Properly duplicate PerlIO::encoding objects

PerlIO::encoding objects are usually initialized by calling Perl methods,
essentially from the pushed() and getarg() callbacks. During cloning, the
PerlIO API will by default call these methods to initialize the duplicate
struct when the PerlIOBase parent struct is itself duplicated. This does
not behave so well because the perl interpreter is not ready to call
methods at this point, for the stacks are not set up yet.

The proper way to duplicate the PerlIO::encoding object is to call sv_dup()
on its members from the dup() PerlIO callback. So the only catch is to make
the getarg() and pushed() calls implied by the duplication of the underlying
PerlIOBase object aware that they are called during cloning, and make them
wait that the control flow returns to the dup() callback. Fortunately,
getarg() knows since its param argument is then non-null, and its return
value is passed immediately to pushed(), so it is enough to tag this
returned value with a custom magic so that pushed() can see it is being
called during cloning.

This fixes [RT #31923].

3 files changed, 59 insertions, 3 deletions

diff --git a/ext/PerlIO-encoding/encoding.pm b/ext/PerlIO-encoding/encoding.pm
index 4cff76d870..97f05ec589 100644
--- a/ext/PerlIO-encoding/encoding.pm
+++ b/ext/PerlIO-encoding/encoding.pm
@@ -1,7 +1,7 @@
 package PerlIO::encoding;
 
 use strict;
-our $VERSION = '0.21';
+our $VERSION = '0.22';
 our $DEBUG = 0;
 $DEBUG and warn __PACKAGE__, " called by ", join(", ", caller), "\n";
 
diff --git a/ext/PerlIO-encoding/encoding.xs b/ext/PerlIO-encoding/encoding.xs
index 03b8850b12..3575d728ac 100644
--- a/ext/PerlIO-encoding/encoding.xs
+++ b/ext/PerlIO-encoding/encoding.xs
@@ -49,13 +49,23 @@ typedef struct {
 
 #define NEEDS_LINES	1
 
+static const MGVTBL PerlIOEncode_tag = { 0, 0, 0, 0, 0, 0, 0, 0 };
+
 SV *
 PerlIOEncode_getarg(pTHX_ PerlIO * f, CLONE_PARAMS * param, int flags)
 {
     PerlIOEncode *e = PerlIOSelf(f, PerlIOEncode);
-    SV *sv = &PL_sv_undef;
-    PERL_UNUSED_ARG(param);
+    SV *sv;
     PERL_UNUSED_ARG(flags);
+    /* During cloning, return an undef token object so that _pushed() knows
+     * that it should not call methods and wait for _dup() to actually dup the
+     * encoding object. */
+    if (param) {
+	sv = newSV(0);
+	sv_magicext(sv, NULL, PERL_MAGIC_ext, &PerlIOEncode_tag, 0, 0);
+	return sv;
+    }
+    sv = &PL_sv_undef;
     if (e->enc) {
 	dSP;
 	/* Not 100% sure stack swap is right thing to do during dup ... */
@@ -85,6 +95,14 @@ PerlIOEncode_pushed(pTHX_ PerlIO * f, const char *mode, SV * arg, PerlIO_funcs *
     IV  code = PerlIOBuf_pushed(aTHX_ f, mode, Nullsv,tab);
     SV *result = Nullsv;
 
+    if (SvTYPE(arg) >= SVt_PVMG
+		&& mg_findext(arg, PERL_MAGIC_ext, &PerlIOEncode_tag)) {
+	e->enc = NULL;
+	e->chk = NULL;
+	e->inEncodeCall = 0;
+	return code;
+    }
+
     PUSHSTACKi(PERLSI_MAGIC);
     ENTER;
     SAVETMPS;
@@ -566,6 +584,9 @@ PerlIOEncode_dup(pTHX_ PerlIO * f, PerlIO * o,
 	if (oe->enc) {
 	    fe->enc = PerlIO_sv_dup(aTHX_ oe->enc, params);
 	}
+	if (oe->chk) {
+	    fe->chk = PerlIO_sv_dup(aTHX_ oe->chk, params);
+	}
     }
     return f;
 }
diff --git a/ext/PerlIO-encoding/t/threads.t b/ext/PerlIO-encoding/t/threads.t
new file mode 100644
index 0000000000..64f0e559a6
--- /dev/null
+++ b/ext/PerlIO-encoding/t/threads.t
@@ -0,0 +1,35 @@
+#!perl
+
+use strict;
+use warnings;
+
+BEGIN {
+    use Config;
+    if ($Config{extensions} !~ /\bEncode\b/) {
+	print "1..0 # Skip: no Encode\n";
+	exit 0;
+    }
+    unless ($Config{useithreads}) {
+	print "1..0 # Skip: no threads\n";
+	exit 0;
+    }
+}
+
+use threads;
+
+use Test::More tests => 3 + 1;
+
+binmode *STDOUT, ':encoding(UTF-8)';
+
+SKIP: {
+    local $@;
+    my $ret = eval {
+	my $thread = threads->create(sub { pass 'in thread'; return 1 });
+	skip 'test thread could not be spawned' => 3 unless $thread;
+	$thread->join;
+    };
+    is $@, '', 'thread did not croak';
+    is $ret, 1, 'thread returned the right value';
+}
+
+pass 'passes at least one test';