Use libsystemd's sd_notify for communicating redis status to systemd

Instead of replicating a subset of libsystemd's sd_notify(3) internally,
use the dynamic library provided by systemd to communicate with the
service manager.

When systemd supervision was auto-detected or configured, communicate
the actual server status (i.e. "Loading dataset", "Waiting for
master<->replica sync") to systemd, instead of declaring readiness right
after initializing the server process.

src/replication.c

@@ -1665,6 +1665,11 @@ void readSyncBulkPayload(connection *conn) {
     if (server.repl_backlog == NULL) createReplicationBacklog();
     serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Finished with success");
 
+    if (server.supervised_mode == SUPERVISED_SYSTEMD) {
+        redisCommunicateSystemd("STATUS=MASTER <-> REPLICA sync: Finished with success. Ready to accept connections.\n");
+        redisCommunicateSystemd("READY=1\n");
+    }
+
     /* Restart the AOF subsystem now that we finished the sync. This
      * will trigger an AOF rewrite, and when done will start appending
      * to the new file. */

src/server.c

@@ -55,6 +55,7 @@
 #include <sys/utsname.h>
 #include <locale.h>
 #include <sys/socket.h>
+#include <dlfcn.h>
 
 /* Our shared "common" objects */
 
@@ -3651,6 +3652,8 @@ int prepareForShutdown(int flags) {
     int nosave = flags & SHUTDOWN_NOSAVE;
 
     serverLog(LL_WARNING,"User requested shutdown...");
+    if (server.supervised_mode == SUPERVISED_SYSTEMD)
+        redisCommunicateSystemd("STOPPING=1\n");
 
     /* Kill all the Lua debugger forked sessions. */
     ldbKillForkedSessions();
@@ -3692,6 +3695,8 @@ int prepareForShutdown(int flags) {
     /* Create a new RDB file before exiting. */
     if ((server.saveparamslen > 0 && !nosave) || save) {
         serverLog(LL_NOTICE,"Saving the final RDB snapshot before exiting.");
+        if (server.supervised_mode == SUPERVISED_SYSTEMD)
+            redisCommunicateSystemd("STATUS=Saving the final RDB snapshot\n");
         /* Snapshotting. Perform a SYNC SAVE and exit */
         rdbSaveInfo rsi, *rsiptr;
         rsiptr = rdbPopulateSaveInfo(&rsi);
@@ -3702,6 +3707,8 @@ int prepareForShutdown(int flags) {
              * saving aborted, handling special stuff like slaves pending for
              * synchronization... */
             serverLog(LL_WARNING,"Error trying to save the DB, can't exit.");
+            if (server.supervised_mode == SUPERVISED_SYSTEMD)
+                redisCommunicateSystemd("STATUS=Error trying to save the DB, can't exit.\n");
             return C_ERR;
         }
     }
@@ -4867,61 +4874,40 @@ int redisSupervisedUpstart(void) {
     return 1;
 }
 
-int redisSupervisedSystemd(void) {
+int redisCommunicateSystemd(const char *sd_notify_msg) {
     const char *notify_socket = getenv("NOTIFY_SOCKET");
-    int fd = 1;
-    struct sockaddr_un su;
-    struct iovec iov;
-    struct msghdr hdr;
-    int sendto_flags = 0;
+    int (*dl_sd_notify)(int unset_environment, const char *state);
+    char *error;
+    void *handle;
 
     if (!notify_socket) {
         serverLog(LL_WARNING,
                 "systemd supervision requested, but NOTIFY_SOCKET not found");
         return 0;
     }
 
-    if ((strchr("@/", notify_socket[0])) == NULL || strlen(notify_socket) < 2) {
-        return 0;
-    }
-
-    serverLog(LL_NOTICE, "supervised by systemd, will signal readiness");
-    if ((fd = socket(AF_UNIX, SOCK_DGRAM, 0)) == -1) {
+    handle = dlopen("libsystemd.so.0", RTLD_LAZY);
+    if (!handle) {
         serverLog(LL_WARNING,
-                "Can't connect to systemd socket %s", notify_socket);
+                "systemd supervision requested, but could not dlopen() libsystemd.so");
+        (void) dlerror();
         return 0;
     }
+    (void) dlerror();
 
