Define a third kind of COW state; STATIC

Previously, when IsCOW flag was set there were two cases:
  SvLEN()==0:
      PV is really a shared HEK

  SvLEN()!=0:
      PV is a COW structure with 1..256 refcount stored in its extra final byte

This change adds a third state:

  SvLEN()==0 && SvFLAGS() & SVppv_STATIC:
      PV is a shared static const pointer and must not be modified

sv_setsv_flags() and sv_setsv_cow() will preserve this state

sv_uncow() will copy it out to a regular string buffer

sv_dup() will preserve the static pointer into cloned threads

1 files changed, 37 insertions, 0 deletions

diff --git a/t/op/bool.t b/t/op/bool.t
new file mode 100644
index 0000000000..21f6d3b7d5
--- /dev/null
+++ b/t/op/bool.t
@@ -0,0 +1,37 @@
+#!./perl
+
+BEGIN {
+    chdir 't' if -d 't';
+    require './test.pl';
+    set_up_inc('../lib');
+}
+
+use strict;
+use warnings;
+
+my $truevar  = (5 == 5);
+my $falsevar = (5 == 6);
+
+cmp_ok($truevar, '==', 1);
+cmp_ok($truevar, 'eq', "1");
+
+cmp_ok($falsevar, '==', 0);
+cmp_ok($falsevar, 'eq', "");
+
+{
+    # Check that boolean COW string buffer is safe to copy into new SVs and
+    # doesn't get corrupted by inplace mutations
+    my $x = $truevar;
+    $x =~ s/1/t/;
+
+    cmp_ok($x, 'eq', "t");
+    cmp_ok($truevar, 'eq', "1");
+
+    my $y = $truevar;
+    substr($y, 0, 1, "T");
+
+    cmp_ok($y, 'eq', "T");
+    cmp_ok($truevar, 'eq', "1");
+}
+
+done_testing();