[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.

2 files changed, 45 insertions, 17 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();
diff --git a/sv.c b/sv.c
index 43488a21d2..fcc076177d 100644
--- a/sv.c
+++ b/sv.c
@@ -3223,6 +3223,8 @@ especially if it could return the position of the first one.
 
 */
 
+static void S_sv_uncow(pTHX_ SV * const sv, const U32 flags);
+
 STRLEN
 Perl_sv_utf8_upgrade_flags_grow(pTHX_ SV *const sv, const I32 flags, STRLEN extra)
 {
@@ -3251,7 +3253,7 @@ Perl_sv_utf8_upgrade_flags_grow(pTHX_ SV *const sv, const I32 flags, STRLEN extr
     }
 
     if (SvIsCOW(sv)) {
-        sv_force_normal_flags(sv, 0);
+        S_sv_uncow(aTHX_ sv, 0);
     }
 
     if (PL_encoding && !(flags & SV_UTF8_NO_ENCODING)) {
@@ -3510,7 +3512,7 @@ Perl_sv_utf8_downgrade(pTHX_ SV *const sv, const bool fail_ok)
 	    int mg_flags = SV_GMAGIC;
 
             if (SvIsCOW(sv)) {
-                sv_force_normal_flags(sv, 0);
+                S_sv_uncow(aTHX_ sv, 0);
             }
 	    if (SvTYPE(sv) >= SVt_PVMG && SvMAGIC(sv)) {
 		/* update pos */
@@ -4871,18 +4873,14 @@ with flags set to 0.
 =cut
 */
 
-void
-Perl_sv_force_normal_flags(pTHX_ SV *const sv, const U32 flags)
+static void
+S_sv_uncow(pTHX_ SV * const sv, const U32 flags)
 {
     dVAR;
 
-    PERL_ARGS_ASSERT_SV_FORCE_NORMAL_FLAGS;
-
+    assert(SvIsCOW(sv));
+    {
 #ifdef PERL_ANY_COW
-    if (SvREADONLY(sv)) {
-	    Perl_croak_no_modify();
-    }
-    else if (SvIsCOW(sv)) {
 	const char * const pvx = SvPVX_const(sv);
 	const STRLEN len = SvLEN(sv);
 	const STRLEN cur = SvCUR(sv);
@@ -4935,13 +4933,7 @@ Perl_sv_force_normal_flags(pTHX_ SV *const sv, const U32 flags)
                 sv_dump(sv);
             }
 	}
-    }
 #else
-    if (SvREADONLY(sv)) {
-	    Perl_croak_no_modify();
-    }
-    else
-	if (SvIsCOW(sv)) {
 	    const char * const pvx = SvPVX_const(sv);
 	    const STRLEN len = SvCUR(sv);
 	    SvIsCOW_off(sv);
@@ -4956,8 +4948,19 @@ Perl_sv_force_normal_flags(pTHX_ SV *const sv, const U32 flags)
 		*SvEND(sv) = '\0';
 	    }
 	    unshare_hek(SvSHARED_HEK_FROM_PV(pvx));
-	}
 #endif
+    }
+}
+
+void
+Perl_sv_force_normal_flags(pTHX_ SV *const sv, const U32 flags)
+{
+    PERL_ARGS_ASSERT_SV_FORCE_NORMAL_FLAGS;
+
+    if (SvREADONLY(sv))
+	Perl_croak_no_modify();
+    else if (SvIsCOW(sv))
+	S_sv_uncow(aTHX_ sv, flags);
     if (SvROK(sv))
 	sv_unref_flags(sv, flags);
     else if (SvFAKE(sv) && isGV_with_GP(sv))