-    memset(&su, 0, sizeof(su));
-    su.sun_family = AF_UNIX;
-    strncpy (su.sun_path, notify_socket, sizeof(su.sun_path) -1);
-    su.sun_path[sizeof(su.sun_path) - 1] = '\0';
-
-    if (notify_socket[0] == '@')
-        su.sun_path[0] = '\0';
+    *(void **)(&dl_sd_notify) = dlsym(handle, "sd_notify");
+    error = dlerror();
 
-    memset(&iov, 0, sizeof(iov));
-    iov.iov_base = "READY=1";
-    iov.iov_len = strlen("READY=1");
-
-    memset(&hdr, 0, sizeof(hdr));
-    hdr.msg_name = &su;
-    hdr.msg_namelen = offsetof(struct sockaddr_un, sun_path) +
-        strlen(notify_socket);
-    hdr.msg_iov = &iov;
-    hdr.msg_iovlen = 1;
-
-    unsetenv("NOTIFY_SOCKET");
-#ifdef HAVE_MSG_NOSIGNAL
-    sendto_flags |= MSG_NOSIGNAL;
-#endif
-    if (sendmsg(fd, &hdr, sendto_flags) < 0) {
-        serverLog(LL_WARNING, "Can't send notification to systemd");
-        close(fd);
+    if (error != NULL) {
+        serverLog(LL_WARNING,
+                "systemd supervision requested, but could not load sd_notify(3) from libsystemd.so");
+        dlclose(handle);
         return 0;
     }
-    close(fd);
-    return 1;
+
+    (void) (*dl_sd_notify)(0, sd_notify_msg);
+    dlclose(handle);
+    return 0;
 }
 
 int redisIsSupervised(int mode) {
@@ -4932,12 +4918,17 @@ int redisIsSupervised(int mode) {
         if (upstart_job) {
             redisSupervisedUpstart();
         } else if (notify_socket) {
-            redisSupervisedSystemd();
+            server.supervised_mode = SUPERVISED_SYSTEMD;
+            serverLog(LL_WARNING,
+                "WARNING auto-supervised by systemd - you MUST set appropriate values for TimeoutStartSec and TimeoutStopSec in your service unit.");
+            return redisCommunicateSystemd("STATUS=Redis is loading...\n");
         }
     } else if (mode == SUPERVISED_UPSTART) {
         return redisSupervisedUpstart();
     } else if (mode == SUPERVISED_SYSTEMD) {
-        return redisSupervisedSystemd();
+        serverLog(LL_WARNING,
+            "WARNING supervised by systemd - you MUST set appropriate values for TimeoutStartSec and TimeoutStopSec in your service unit.");
+        return redisCommunicateSystemd("STATUS=Redis is loading...\n");
     }
 
     return 0;
@@ -5130,6 +5121,15 @@ int main(int argc, char **argv) {
             serverLog(LL_NOTICE,"Ready to accept connections");
         if (server.sofd > 0)
             serverLog(LL_NOTICE,"The server is now ready to accept connections at %s", server.unixsocket);
+        if (server.supervised_mode == SUPERVISED_SYSTEMD) {
+            /* XXX TODO is this sufficient to pass readiness control off to readSyncBulkPayload()? */
+            if (!server.masterhost) {
+                redisCommunicateSystemd("STATUS=Ready to accept connections\n");
+                redisCommunicateSystemd("READY=1\n");
+            } else {
+                redisCommunicateSystemd("STATUS=Waiting for MASTER <-> REPLICA sync\n");
+            }
+        }
     } else {
         InitServerLast();
         sentinelIsRunning();

src/server.h

@@ -1620,6 +1620,7 @@ uint64_t crc64(uint64_t crc, const unsigned char *s, uint64_t l);
 void exitFromChild(int retcode);
 size_t redisPopcount(void *s, long count);
 void redisSetProcTitle(char *title);
+int redisCommunicateSystemd(const char *sd_notify_msg);
 
 /* networking.c -- Networking and Client related operations */
 client *createClient(connection *conn);