RT #118213: handle $r=qr/.../; /$r/p properly

In the case where a qr// regex is directly used by PMOP (rather than being
interpolated with some other stuff and a new regex created, such as
/a$r/p), then the PMf_KEEPCOPY flag will be set on the PMOP, but the
corresponding RXf_PMf_KEEPCOPY flag *won't* be set on the regex.

Since most of the regex handling for copying the string and extracting out
${^PREMATCH} etc is done based on the RXf_PMf_KEEPCOPY flag in the regex,
this is a bit of a problem.

Prior to 5.18.0 this wasn't so noticeable, since various other bugs around
//p handling meant that ${$PREMATCH} etc often accidentally got set
anyway. 5.18.0 fixed these bugs, and so as a side-effect, exposed the
PMOP verses regex flag issue. In particular, this stopped working in
5.18.0:

    my $pat = qr/a/;
    'aaaa' =~ /$pat/gp or die;
    print "MATCH=[${^MATCH}]\n";

(prints 'a' in 5.16.0, undef in 5.18.0).
The presence /g caused the engine to copy the string anyway by luck.

We can't just set the RXf_PMf_KEEPCOPY flag on the regex if we see the
PMf_KEEPCOPY flag on the PMOP, otherwise stuff like this will be wrong:

    $r = qr/..../;
    /$r/p; 		# set RXf_PMf_KEEPCOPY on $r
    /$r/;		# does a /p match by mistake

Since for 5.19.x onwards COW is enabled by default (and cheap copies are
always made regardless of /p), then this fix is mainly for PERL_NO_COW
builds and for backporting to 5.18.x. (Although it still applies to
strings that can't be COWed for whatever reason).

Since we can't set a flag in the rx, we fix this by:

1) when calling the regex engine (which may attempt to copy part or all of
the capture string), make sure we pass REXEC_COPY_STR, but neither of
REXEC_COPY_SKIP_PRE, REXEC_COPY_SKIP_POST when we call regexec() from
pp_match or pp_subst when the corresponding PMOP has PMf_KEEPCOPY set.

2) in Perl_reg_numbered_buff_fetch() etc, check for PMf_KEEPCOPY in
PL_curpm as well as for RXf_PMf_KEEPCOPY in the current rx before deciding
whether to process ${^PREMATCH} etc.

As well as adding new tests to t/re/reg_pmod.t, I also changed the
string to be matched against from being '12...' to '012...', to ensure that
the lengths of ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} would all be
different.

pp_hot.c

@@ -1398,6 +1398,7 @@ PP(pp_match)
     if (       RX_NPARENS(rx)
             || PL_sawampersand
             || (RX_EXTFLAGS(rx) & (RXf_EVAL_SEEN|RXf_PMf_KEEPCOPY))
+            || (dynpm->op_pmflags & PMf_KEEPCOPY)
     )
 #endif
     {
@@ -1409,6 +1410,11 @@ PP(pp_match)
         if (! (global && gimme == G_ARRAY))
             r_flags |= REXEC_COPY_SKIP_POST;
     };
+#ifdef PERL_SAWAMPERSAND
+    if (dynpm->op_pmflags & PMf_KEEPCOPY)
+        /* handle KEEPCOPY in pmop but not rx, eg $r=qr/a/; /$r/p */
+        r_flags &= ~(REXEC_COPY_SKIP_PRE|REXEC_COPY_SKIP_POST);
+#endif
 
     s = truebase;
 
@@ -2108,6 +2114,7 @@ PP(pp_subst)
     r_flags = (    RX_NPARENS(rx)
                 || PL_sawampersand
                 || (RX_EXTFLAGS(rx) & (RXf_EVAL_SEEN|RXf_PMf_KEEPCOPY))
+                || (rpm->op_pmflags & PMf_KEEPCOPY)
               )
           ? REXEC_COPY_STR
           : 0;

regcomp.c

@@ -6726,13 +6726,23 @@ Perl_reg_numbered_buff_fetch(pTHX_ REGEXP * const r, const I32 paren,
 
     PERL_ARGS_ASSERT_REG_NUMBERED_BUFF_FETCH;
 
-    if ( (    n == RX_BUFF_IDX_CARET_PREMATCH
+    if (      n == RX_BUFF_IDX_CARET_PREMATCH
            || n == RX_BUFF_IDX_CARET_FULLMATCH
            || n == RX_BUFF_IDX_CARET_POSTMATCH
-         )
-         && !(rx->extflags & RXf_PMf_KEEPCOPY)
-    )
-        goto ret_undef;
+       )
+    {
+        bool keepcopy = cBOOL(rx->extflags & RXf_PMf_KEEPCOPY);
+        if (!keepcopy) {
+            /* on something like
+             *    $r = qr/.../;
+             *    /$qr/p;
+             * the KEEPCOPY is set on the PMOP rather than the regex */
+            if (PL_curpm && r == PM_GETRE(PL_curpm))
+                 keepcopy = cBOOL(PL_curpm->op_pmflags & PMf_KEEPCOPY);
+        }
+        if (!keepcopy)
+            goto ret_undef;
+    }
 
     if (!rx->subbeg)
         goto ret_undef;
@@ -6838,13 +6848,27 @@ Perl_reg_numbered_buff_length(pTHX_ REGEXP * const r, const SV * const sv,
 
