add a new function, Perl_perly_sighandler()

This function implements the body of what used to be Perl_sighandler(),
the latter becoming a thin wrapper round Perl_perly_sighandler().

The main reason for this change is that it allows us to add an extra
arg, 'safe' to the function without breaking backcompat. This arg
indicates whether the function is being called directly from the OS
signal handler (safe==0), or deferred via Perl_despatch_signals()
(safe==1).

This allows an infelicity in the code to be fixed - it was formerly
trying to determine the route it had been called by (and hence whether a
'safe' route) by seeing if either of the sig/uap parameters was
non-null. It turns out that this was highly dogdy, and only worked by
luck. The safe caller did indeed pass NULL args, but due to a bug
(shortly to be fixed), sometimes the kernel thinks its calling a 1-arg
sig handler when its actually calling a 3-arg one. This means that the
sig/uap args are random garbage, and happen to be non-zero only by happy
coincidence on the OS/platforms so far.

Also, it turns out that the call via Perl_csighandler() was getting it
wrong: its explicit (NULL,NULL) args made it look like a safe signal
call. This will be corrected in the next commit, but for this commit the
old wrong behaviour is preserved.

See RT #82040 for details of when/why the original dodgy 'safe' check
was
added.

1 files changed, 55 insertions, 9 deletions

diff --git a/mg.c b/mg.c
index 998b4d8f92..a934ea7e2f 100644
--- a/mg.c
+++ b/mg.c
@@ -1533,11 +1533,20 @@ Perl_csighandler(int sig)
 	   (PL_signals & PERL_SIGNALS_UNSAFE_FLAG))
 	/* Call the perl level handler now--
 	 * with risk we may be in malloc() or being destructed etc. */
+    {
+        if (PL_sighandlerp == Perl_sighandler)
+            /* default handler, so can call perly_sighandler() directly
+             * rather than via Perl_sighandler, passing the extra
+             * 'safe = false' arg
+             */
+            Perl_perly_sighandler(sig, NULL, NULL, 1 /* XXX tmp safe */);
+        else
 #ifdef PERL_USE_3ARG_SIGHANDLER
-	(*PL_sighandlerp)(sig, NULL, NULL);
+            (*PL_sighandlerp)(sig, NULL, NULL);
 #else
-	(*PL_sighandlerp)(sig);
+            (*PL_sighandlerp)(sig);
 #endif
+    }
     else {
 	if (!PL_psig_pend) return;
 	/* Set a flag to say this signal is pending, that is awaiting delivery after
@@ -1615,11 +1624,19 @@ Perl_despatch_signals(pTHX)
 	    }
 #endif
  	    PL_psig_pend[sig] = 0;
+            if (PL_sighandlerp == Perl_sighandler)
+                /* default handler, so can call perly_sighandler() directly
+                 * rather than via Perl_sighandler, passing the extra
+                 * 'safe = true' arg
+                 */
+                Perl_perly_sighandler(sig, NULL, NULL, 1 /* safe */);
+            else
 #ifdef PERL_USE_3ARG_SIGHANDLER
-	    (*PL_sighandlerp)(sig, NULL, NULL);
+                (*PL_sighandlerp)(sig, NULL, NULL);
 #else
-	    (*PL_sighandlerp)(sig);
+                (*PL_sighandlerp)(sig);
 #endif
+
 #ifdef HAS_SIGPROCMASK
 	    if (!was_blocked)
 		LEAVE;
@@ -3319,12 +3336,35 @@ Perl_whichsig_pvn(pTHX_ const char *sig, STRLEN len)
     return -1;
 }
 
-Signal_t
 #ifdef PERL_USE_3ARG_SIGHANDLER
+
+Signal_t
 Perl_sighandler(int sig, Siginfo_t *sip, void *uap)
+{
+    Perl_perly_sighandler(sig, sip, uap, 0);
+}
+
 #else
+
+Signal_t
 Perl_sighandler(int sig)
+{
+    Perl_perly_sighandler(sig, NULL, NULL, 0);
+}
+
 #endif
+
+/* Invoke the perl-level signal handler. This function is called either
+ * directly from one of the C-level signals handlers (Perl_sighandler or
+ * Perl_csighandler), or for safe signals, later from
+ * Perl_despatch_signals() at a suitable safe point during execution.
+ *
+ * 'safe' is a boolean indicating the latter call path.
+ */
+
+Signal_t
+Perl_perly_sighandler(int sig, Siginfo_t *sip PERL_UNUSED_DECL,
+                    void *uap PERL_UNUSED_DECL, bool safe)
 {
 #ifdef PERL_GET_SIG_CONTEXT
     dTHXa(PERL_GET_SIG_CONTEXT);
@@ -3456,10 +3496,13 @@ Perl_sighandler(int sig)
 	 * blocked by the system when we entered.
 	 */
 #ifdef HAS_SIGPROCMASK
-#if defined(HAS_SIGACTION) && defined(SA_SIGINFO)
-	    if (sip || uap)
-#endif
-	    {
+	    if (!safe) {
+                /* safe signals called via dispatch_signals() set up a
+                 * savestack destructor, unblock_sigmask(), to
+                 * automatically unblock the handler at the end. If
+                 * instead we get here directly, we have to do it
+                 * ourselves
+                 */
 		sigset_t set;
 		sigemptyset(&set);
 		sigaddset(&set,sig);
@@ -3467,6 +3510,9 @@ Perl_sighandler(int sig)
 	    }
 #else
 	    /* Not clear if this will work */
+            /* XXX not clear if this should be protected by 'if (safe)'
+             * too */
+
 	    (void)rsignal(sig, SIG_IGN);
 	    (void)rsignal(sig, PL_csighandlerp);
 #endif