Merge branch 'lukas/erts/fix_signalhandler_errno_restore/OTP-13868' into maint

* lukas/erts/fix_signalhandler_errno_restore/OTP-13868:
  erts: Fix child setup signal hander bug

2 files changed, 19 insertions, 7 deletions

diff --git a/erts/emulator/sys/unix/erl_child_setup.c b/erts/emulator/sys/unix/erl_child_setup.c
index 6b9ddd8da4..69fc6c2879 100644
--- a/erts/emulator/sys/unix/erl_child_setup.c
+++ b/erts/emulator/sys/unix/erl_child_setup.c
@@ -348,7 +348,7 @@ child_error:
  * for posterity. */
 
 static void handle_sigchld(int sig) {
-    int buff[2], res;
+    int buff[2], res, __preverrno = errno;
 
     sys_sigblock(SIGCHLD);
 
@@ -362,6 +362,16 @@ static void handle_sigchld(int sig) {
     }
 
     sys_sigrelease(SIGCHLD);
+
+    /* We save and restore the original errno as otherwise
+       the thread we are running in may end up with an
+       unexpected errno. An example of when this happened
+       was when the select in main had gotten an EINTR but
+       before the errno was checked the signal handler
+       was called and set errno to ECHILD from waitpid
+       which caused erl_child_setup to abort as it does
+       not expect ECHILD to be set after select */
+    errno = __preverrno;
 }
 
 #if defined(__ANDROID__)
@@ -423,7 +433,7 @@ main(int argc, char *argv[])
     sigemptyset(&sa.sa_mask);
     sa.sa_flags = SA_RESTART | SA_NOCLDSTOP;
     if (sigaction(SIGCHLD, &sa, 0) == -1) {
-        perror(0);
+        perror(NULL);
         exit(1);
     }
 
diff --git a/erts/emulator/sys/unix/sys.c b/erts/emulator/sys/unix/sys.c
index 6fb86f6dda..089efec3e8 100644
--- a/erts/emulator/sys/unix/sys.c
+++ b/erts/emulator/sys/unix/sys.c
@@ -715,11 +715,13 @@ static RETSIGTYPE suspend_signal(void)
 static RETSIGTYPE suspend_signal(int signum)
 #endif
 {
-   int res;
-   int buf[1];
-   do {
-     res = read(sig_suspend_fds[0], buf, sizeof(int));
-   } while (res < 0 && errno == EINTR);
+    int res, buf[1], __errno = errno;
+    do {
+        res = read(sig_suspend_fds[0], buf, sizeof(int));
+    } while (res < 0 && errno == EINTR);
+
+    /* restore previous errno in case read changed it */
+    errno = __errno;
 }
 #endif /* #ifdef ERTS_SYS_SUSPEND_SIGNAL */