Reset context when updating default server list

libmemcached does not have a function for removing servers from the current
context so we must recreate the context from scratch when the list of
default servers changes.

The new functionality also makes it possible to implement a
memcache_server_delete() function at some point as requested in [ohmu#4] but
it's not implemented in this commit.

pgmemcache.c

@@ -17,6 +17,7 @@ PG_MODULE_MAGIC;
 #endif
 
 /* Internal functions */
+static void pgmemcache_reset_context(void);
 static void assign_default_servers_guc(const char *newval, void *extra);
 static void assign_default_behavior_guc(const char *newval, void *extra);
 static memcached_behavior get_memcached_behavior_flag(const char *flag);
@@ -27,7 +28,6 @@ static Datum memcache_atomic_op(bool increment, PG_FUNCTION_ARGS);
 static Datum memcache_set_cmd(int type, PG_FUNCTION_ARGS);
 static memcached_return do_server_add(const char *host_str);
 
-
 /* Per-backend global state. */
 static struct memcache_global_s
 {
@@ -41,19 +41,6 @@ static struct memcache_global_s
 
 void _PG_init(void)
 {
-  int rc;
-
-  globals.mc = memcached_create(NULL);
-
-  /* Use memcache binary protocol by default as required for
-     memcached_(increment|decrement)_with_initial. */
-  rc = memcached_behavior_set(globals.mc,
-                              MEMCACHED_BEHAVIOR_BINARY_PROTOCOL,
-                              1);
-  if (rc != MEMCACHED_SUCCESS)
-    elog(WARNING, "pgmemcache: memcached_behavior_set(BINARY_PROTOCOL): %s",
-                  memcached_strerror(globals.mc, rc));
-
   DefineCustomStringVariable("pgmemcache.default_servers",
                              "Comma-separated list of memcached servers to connect to.",
                              "Specified as a comma-separated list of host:port (port is optional).",
@@ -106,23 +93,7 @@ void _PG_init(void)
                              NULL,
                              NULL);
 
-#if LIBMEMCACHED_WITH_SASL_SUPPORT
-  /* XXX: does this work?  We should probably add an assign_hook for these
-   * so that memcache sasl data is updated when the values are changed. */
-  if (globals.sasl_authentication_username != NULL && strlen(globals.sasl_authentication_username) > 0 &&
-      globals.sasl_authentication_password != NULL && strlen(globals.sasl_authentication_password) > 0)
-    {
-      int rc = memcached_set_sasl_auth_data(globals.mc,
-                                            globals.sasl_authentication_username,
-                                            globals.sasl_authentication_password);
-      if (rc != MEMCACHED_SUCCESS)
-        elog(ERROR, "pgmemcache: memcached_set_sasl_auth_data: %s",
-                    memcached_strerror(globals.mc, rc));
-      rc = sasl_client_init(NULL);
-      if (rc != SASL_OK)
-        elog(ERROR, "pgmemcache: sasl_client_init failed: %d", rc);
-    }
-#endif
+  pgmemcache_reset_context();
 }
 
 /* This is called when we're being unloaded from a process. Note that
@@ -153,10 +124,57 @@ static time_t interval_to_time_t(Interval *span)
   return (time_t) result;
 }
 
+static void pgmemcache_reset_context(void)
+{
+  int rc;
+
+  if (globals.mc)
+    {
+      memcached_free(globals.mc);
+      globals.mc = NULL;
+    }
+
+  globals.mc = memcached_create(NULL);
+
+  /* Always use the memcache binary protocol as required for
+     memcached_(increment|decrement)_with_initial. */
+  rc = memcached_behavior_set(globals.mc,
+                              MEMCACHED_BEHAVIOR_BINARY_PROTOCOL,
+                              1);
+  if (rc != MEMCACHED_SUCCESS)
+    elog(WARNING, "pgmemcache: memcached_behavior_set(BINARY_PROTOCOL): %s",
+                  memcached_strerror(globals.mc, rc));
+
+  assign_default_behavior_guc(globals.default_behavior, NULL);
+
+#if LIBMEMCACHED_WITH_SASL_SUPPORT
+  /* XXX: does this work?  We should probably add an assign_hook for these
+   * so that memcache sasl data is updated when the values are changed. */
+  if (globals.sasl_authentication_username != NULL && strlen(globals.sasl_authentication_username) > 0 &&
+      globals.sasl_authentication_password != NULL && strlen(globals.sasl_authentication_password) > 0)
+    {
+      int rc = memcached_set_sasl_auth_data(globals.mc,
+                                            globals.sasl_authentication_username,
+                                            globals.sasl_authentication_password);
+      if (rc != MEMCACHED_SUCCESS)
+        elog(ERROR, "pgmemcache: memcached_set_sasl_auth_data: %s",
+                    memcached_strerror(globals.mc, rc));
+      rc = sasl_client_init(NULL);
+      if (rc != SASL_OK)
+        elog(ERROR, "pgmemcache: sasl_client_init failed: %d", rc);
+    }
+#endif
+}
+
 static void assign_default_servers_guc(const char *newval, void *extra)
 {
   if (newval)
-    do_server_add(newval);
+    {
+      /* there is no way to remove servers from a memcache context, so
+       * recreate it from scratch when the server list changes */
+      pgmemcache_reset_context();
+      do_server_add(newval);
+    }
 }
 
 static void assign_default_behavior_guc(const char *newval, void *extra)