Add respawn checking feature

Add a feature which allows arbitrary checks to be performed whenever the child
process is about to be respawned. Currently implemented checks allow running of
a command (via system(3)) or testing the existence of a file.

This was developed to be used to stop respawns of a daemon that was started in
response to a udev hotplug event, but it is generally applicable to many other
scenarios.

src/daemonitor/800_checks.c

@@ -0,0 +1,186 @@
+/* daemonitor/src/daemonitor/800_checks.c
+ *
+ *  Copyright: ©2012, Laurence Withers.
+ *  Author: Laurence Withers <l@lwithers.me.uk>
+ *  License: GPLv3
+ */
+
+
+
+/* RESPAWN CHECKS **************************************************************
+ *
+ *  This file implements functionality for various checks that may be carried
+ *  out whenever the child process needs to be respawned. An arbitrary number of
+ *  checks may be registered; if any single check fails, the entire test counts
+ *  as failing. The result of the checks is found by calling run_checks().
+ *
+ *  A failed check will log details at LOG_NOTICE (or above).
+ */
+
+
+
+/* PUBLIC API *****************************************************************/
+
+
+
+/* check_command_register()
+ *  cmd: A command (passed to system(3)) to run. If it exits with code 0, the
+ *       check passes.
+ */
+void check_command_register(const char* cmd);
+
+
+
+/* check_file_register()
+ *  path: A file to test. If it exists, the check passes.
+ */
+void check_file_register(const char* cmd);
+
+
+
+/* run_checks()
+ *  Returns 0 if all checks succeeded (including when none are registered) and
+ *  non-0 if any check failed.
+ */
+int run_checks(void);
+
+
+
+/* IMPLEMENTATION *************************************************************/
+
+
+
+enum check_type {
+    check_type_command,
+    check_type_file,
+};
+
+
+
+/* struct check
+ * check_head
+ *  A linked list of the checks that are to be run.
+ */
+struct check {
+    enum check_type type;
+
+    /* can be the argument for system(3) or the path to check for a file */
+    const char* arg;
+
+    /* linked list structure */
+    struct check* next;
+};
+
+struct check* check_head = 0;
+
+
+
+/* check_generic_register()
+ *  Instantiates a ‘struct check’ and adds it to the list at ‘check_head’.
+ */
+void
+check_generic_register(enum check_type type, const char* arg)
+{
+    struct check* c;
+
+    c = malloc(sizeof(*c));
+    c->type = type;
+    c->arg = arg;
+    c->next = check_head;
+    check_head = c;
+}
+
+
+
+void
+check_command_register(const char* cmd)
+{
+    check_generic_register(check_type_command, cmd);
+}
+
+
+
+int
+check_command_run(const struct check* iter)
+{
+    int ret;
+
+    ret = system(iter->arg);
+    if(ret == -1) {
+        LOG(LOG_ERR, "Unable to run check command \"%s\": %m.", iter->arg);
+        return 0; /* don't exit just because system(3) failed */
+    }
+
+    if(WIFSIGNALED(ret)) {
+        LOG(LOG_WARNING, "Check command \"%s\" killed by signal (%s). Exiting.",
+            iter->arg, strsignal(WTERMSIG(ret)));
+        return -1;
+    }
+
+    if(!WIFEXITED(ret)) {
+        LOG(LOG_WARNING, "Unrecognised exit code 0x%X from check command "
+            "\"%s\".", ret, iter->arg);
+        return 0;
+    }
+
+    if(WEXITSTATUS(ret)) {
+        LOG(LOG_NOTICE, "Check command \"%s\" signalled failure (exit code "
+            "%d). Exiting.", iter->arg, WEXITSTATUS(ret));
+        return -1;
+    }
+
+    return 0;
+}
+
+
+
+void
+check_file_register(const char* path)
+{
+    check_generic_register(check_type_file, path);
+}
+
+
+
+int
+check_file_run(const struct check* iter)
+{
+    int ret;
+
+    ret = access(iter->arg, F_OK);
+    if(ret) {
+        LOG(LOG_NOTICE, "Check file \"%s\" does not exist. Exiting.",
+            iter->arg);
+        return -1;
+    }
+
+    return 0;
+}
+
+
+
+int
+run_checks(void)
+{
+    const struct check* iter;
+
+    for(iter = check_head; iter; iter = iter->next) {
+        switch(iter->type) {
+        case check_type_command:
+            if(check_command_run(iter)) return -1;
+            break;
+
+        case check_type_file:
+            if(check_file_run(iter)) return -1;
+            break;
+        }
+    }
+
+    return 0;
+}
+
+
+
+/* options for text editors
+vim: expandtab:ts=4:sw=4
+*/

src/daemonitor/999_main.c

@@ -1,8 +1,8 @@
 /* daemonitor/src/daemonitor/999_main.c
  *
- *  (c)2007, Laurence Withers, <l@lwithers.me.uk>.
- *  Released under the GNU GPLv3. See file COPYING or
- *  http://www.gnu.org/copyleft/gpl.html for details.
+ *  Copyright: ©2007–2012, Laurence Withers.
+ *  Author: Laurence Withers <l@lwithers.me.uk>
+ *  License: GPLv3
  */
 
 
@@ -37,6 +37,10 @@ void usage(void)
         "                              " DEFAULT_RESTART_INTERVAL_S "). May be 0 to restart immediately.\n"
         " -E, --environment <path>  Name of file containing environment variables for\n"
         "                           child process, one NAME=VALUE entry per line.\n"
+        " -C, --check-command <cmd> Runs <cmd> via /bin/sh before restarting a service\n"
+        "                           that has exited. Stops if <cmd> fails.\n"
+        " -F, --check-file <path>   Checks that <path> exists before restarting a service\n"
+        "                           that has exited. Stops if it does not exist.\n"
     "", stderr);
 }
 
@@ -53,6 +57,8 @@ struct option options[] = {
     { "pidfile", required_argument, 0, 'p' },
     { "restart-interval", required_argument, 0, 'R' },
     { "environment", required_argument, 0, 'E' },
+    { "check-command", required_argument, 0, 'C' },
+    { "check-file", required_argument, 0, 'F' },
     { 0, 0, 0, 0 }
 };
 
@@ -61,7 +67,7 @@ struct option options[] = {
 /* optstr
  *  The list of short commandline options for getopt(3). Sorted alphabetically, lowercase first.
  */
-const char* optstr = "dhl:p:E:R:V";
+const char* optstr = "dhl:p:C:E:F:R:V";
 
 
 
@@ -90,6 +96,8 @@ int main(int argc, char* argv[])
         case 'p': pidfile_argument = optarg; break;
         case 'R': restart_argument = optarg; break;
         case 'E': set_environment_file(optarg); break;
+        case 'C': check_command_register(optarg); break;
+        case 'F': check_file_register(optarg); break;
         }
     }
   opts_done:
@@ -150,6 +158,8 @@ int main(int argc, char* argv[])
             LOG(LOG_NOTICE, "Sleeping for %d seconds.", restart_interval);
             safe_sleep_fixed(restart_interval, 0);
         }
+
+        if(run_checks()) return 0;
     }
 
     return 0;
@@ -158,6 +168,5 @@ int main(int argc, char* argv[])
 
 
 /* options for text editors
-kate: replace-trailing-space-save true; space-indent true; tab-width 4;
 vim: expandtab:ts=4:sw=4
 */