op.c - work around Module::Install::DSL issue

This converts INIT {} blocks from the Module::Install::DSL
namespace into BEGIN blocks. This works around the bug reported in
GH Issue #16300. (Hopefully, not fully tested yet.) Which in turn
should allow us to close the bug in #2754.

See also PR: #20168 and Issue: #20161 both of which are blocked by
this.

op.c

@@ -10711,16 +10711,19 @@ S_process_special_blocks(pTHX_ I32 floor, const char *const fullname,
 {
     const char *const colon = strrchr(fullname,':');
     const char *const name = colon ? colon + 1 : fullname;
+    int is_module_install_hack = 0;
 
     PERL_ARGS_ASSERT_PROCESS_SPECIAL_BLOCKS;
 
     if (*name == 'B') {
-        if (strEQ(name, "BEGIN")) {
+        module_install_hack:
+        if (strEQ(name, "BEGIN") || is_module_install_hack) {
             const I32 oldscope = PL_scopestack_ix;
             SV *max_nest_sv = NULL;
             IV max_nest_iv;
             dSP;
             (void)CvGV(cv);
+            is_module_install_hack = 0;
             if (floor) LEAVE_SCOPE(floor);
             ENTER;
 
@@ -10846,6 +10849,30 @@ S_process_special_blocks(pTHX_ I32 floor, const char *const fullname,
                 return FALSE;
         } else if (*name == 'I') {
             if (strEQ(name, "INIT")) {
+#ifdef MI_INIT_WORKAROUND_PACK
+                {
+                    HV *hv= CvSTASH(cv);
+                    STRLEN len = hv ? HvNAMELEN(hv) : 0;
+                    char *pv= (len == sizeof(MI_INIT_WORKAROUND_PACK)-1)
+                            ? HvNAME_get(hv) : NULL;
+                    if ( pv && strEQ(pv,MI_INIT_WORKAROUND_PACK) )
+                    {
+                        /* old versions of Module::Install::DSL contain code
+                         * that creates an INIT in eval, which expect to run
+                         * after an exit(0) in BEGIN. This unfortunately
+                         * breaks a lot of code in the CPAN river. So we magically
+                         * convert INIT blocks from Module::Install::DSL to
+                         * be BEGIN blocks. Which works out, since the INIT
+                         * blocks it creates are eval'ed so are late.
+                         */
+                        Perl_warn(aTHX_ "Treating %s::INIT block as BEGIN block as workaround",
+                                MI_INIT_WORKAROUND_PACK);
+                        is_module_install_hack = 1;
+                        goto module_install_hack;
+                    }
+
+                }
+#endif
                 if (PL_main_start)
                     /* diag_listed_as: Too late to run %s block */
                     Perl_ck_warner(aTHX_ packWARN(WARN_VOID),

op.h

@@ -1166,6 +1166,7 @@ struct op_argcheck_aux {
     char slurpy;     /* presence of slurpy: may be '\0', '@' or '%' */
 };
 
+#define MI_INIT_WORKAROUND_PACK "Module::Install::DSL"
 
 /*
  * ex: set ts=8 sts=4 sw=4 et:

pod/perldiag.pod

@@ -6414,6 +6414,15 @@ C<$tr> or C<$y> may cause this error.
 (F) The lexer couldn't find the final delimiter of a tr///, tr[][],
 y/// or y[][] construct.
 
+=item Treating %s::INIT block as BEGIN block as workaround
+
+(S) A package is using an old version of C<Module::Install::DSL> to
+install, which makes unsafe assumptions about when INIT blocks will be
+called. Because C<Module::Install::DSL> is used to install other modules
+and is difficult to upgrade we have a special workaround in place to
+deal with this. Unless you are a maintainer of an affected module you
+can ignore this warning. We emit it only as a sanity check.
+
 =item '%s' trapped by operation mask
 
 (F) You tried to use an operator from a Safe compartment in which it's

t/op/blocks.t

@@ -6,7 +6,7 @@ BEGIN {
     set_up_inc('../lib');
 }
 
-plan tests => 22;
+plan tests => 23;
 
 my @expect = qw(
 b1
@@ -265,3 +265,7 @@ TODO: {
 
 fresh_perl_is('eval "BEGIN {goto end}"; end:', '', {}, 'RT #113934: goto out of BEGIN causes assertion failure');
 
+fresh_perl_is('package Module::Install::DSL; BEGIN { eval "INIT { print q(INIT fired in eval) }" }',
+    "Treating Module::Install::DSL::INIT block as BEGIN block as workaround at (eval 1) line 1.\n"
+    ."INIT fired in eval", {},
+   'GH Issue #16300: Module::Install::DSL workaround');