7c0f51 100644 (file)
--- a/perl/Git/LoadCPAN.pm
+++ b/perl/Git/LoadCPAN.pm
@@ -19,13 +19,25 @@ =head1 DESCRIPTION
 will fall back on C<FromCPAN::Module::Name> shipped with Git itself.
 
 Usually distributors will not ship with Git's Git::FromCPAN tree at
-all, preferring to use their own packaging of CPAN modules instead.
+all via the C<NO_PERL_CPAN_FALLBACKS> option, preferring to use their
+own packaging of CPAN modules instead.
 
 This module is only intended to be used for code shipping in the
 C<git.git> repository. Use it for anything else at your peril!
 
 =cut
 
+# NO_PERL_CPAN_FALLBACKS_STR evades the sed search-replace from the
+# Makefile, and allows for detecting whether the module is loaded from
+# perl/Git as opposed to perl/build/Git, which is useful for one-off
+# testing without having Error.pm et al installed.
+use constant NO_PERL_CPAN_FALLBACKS_STR => '@@' . 'NO_PERL_CPAN_FALLBACKS' . '@@';
+use constant NO_PERL_CPAN_FALLBACKS => (
+       q[@@NO_PERL_CPAN_FALLBACKS@@] ne ''
+       and
+       q[@@NO_PERL_CPAN_FALLBACKS@@] ne NO_PERL_CPAN_FALLBACKS_STR
+);
+
 sub import {
        shift;
        my $caller = caller;
@@ -45,6 +57,25 @@ sub import {
        } or do {
                my $error = $@ || "Zombie Error";
 
+               if (NO_PERL_CPAN_FALLBACKS) {
+                       chomp(my $error = sprintf <<'THEY_PROMISED', $module);
+BUG: The '%s' module is not here, but NO_PERL_CPAN_FALLBACKS was set!
+
+Git needs this Perl module from the CPAN, and will by default ship
+with a copy of it. This Git was built with NO_PERL_CPAN_FALLBACKS,
+meaning that whoever built it promised to provide this module.
+
+You're seeing this error because they broke that promise, and we can't
+load our fallback version, since we were asked not to install it.
+
+If you're seeing this error and didn't package Git yourself the
+package you're using is broken, or your system is broken. This error
+won't appear if Git is built without NO_PERL_CPAN_FALLBACKS (instead
+we'll use our fallback version of the module).
+THEY_PROMISED
+                       die $error;
+               }
+
                my $Git_LoadCPAN_pm_path = $INC{"Git/LoadCPAN.pm"} || die "BUG: Should have our own path from %INC!";
 
                require File::Basename;