     PERL_ARGS_ASSERT_REG_NUMBERED_BUFF_LENGTH;
 
+    if (   paren == RX_BUFF_IDX_CARET_PREMATCH
+        || paren == RX_BUFF_IDX_CARET_FULLMATCH
+        || paren == RX_BUFF_IDX_CARET_POSTMATCH
+    )
+    {
+        bool keepcopy = cBOOL(rx->extflags & RXf_PMf_KEEPCOPY);
+        if (!keepcopy) {
+            /* on something like
+             *    $r = qr/.../;
+             *    /$qr/p;
+             * the KEEPCOPY is set on the PMOP rather than the regex */
+            if (PL_curpm && r == PM_GETRE(PL_curpm))
+                 keepcopy = cBOOL(PL_curpm->op_pmflags & PMf_KEEPCOPY);
+        }
+        if (!keepcopy)
+            goto warn_undef;
+    }
+
     /* Some of this code was originally in C<Perl_magic_len> in F<mg.c> */
     switch (paren) {
       case RX_BUFF_IDX_CARET_PREMATCH: /* ${^PREMATCH} */
-         if (!(rx->extflags & RXf_PMf_KEEPCOPY))
-            goto warn_undef;
-        /*FALLTHROUGH*/
-
       case RX_BUFF_IDX_PREMATCH:       /* $` */
         if (rx->offs[0].start != -1) {
 			i = rx->offs[0].start;
@@ -6857,8 +6881,6 @@ Perl_reg_numbered_buff_length(pTHX_ REGEXP * const r, const SV * const sv,
         return 0;
 
       case RX_BUFF_IDX_CARET_POSTMATCH: /* ${^POSTMATCH} */
-         if (!(rx->extflags & RXf_PMf_KEEPCOPY))
-            goto warn_undef;
       case RX_BUFF_IDX_POSTMATCH:       /* $' */
 	    if (rx->offs[0].end != -1) {
 			i = rx->sublen - rx->offs[0].end;
@@ -6870,13 +6892,7 @@ Perl_reg_numbered_buff_length(pTHX_ REGEXP * const r, const SV * const sv,
 	    }
         return 0;
 
-      case RX_BUFF_IDX_CARET_FULLMATCH: /* ${^MATCH} */
-         if (!(rx->extflags & RXf_PMf_KEEPCOPY))
-            goto warn_undef;
-        /*FALLTHROUGH*/
-
-      /* $& / ${^MATCH}, $1, $2, ... */
-      default:
+      default: /* $& / ${^MATCH}, $1, $2, ... */
 	    if (paren <= (I32)rx->nparens &&
             (s1 = rx->offs[paren].start) != -1 &&
             (t1 = rx->offs[paren].end) != -1)

t/re/reg_pmod.t

@@ -11,9 +11,10 @@ use warnings;
 
 our @tests = (
     # /p      Pattern   PRE     MATCH   POST
-    [ '/p',   "345",    "12-", "345",  "-6789"],
-    [ '(?p)', "345",    "12-", "345",  "-6789"],
-    [ '(?p:)',"345",    "12-", "345",  "-6789"],
+    [ '/p',   "345",    "012-", "345",  "-6789"],
+    [ '/$r/p',"345",    "012-", "345",  "-6789"],
+    [ '(?p)', "345",    "012-", "345",  "-6789"],
+    [ '(?p:)',"345",    "012-", "345",  "-6789"],
     [ '',     "(345)",  undef,  undef,  undef ],
     [ '',     "345",    undef,  undef,  undef ],
 );
@@ -26,8 +27,10 @@ sub _u($$) { "$_[0] is ".(defined $_[1] ? "'$_[1]'" : "undef") }
 
 foreach my $test (@tests) {
     my ($p, $pat,$l,$m,$r) = @$test;
+    my $qr = qr/$pat/;
     for my $sub (0,1) {
 	my $test_name = $p eq '/p'   ? "/$pat/p"
+		      : $p eq '/$r/p'? $p
 		      : $p eq '(?p)' ? "/(?p)$pat/"
 		      : $p eq '(?p:)'? "/(?p:$pat)/"
 		      :                "/$pat/";
@@ -36,16 +39,18 @@ foreach my $test (@tests) {
 	#
 	# Cannot use if/else due to the scope invalidating ${^MATCH} and friends.
 	#
-	$_ = '12-345-6789';
+	$_ = '012-345-6789';
 	my $ok =
 		$sub ?
 			(   $p eq '/p'   ? s/$pat/abc/p
+			  : $p eq '/$r/p'? s/$qr/abc/p
 			  : $p eq '(?p)' ? s/(?p)$pat/abc/
 			  : $p eq '(?p:)'? s/(?p:$pat)/abc/
 			  :                s/$pat/abc/
 			)
 		     :
 			(   $p eq '/p'   ? /$pat/p
+			  : $p eq '/$r/p'? /$qr/p
 			  : $p eq '(?p)' ? /(?p)$pat/
 			  : $p eq '(?p:)'? /(?p:$pat)/
 			  :                /$pat/