Make sure truncated in-memory files have trailing null

Just spotted this....

If the target scalar contains something already, then setting SvCUR to
0 is not sufficient.

2 files changed, 7 insertions, 2 deletions

diff --git a/ext/PerlIO-scalar/scalar.xs b/ext/PerlIO-scalar/scalar.xs
index fd8ac67d4b..552345dfce 100644
--- a/ext/PerlIO-scalar/scalar.xs
+++ b/ext/PerlIO-scalar/scalar.xs
@@ -55,6 +55,7 @@ PerlIOScalar_pushed(pTHX_ PerlIO * f, const char *mode, SV * arg,
     {
 	sv_force_normal(s->var);
 	SvCUR_set(s->var, 0);
+	if (SvPOK(s->var)) *SvPVX(s->var) = 0;
     }
     if (SvUTF8(s->var) && !sv_utf8_downgrade(s->var, TRUE)) {
 	if (ckWARN(WARN_UTF8))
diff --git a/ext/PerlIO-scalar/t/scalar.t b/ext/PerlIO-scalar/t/scalar.t
index 4de54f7ed0..a1d2b5354a 100644
--- a/ext/PerlIO-scalar/t/scalar.t
+++ b/ext/PerlIO-scalar/t/scalar.t
@@ -16,7 +16,7 @@ use Fcntl qw(SEEK_SET SEEK_CUR SEEK_END); # Not 0, 1, 2 everywhere.
 
 $| = 1;
 
-use Test::More tests => 112;
+use Test::More tests => 113;
 
 my $fh;
 my $var = "aaa\n";
@@ -340,7 +340,7 @@ sub has_trailing_nul(\$) {
 }
 SKIP: {
     if ($Config::Config{'extensions'} !~ m!\bPerlIO/scalar\b!) {
-	skip "no B", 3;
+	skip "no B", 4;
     }
     require B;
 
@@ -359,6 +359,10 @@ SKIP: {
     print $fh "abc";
     ok has_trailing_nul $memfile,
 	 'write appends null when growing string after seek past end';
+
+    open $fh, ">", \($memfile = "hello");
+    ok has_trailing_nul $memfile,
+	 'initial truncation in ">" mode provides trailing null';
 }
 
 # [perl #112780] Cloning of in-memory handles