fix COW match capture optimisation

When an SV matches, a new SV is created which is a COW copy of the original
SV, and stored in prog->saved_copy, then prog->subbeg is set to point to
the (shared) PVX buffer.

Earlier in this branch I introduced an optimisation that skipped freeing
the old saved_copy and creating a new COW SV each time, if the old
saved_copy SV was already a shared copy of the SV being matched.

So far so good, except that I implemented it wrongly: if non-COW
matches (which malloc() subbeg) are interspersed with COW matches,
then the subbeg of the COW and the malloced subbeg get mixed up and
AddressSanitizer throws a wobbly.

The fix is simple: in the optimised branch, we still need to free subbeg
if RXp_MATCH_COPIED is true, then reassign it.

1 files changed, 15 insertions, 1 deletions

diff --git a/t/re/pat.t b/t/re/pat.t
index a89249087e..020068cc25 100644
--- a/t/re/pat.t
+++ b/t/re/pat.t
@@ -20,7 +20,7 @@ BEGIN {
     require './test.pl';
 }
 
-plan tests => 689;  # Update this when adding/deleting tests.
+plan tests => 694;  # Update this when adding/deleting tests.
 
 run_tests() unless caller;
 
@@ -1435,6 +1435,20 @@ EOP
         }
     }
 
+    # this mixture of readonly (not COWable) and COWable strings
+    # messed up the capture buffers under COW. The actual test results
+    # are incidental; the issue is was an AddressSanitizer failure
+    {
+	my $c ='AB';
+	my $res = '';
+	for ($c, 'C', $c, 'DE') {
+	    ok(/(.)/, "COWable match");
+	    $res .= $1;
+	}
+	is($res, "ACAD");
+    }
+
+
 } # End of sub run_tests
 
 1;