Persist changes to the list of monitored servers

When a server is added to a monitor, an supplementary configuration file
is generated to persist this information. This will allow dynamic
modifications to server lists which will survive restarts and unexpected
downtime.

The monitor will only add new servers to its list of monitored
servers. This prevents duplicate entries in the list and makes it safe to
persist all used servers to the supplementary configuration file instead of only the ones that are not listed in the main configuration.

server/core/monitor.c

@@ -40,6 +40,7 @@
 #include <mysqld_error.h>
 #include <maxscale/mysql_utils.h>
 #include <maxscale/alloc.h>
+#include <maxscale/gwdirs.h>
 
 /*
  *  Create declarations of the enum for monitor events and also the array of
@@ -244,46 +245,62 @@ monitorStopAll()
 void
 monitorAddServer(MONITOR *mon, SERVER *server)
 {
-    MONITOR_SERVERS *db = (MONITOR_SERVERS *)MXS_MALLOC(sizeof(MONITOR_SERVERS));
-    MXS_ABORT_IF_NULL(db);
-
-    db->server = server;
-    db->con = NULL;
-    db->next = NULL;
-    db->mon_err_count = 0;
-    db->log_version_err = true;
-    /** Server status is uninitialized */
-    db->mon_prev_status = -1;
-    /* pending status is updated by get_replication_tree */
-    db->pending_status = 0;
-
-    monitor_state_t old_state = mon->state;
-
-    if (old_state == MONITOR_STATE_RUNNING)
-    {
-        monitorStop(mon);
-    }
-
+    bool new_server = true;
     spinlock_acquire(&mon->lock);
 
-    if (mon->databases == NULL)
+    for (MONITOR_SERVERS *db = mon->databases; db; db = db->next)
     {
-        mon->databases = db;
-    }
-    else
-    {
-        MONITOR_SERVERS *ptr = mon->databases;
-        while (ptr->next != NULL)
+        if (db->server == server)
         {
-            ptr = ptr->next;
+            new_server = false;
         }
-        ptr->next = db;
     }
+
     spinlock_release(&mon->lock);
 
-    if (old_state == MONITOR_STATE_RUNNING)
+    if (new_server)
     {
-        monitorStart(mon, mon->parameters);
+        MONITOR_SERVERS *db = (MONITOR_SERVERS *)MXS_MALLOC(sizeof(MONITOR_SERVERS));
+        MXS_ABORT_IF_NULL(db);
+
+        db->server = server;
+        db->con = NULL;
+        db->next = NULL;
+        db->mon_err_count = 0;
+        db->log_version_err = true;
+        /** Server status is uninitialized */
+        db->mon_prev_status = -1;
+        /* pending status is updated by get_replication_tree */
+        db->pending_status = 0;
+
+        monitor_state_t old_state = mon->state;
+
+        if (old_state == MONITOR_STATE_RUNNING)
+        {
+            monitorStop(mon);
+        }
+
+        spinlock_acquire(&mon->lock);
+
+        if (mon->databases == NULL)
+        {
+            mon->databases = db;
+        }
+        else
+        {
+            MONITOR_SERVERS *ptr = mon->databases;
+            while (ptr->next != NULL)
+            {
+                ptr = ptr->next;
+            }
+            ptr->next = db;
+        }
+        spinlock_release(&mon->lock);
+
+        if (old_state == MONITOR_STATE_RUNNING)
+        {
+            monitorStart(mon, mon->parameters);
+        }
     }
 }
 
@@ -1152,3 +1169,86 @@ bool monitor_server_in_use(const SERVER *server)
 
     return rval;
 }
+
+/**
+ * Creates a monitor configuration at the location pointed by @c filename
+ *
+ * @param monitor Monitor to serialize into a configuration
+ * @param filename Filename where configuration is written
+ * @return True on success, false on error
+ */
+static bool create_monitor_config(const MONITOR *monitor, const char *filename)
+{
+    int file = open(filename, O_EXCL | O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
+
+    if (file == -1)
+    {
+        char errbuf[MXS_STRERROR_BUFLEN];
+        MXS_ERROR("Failed to open file '%s' when serializing monitor '%s': %d, %s",
+                  filename, monitor->name, errno, strerror_r(errno, errbuf, sizeof(errbuf)));
+        return false;
+    }
+
+    /**
+     * Only additional parameters are added to the configuration. This prevents
+     * duplication or addition of parameters that don't support it.
+     *
+     * TODO: Check for return values on all of the dprintf calls
+     */
+    dprintf(file, "[%s]\n", monitor->name);
+
+    if (monitor->databases)
+    {
+        dprintf(file, "servers=");
+        for (MONITOR_SERVERS *db = monitor->databases; db; db = db->next)
+        {
+            if (db != monitor->databases)
+            {
+                dprintf(file, ",");
+            }
+            dprintf(file, "%s", db->server->unique_name);
+        }
+        dprintf(file, "\n");
+    }
+
+    close(file);
+
+    return true;
+}
+
+bool monitor_serialize_servers(const MONITOR *monitor)
+{
+    bool rval = false;
+    char filename[PATH_MAX];
+    snprintf(filename, sizeof(filename), "%s/%s.cnf.tmp", get_config_persistdir(),
+             monitor->name);
+
+    if (unlink(filename) == -1 && errno != ENOENT)
+    {
+        char err[MXS_STRERROR_BUFLEN];
+        MXS_ERROR("Failed to remove temporary monitor configuration at '%s': %d, %s",
+                  filename, errno, strerror_r(errno, err, sizeof(err)));
+    }
+    else if (create_monitor_config(monitor, filename))
+    {
+        char final_filename[PATH_MAX];
+        strcpy(final_filename, filename);
+
+        char *dot = strrchr(final_filename, '.');
+        ss_dassert(dot);
+        *dot = '\0';
+
+        if (rename(filename, final_filename) == 0)
+        {
+            rval = true;
+        }
+        else
+        {
+            char err[MXS_STRERROR_BUFLEN];
+            MXS_ERROR("Failed to rename temporary monitor configuration at '%s': %d, %s",
+                      filename, errno, strerror_r(errno, err, sizeof(err)));
+        }
+    }
+
+    return rval;
+}