core: fix wal saving an empty xlog during startup failure

After the commit a392eb76 ("box: destroy its modules even if box.cfg
not done") wal_free() on exit is called even if box.cfg() isn't finished
yet, and thus wal isn't yet enabled. This leads to a bug when wal would
unconditionally write a 00...0.xlog regardless of the actual xlog
directory contents or the actual vclock.

Let's fix this by not writing an empty xlog on exit if wal writer's
vclock is zero. It means the writer either wasn't enabled yet, or was
enabled, but hasn't written anything. In both cases writing the empty
xlog is extraneous.

Closes #8704

NO_DOC=bugfix

(cherry picked from commit f78908c9)

changelogs/unreleased/gh-8704-wal-exit-empty-xlog.md

+## bugfix/core
+
+* Fixed the node writing an empty `00000000000000000000.xlog` file regardless of
+  the actual vclock when interrupted during the initial `box.cfg()` call
+  (gh-8704).

src/box/wal.c

@@ -1206,7 +1206,7 @@ wal_writer_f(va_list ap)
 	 * have to rescan the last WAL to find the instance vclock.
 	 * Don't create a WAL if the last one is empty.
 	 */
-	if (writer->wal_mode != WAL_NONE &&
+	if (writer->wal_mode != WAL_NONE && vclock_sum(&writer->vclock) > 0 &&
 	    (!xlog_is_open(&writer->current_wal) ||
 	     vclock_compare(&writer->vclock,
 			    &writer->current_wal.meta.vclock) > 0)) {

test/box-luatest/gh_8704_extra_empty_xlog_on_exit_test.lua

+local t = require('luatest')
+local server = require('luatest.server')
+local replica_set = require('luatest.replica_set')
+
+local g = t.group('gh-8704-empty-xlog-on-exit')
+
+local fio = require('fio')
+
+g.before_each(function(cg)
+    cg.rs = replica_set:new{}
+    cg.server = cg.rs:build_and_add_server{
+        alias = 'server',
+        box_cfg = {
+            replication = server.build_listen_uri('nonexistent_uri', cg.rs.id),
+        },
+    }
+end)
+
+g.after_each(function(cg)
+    cg.rs:drop()
+end)
+
+g.test_no_empty_xlog_on_startup_failure = function(cg)
+    cg.server:start{wait_until_ready = false}
+    local logfile = fio.pathjoin(cg.server.workdir, 'server.log')
+    t.helpers.retrying({}, function()
+        t.assert(cg.server:grep_log('connecting to 1 replicas', nil,
+                                    {filename = logfile}),
+                 'Server is started')
+    end)
+    cg.server:stop()
+    local xlog_pattern = fio.pathjoin(cg.server.workdir, '*.xlog')
+    t.assert_equals(#fio.glob(xlog_pattern), 0,
+                    'No xlogs left after a failed startup')
+    local snap_pattern = fio.pathjoin(cg.server.workdir, '*.snap')
+    t.assert_equals(#fio.glob(snap_pattern), 0,
+                    'No snapshots left after a failed startup')
+end