submission: Allocate each backend on its own pool.

This allows allocating per-backend module data.

src/submission/submission-backend-relay.c

@@ -992,9 +992,11 @@ submission_backend_relay_create(
 	struct mail_user *user = client->user;
 	struct ssl_iostream_settings ssl_set;
 	struct smtp_client_settings smtp_set;
+	pool_t pool;
 
-	backend = i_new(struct submission_backend_relay, 1);
-	submission_backend_init(&backend->backend, client,
+	pool = pool_alloconly_create("submission relay backend", 1024);
+	backend = p_new(pool, struct submission_backend_relay, 1);
+	submission_backend_init(&backend->backend, pool, client,
 				&backend_relay_vfuncs);
 
 	i_zero(&ssl_set);
@@ -1076,7 +1078,6 @@ static void backend_relay_destroy(struct submission_backend *_backend)
 		smtp_client_transaction_destroy(&backend->trans);
 	if (backend->conn != NULL)
 		smtp_client_connection_close(&backend->conn);
-	i_free(backend);
 }
 
 static void backend_relay_ready_cb(const struct smtp_reply *reply,

src/submission/submission-backend.c

@@ -11,9 +11,10 @@
 #include "submission-backend.h"
 
 void submission_backend_init(struct submission_backend *backend,
-			     struct client *client,
+			     pool_t pool, struct client *client,
 			     const struct submission_backend_vfuncs *vfunc)
 {
+	backend->pool = pool;
 	backend->client = client;
 	backend->v = *vfunc;
 
@@ -32,6 +33,7 @@ static void submission_backend_destroy(struct submission_backend *backend)
 
 	DLLIST_REMOVE(&client->backends, backend);
 	backend->v.destroy(backend);
+	pool_unref(&backend->pool);
 }
 
 void submission_backends_destroy_all(struct client *client)

src/submission/submission-backend.h

@@ -80,6 +80,7 @@ struct submission_backend_vfuncs {
 };
 
 struct submission_backend {
+	pool_t pool;
 	struct client *client;
 
 	struct submission_backend *prev, *next;
@@ -98,7 +99,7 @@ struct submission_backend {
 };
 
 void submission_backend_init(struct submission_backend *backend,
-			     struct client *client,
+			     pool_t pool, struct client *client,
 			     const struct submission_backend_vfuncs *vfunc);
 void submission_backends_destroy_all(struct client *client);