update PL_main_thread on fork()

85e97066 modified the perl signal handler to forward signals to the
main thread if it received a signal in a non-perl thread, which
required saving the id of the main perl thread.

Unfortunately I forgot to handle a possible change in the main thread
id on a fork, this fixes that by re-saving the new main thread id
immediately after a fork (via pthread_atfork())

On Linux it appears that the main thread id returned by pthread_seld()
is constant between processes, but this may not be true on other
platforms.

Discussed at:

https://github.com/Perl/perl5/issues/23326#issuecomment-3050481975

embed.fnc

@@ -711,6 +711,7 @@ Apx	|void	|apply_attrs_string					\
 Adp	|OP *	|apply_builtin_cv_attributes				\
 				|NN CV *cv				\
 				|NULLOK OP *attrlist
+CTp	|void	|atfork_child
 CTp	|void	|atfork_lock
 CTp	|void	|atfork_unlock
 Cop	|SV **	|av_arylen_p	|NN AV *av

embed.h

@@ -131,6 +131,7 @@
 # define amagic_deref_call(a,b)                 Perl_amagic_deref_call(aTHX_ a,b)
 # define apply_attrs_string(a,b,c,d)            Perl_apply_attrs_string(aTHX_ a,b,c,d)
 # define apply_builtin_cv_attributes(a,b)       Perl_apply_builtin_cv_attributes(aTHX_ a,b)
+# define atfork_child                           Perl_atfork_child
 # define atfork_lock                            Perl_atfork_lock
 # define atfork_unlock                          Perl_atfork_unlock
 # define av_clear(a)                            Perl_av_clear(aTHX_ a)

ext/ExtUtils-Miniperl/lib/ExtUtils/Miniperl.pm

@@ -5,7 +5,7 @@ use Exporter 'import';
 use ExtUtils::Embed 1.31, qw(xsi_header xsi_protos xsi_body);
 
 our @EXPORT = qw(writemain);
-our $VERSION = '1.14';
+our $VERSION = '1.15';
 
 # blead will run this with miniperl, hence we can't use autodie or File::Temp
 my $temp;
@@ -122,7 +122,7 @@ main(int argc, char **argv, char **env)
      * --GSAR 2001-07-20 */
     PTHREAD_ATFORK(Perl_atfork_lock,
                    Perl_atfork_unlock,
-                   Perl_atfork_unlock);
+                   Perl_atfork_child);
 #endif
 
     PERL_SYS_FPU_INIT;

ext/XS-APItest/t/thread.t

@@ -1,6 +1,7 @@
 #!perl
 use warnings;
 use strict;
+use Test2::IPC;
 use Test2::Tools::Basic;
 use Config;
 
@@ -14,4 +15,27 @@ use XS::APItest qw(thread_id_matches);
 ok(thread_id_matches(),
    "check main thread id saved and is current thread");
 
+# This test isn't too useful on Linux, it passes without the fix.
+#
+# thread ids are unique only within a process, so it's valid for Linux
+# pthread_self() to return the same id for the main thread after a
+# fork.
+#
+# This may be different on other POSIX-likes.
+SKIP:
+{
+    $Config{d_fork}
+      or skip "Need fork", 1;
+    my $pid = fork;
+    defined $pid
+      or skip "Fork failed", 1;
+    if ($pid == 0) {
+        ok(thread_id_matches(), "check main thread id is updated by fork");
+        exit;
+    }
+    else {
+        waitpid($pid, 0);
+    }
+}
+
 done_testing();

miniperlmain.c

@@ -96,7 +96,7 @@ main(int argc, char **argv, char **env)
      * --GSAR 2001-07-20 */
     PTHREAD_ATFORK(Perl_atfork_lock,
                    Perl_atfork_unlock,
-                   Perl_atfork_unlock);
+                   Perl_atfork_child);
 #endif
 
     PERL_SYS_FPU_INIT;

proto.h

@@ -212,6 +212,10 @@ Perl_apply_builtin_cv_attributes(pTHX_ CV *cv, OP *attrlist);
 #define PERL_ARGS_ASSERT_APPLY_BUILTIN_CV_ATTRIBUTES \
         assert(cv); assert(SvTYPE(cv) == SVt_PVCV || SvTYPE(cv) == SVt_PVFM)
 
+PERL_CALLCONV void
+Perl_atfork_child(void);
+#define PERL_ARGS_ASSERT_ATFORK_CHILD
+
 PERL_CALLCONV void
 Perl_atfork_lock(void);
 #define PERL_ARGS_ASSERT_ATFORK_LOCK

util.c

@@ -2872,6 +2872,18 @@ Perl_atfork_unlock(void)
 #endif
 }
 
+void
+Perl_atfork_child(void) {
+#ifdef USE_ITHREADS
+    /* so we can resend signals received in a non-perl thread to the
+       new main thread
+    */
+    PTHREAD_INIT_SELF(PL_main_thread);
+#endif
+
+    Perl_atfork_unlock();
+}
+
 /*
 =for apidoc_section $concurrency
 =for apidoc my_fork