perl: fix parallel build race condition in target build

We have received reports of builds of perl occasionally failing when
building with many parallel jobs, with a log like the following:

    LD_LIBRARY_PATH=[...]/perl/perl-5.40.0 ./miniperl -Ilib make_ext.pl \
        dist/constant/pm_to_blib  MAKE="make" LIBPERL_A=libperl.so
    File/Path.pm did not return a true value at [...]/hostpkg/usr/lib/perl5/5.40.0/ExtUtils/MakeMaker.pm line 13.
    BEGIN failed--compilation aborted at [...]/hostpkg/usr/lib/perl5/5.40.0/ExtUtils/MakeMaker.pm line 13.
    Compilation failed in require at Makefile.PL line 3.
    BEGIN failed--compilation aborted at Makefile.PL line 3.
    Unsuccessful Makefile.PL(dist/constant): code=65280 at make_ext.pl line 532.

The failing extension (dist/constant in the above log) would differ
between runs.

The cause of the issue is the `-Ilib` in the command line of miniperl.
In the host build, `./miniperl -I lib` will use the following include
path:

    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/AutoLoader/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/dist/Carp/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/dist/PathTools
    [..]/build_dir/hostpkg/perl/perl-5.40.0/dist/PathTools/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/ExtUtils-Install/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/ExtUtils-MakeMaker/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/ExtUtils-Manifest/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/File-Path/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/ext/re
    [..]/build_dir/hostpkg/perl/perl-5.40.0/dist/Term-ReadLine/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/dist/Exporter/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/ext/File-Find/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/Text-Tabs/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/dist/constant/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/version/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/Getopt-Long/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/Text-ParseWords/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/cpan/ExtUtils-PL2Bat/lib
    [..]/build_dir/hostpkg/perl/perl-5.40.0/lib
    .

Various dependencies of the extension build scripts (Makefile.PL) -
including File-Path, which failed to be loaded in the error log - are
included in the path by buildcustomize.pl, as these extensions are only
installed to `lib` as the build proceeds.

However, in a target build, miniperl is just a symlink to the previously
built host perl. As the host perl does not implicitly load
`buildcustomize.pl`, we get the following include path for
`./miniperl -Ilib`:

    lib
    [..]/staging_dir/hostpkg/usr/lib/perl5/site_perl/5.40.0/x86_64-linux
    [..]/staging_dir/hostpkg/usr/lib/perl5/site_perl/5.40.0
    [..]/staging_dir/hostpkg/usr/lib/perl5/5.40.0/x86_64-linux
    [..]/staging_dir/hostpkg/usr/lib/perl5/5.40.0

The host perl's install location is used as the default include path
which provides File-Path etc. for the target build; however, as more
and more libraries get installed into `lib` during the extension build,
they may get loaded from there instead, as `lib` is at the beginning of
the include path. When multiple extensions are built in parallel, a
Makefile.PL may attempt to load File/Path from `lib` after the file has
been created, but before its contents have been written fully, resulting
in the build to fail.

In fact, we should not load anything from `lib` during the target build,
as it is the staging directory for the target, including native
extensions built for the target architecture - with one exception: The
build scripts expect to find target information in the `Config` module,
so simply removing `lib` from the include path completely would break
the build.

Solve the issue by creating an alternative lib directory `lib_build`,
symlinking `Config.pm` and its dependencies in it, and replacing the
`-Ilib` argument with `-Ilib_build` using a wrapper script around the
host perl executable. This is similar to the approach seen in perl's own
obsolete/broken cross compile scripts (`Cross/Makefile`).

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

lang/perl/Makefile

@@ -92,6 +92,21 @@ endef
 
 # Target perl
 define Build/Configure
+	# We don't want to pass -Ilib to host perl in the target build (as lib
+	# contains the target libraries, and files may currently be written
+	# while being imported in parallel builds). We do however need the
+	# target versions of the Config modules at the beginning of the include
+	# path for the build scripts' use.
+	#
+	# Create an alternative lib_build directory that will be added to the
+	# include path instead of lib (using hostperl-wrapper), containing only
+	# the config modules.
+	$(INSTALL_DIR) $(PKG_BUILD_DIR)/lib_build
+	ln -sf ../lib/Config.pm ../lib/Config_heavy.pl ../lib/Config_git.pl $(PKG_BUILD_DIR)/lib_build/
+
+	install -m0755 files/hostperl-wrapper $(PKG_BUILD_DIR)/hostperl-wrapper
+	sed -i "s'@HOST_PERL@'$(HOST_PERL_PREFIX)/bin/perl'g" $(PKG_BUILD_DIR)/hostperl-wrapper
+
 	$(PERL_CMD) files/perlconfig.pl -Dowrt:target_cc='$(TARGET_CC)' \
 	                                -Dowrt:gccversion=$(CONFIG_GCC_VERSION) \
 	                                -Dowrt:target_cross='$(TARGET_CROSS)' \

lang/perl/files/base.config

@@ -650,7 +650,7 @@ hint='recommended'
 hostcat='cat /etc/hosts'
 hostgenerate="$owrt:host_perl_prefix/bin/generate_uudmap"
 hostosname=''
-hostperl="$owrt:host_perl_prefix/bin/perl"
+hostperl="./hostperl-wrapper"
 html1dir=' '
 html1direxp=''
 html3dir=' '

lang/perl/files/hostperl-wrapper

@@ -0,0 +1,14 @@
+#!@HOST_PERL@
+
+foreach (@ARGV) {
+	# Stop parsing options if we encounter a non-option argument or --
+	last if $_ eq '--' || $_ !~ m/^-/;
+
+	# Modify first option of the form -Ilib, -I../lib, ... to refer to lib_build instead
+	if ($_ =~ m@-I(.*/)?lib@) {
+		$_ .= '_build';
+		last;
+	}
+}
+
+exec '@HOST_PERL@', @ARGV