support: Add support_create_timer

It is a simple wrapper over timer_create, timer_settime, and
sigaction.  It will be used to check for large timeout to trigger an
EINTR and to avoid use a large timeout (as for alarm()).

Reviewed-by: Lukasz Majewski <lukma@denx.de>

support/Makefile

@@ -50,6 +50,7 @@ libsupport-routines = \
   support_chroot \
   support_copy_file \
   support_copy_file_range \
+  support_create_timer \
   support_descriptor_supports_holes \
   support_descriptors \
   support_enter_mount_namespace \

support/support.h

@@ -24,6 +24,7 @@
 #define SUPPORT_H
 
 #include <stdbool.h>
+#include <stdint.h>
 #include <stddef.h>
 #include <sys/cdefs.h>
 /* For mode_t.  */
@@ -153,6 +154,16 @@ extern bool support_select_modifies_timeout (void);
    tv_usec larger than 1000000.  */
 extern bool support_select_normalizes_timeout (void);
 
+/* Create a timer that trigger after SEC seconds and NSEC nanoseconds.  If
+   REPEAT is true the timer will repeat indefinitely.  If CALLBACK is not
+   NULL, the function will be called when the timer expires; otherwise a
+   dummy empty function is used instead.
+   This is implemented with POSIX per-process timer with SIGEV_SIGNAL.  */
+timer_t support_create_timer (uint64_t sec, long int nsec, bool repeat,
+			      void (*callback)(int));
+/* Disable the timer TIMER.  */
+void support_delete_timer (timer_t timer);
+
 __END_DECLS
 
 #endif /* SUPPORT_H */

support/support_create_timer.c

@@ -0,0 +1,69 @@
+/* Create a periodic timer.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+#include <support/support.h>
+#include <support/xsignal.h>
+#include <time.h>
+
+static void
+dummy_alrm_handler (int sig)
+{
+}
+
+timer_t
+support_create_timer (uint64_t sec, long int nsec, bool repeat,
+		      void (*callback)(int))
+{
+  struct sigaction sa;
+  sa.sa_handler = callback != NULL ? callback : dummy_alrm_handler;
+  sigemptyset (&sa.sa_mask);
+  sa.sa_flags = 0;
+  xsigaction (SIGALRM, &sa, NULL);
+
+  struct sigevent ev = {
+    .sigev_notify = SIGEV_SIGNAL,
+    .sigev_signo = SIGALRM
+  };
+  timer_t timerid;
+  int r = timer_create (CLOCK_REALTIME, &ev, &timerid);
+  if (r == -1)
+    FAIL_EXIT1 ("timer_create: %m");
+
+  /* Single timer with 0.1s.  */
+  struct itimerspec its =
+    {
+      { .tv_sec = repeat ? sec : 0, .tv_nsec = repeat ? nsec : 0 },
+      { .tv_sec = sec, .tv_nsec = nsec }
+    };
+  r = timer_settime (timerid, 0, &its, NULL);
+  if (r == -1)
+    FAIL_EXIT1 ("timer_settime: %m");
+
+  return timerid;
+}
+
+/* Disable the timer TIMER.  */
+void
+support_delete_timer (timer_t timer)
+{
+  int r = timer_delete (timer);
+  if (r == -1)
+    FAIL_EXIT1 ("timer_delete: %m");
+  xsignal (SIGALRM, SIG_DFL);
+}