Improvements to 31c9a3 - CPAN code did depend on the previous behaviour of blessing filehandles into FileHandle

It turns out that it's not quite as simple as blessing into IO::File.
If you do (just) that, then it breaks any existing code that does
C<require IO::Handle;> to allow it to call methods on file handles,
because they're blessed into IO::File, which isn't loaded. (Note this code
doesn't assume that methods in IO::Seekable are there to be called)

So, it all should work if you also set @IO::File:::ISA correctly?
That way, code that assumes that methods from IO::Handle can be called will
work. However, gv.c now starts complaining (but not failing) if IO::Handle,
IO::Seekable and Exporter aren't present, because it goes looking for
methods in them.

So the solution seems to be to set @IO::File::ISA *and* create (empty)
stashes for the other 3 packages. Patch appended, but not applied.

1 files changed, 24 insertions, 0 deletions

diff --git a/perl.c b/perl.c
index 9f7e831db5..04184be040 100644
--- a/perl.c
+++ b/perl.c
@@ -3863,10 +3863,34 @@ S_init_predump_symbols(pTHX)
     dVAR;
     GV *tmpgv;
     IO *io;
+    AV *isa;
 
     sv_setpvs(get_sv("\"", GV_ADD), " ");
     PL_ofsgv = (GV*)SvREFCNT_inc(gv_fetchpvs(",", GV_ADD|GV_NOTQUAL, SVt_PV));
 
+
+    /* Historically, PVIOs were blessed into IO::Handle, unless
+       FileHandle was loaded, in which case they were blessed into
+       that. Action at a distance.
+       However, if we simply bless into IO::Handle, we break code
+       that assumes that PVIOs will have (among others) a seek
+       method. IO::File inherits from IO::Handle and IO::Seekable,
+       and provides the needed methods. But if we simply bless into
+       it, then we break code that assumed that by loading
+       IO::Handle, *it* would work.
+       So a compromise is to set up the correct @IO::File::ISA,
+       so that code that does C<use IO::Handle>; will still work.
+    */
+		   
+    isa = get_av("IO::File::ISA", GV_ADD | GV_ADDMULTI);
+    av_push(isa, newSVpvs("IO::Handle"));
+    av_push(isa, newSVpvs("IO::Seekable"));
+    av_push(isa, newSVpvs("Exporter"));
+    (void) gv_fetchpvs("IO::Handle::", GV_ADD, SVt_PVGV);
+    (void) gv_fetchpvs("IO::Seekable::", GV_ADD, SVt_PVGV);
+    (void) gv_fetchpvs("Exporter::", GV_ADD, SVt_PVGV);
+
+
     PL_stdingv = gv_fetchpvs("STDIN", GV_ADD|GV_NOTQUAL, SVt_PVIO);
     GvMULTI_on(PL_stdingv);
     io = GvIOp(PL_stdingv);