Make PerlIO marginally reentrant

Currently if an operation on a file handle is interrupted, and if
the signal handler accesses that same file handle (e.g. closes it),
then perl will crash. See [perl #75556].

This commit provides some basic infrastructure to avoid segfaults.
Basically it adds a lock count field to each handle (by re-purposing the
unused flags field in the PL_perlio array), then each time a signal
handler is called, the count is incremented. Then various parts of PerlIO
use a positive count to change behaviour. Most importantly, when layers
are popped, the PerlIOl structure is cleared, but not freed, and is left
in the chain of layers. This means that callers still holding pointers to
the various layers won't access freed structures. It does however mean
that PerlIOl structs may be leaked, and possibly slots in PL_perlio. But
this is better than crashing.

Not much has been done to give sensible behaviour on re-entrancy; for
example, a buffer that has already been written once might get written
again. Fixing this sort of thing would require a large-scale audit of
perlio.c.

1 files changed, 1 insertions, 0 deletions

diff --git a/perliol.h b/perliol.h
index 744ffc8870..019fa8ca04 100644
--- a/perliol.h
+++ b/perliol.h
@@ -90,6 +90,7 @@ struct _PerlIO {
 #define PERLIO_F_FASTGETS	0x00400000
 #define PERLIO_F_TTY		0x00800000
 #define PERLIO_F_NOTREG         0x01000000   
+#define PERLIO_F_CLEARED        0x02000000 /* layer cleared but not freed */
 
 #define PerlIOBase(f)      (*(f))
 #define PerlIOSelf(f,type) ((type *)PerlIOBase(f))