recovery: restore recovery_atfork()

There is still a valid case for fork() after start, it's box.coredump().

The assert that replaced recovery_atfork() call would sporadically fail
even in our own test suite.

Revert the change.

src/box/box.cc

@@ -502,7 +502,8 @@ box_load_cfg()
 void
 box_atfork()
 {
-	assert(recovery == NULL);
+	/* box.coredump() forks to save a core. */
+	recovery_atfork(recovery);
 }
 
 int

src/box/recovery.cc

@@ -242,6 +242,18 @@ recovery_exit(struct recovery_state *r)
 	recovery_delete(r);
 }
 
+void
+recovery_atfork(struct recovery_state *r)
+{
+       xlog_atfork(&r->current_wal);
+       /*
+        * Make sure that atexit() handlers in the child do
+        * not try to stop the non-existent thread.
+        * The writer is not used in the child.
+        */
+       r->writer = NULL;
+}
+
 void
 recovery_apply_row(struct recovery_state *r, struct xrow_header *row)
 {

src/box/recovery.h

@@ -158,6 +158,12 @@ recovery_apply_row(struct recovery_state *r, struct xrow_header *packet);
 int64_t
 recovery_last_checkpoint(struct recovery_state *r);
 
+/**
+ * Ensure we don't corrupt the current WAL file in the child.
+ */
+void
+recovery_atfork(struct recovery_state *r);
+
 #if defined(__cplusplus)
 } /* extern "C" */
 #endif /* defined(__cplusplus) */