[perl #119043] Allow utf8 up/downgrade on ro COWs

Commit 1913067 allowed COW constants to be read-only.  This broke
Glib, so I reverted it with ba36554e02.  That caused this bug to reë-
merge (I hadn’t realised that I had fixed it in 1913067):

perl -e 'for(1..10){for(__PACKAGE__){warn $_; $_++}}'
main at -e line 1.
maio at -e line 1.
maip at -e line 1.
maiq at -e line 1.
mair at -e line 1.

so I reverted the revert two commits ago.

Glib was triggering a read-only error because it called
sv_utf8_upgrade on a read-only COW scalar, and sv_utf8_upgrade does
sv_force_normal on COWs to de-COW them.  sv_force_normal croaks on
read-only scalars.

The real problem here is that sv_force_normal means ‘I am going to
modify this scalar’, yet sv_utf8_upgrade conceptually does not modify
the scalar, but only changes the internal representation.

Having to call sv_force_normal to get the *side effect* of de-COWing
without triggering the various other things it does is no good.

What we need is a separate sv_uncow function that sv_force_normal
uses.  This commit introduces such a function.

1 files changed, 25 insertions, 0 deletions

diff --git a/lib/utf8.t b/lib/utf8.t
index 8e2b8ea00e..e6c94e6dc4 100644
--- a/lib/utf8.t
+++ b/lib/utf8.t
@@ -563,4 +563,29 @@ for my $pos (0..5) {
     is($s, "A\xc8\x81\xe8\xab\x86","(pos $pos) str after  U; utf8::encode");
 }
 
+# [perl #119043] utf8::upgrade should not croak on read-only COWs
+for(__PACKAGE__) {
+    # First make sure we have a COW, otherwise this test is useless.
+    my $copy = $_;
+    my @addrs = unpack "L!L!", pack "pp", $copy, $_;
+    if ($addrs[0] != $addrs[1]) {
+	fail("__PACKAGE__ did not produce a COW - if this change was "
+	    ."intentional, please provide me with another ro COW scalar")
+    }
+    else {
+	eval { utf8::upgrade($_) };
+	is $@, "", 'no error with utf8::upgrade on read-only COW';
+    }
+}
+# This one croaks, but not because the scalar is read-only
+eval "package \x{100};\n" . <<'END'
+    for(__PACKAGE__) {
+	eval { utf8::downgrade($_) };
+	::like $@, qr/^Wide character/,
+	    'right error with utf8::downgrade on read-only COW';
+    }
+    1
+END
+or die $@;
+
 done_testing();