dht: let a node that lost the network try to rejoin it

Dht::onDisconnected() only starts a bootstrap when bootstrapJob is
unset, reading a set handle as "a bootstrap is already scheduled".
That reading is wrong: Scheduler::run() erases the job from its timers
and calls it, but never clears the shared handle the caller holds, and
Scheduler::cancel() leaves the handle alone too when the job is no
longer in the timers. The handle therefore stays set for as long as the
Dht lives, once a bootstrap job has run.

That is harmless while bootstrap() reschedules itself, which it does as
long as the node is disconnected. It is not harmless on the early
return: a job that fires while a ping is still in flight, or while a
dubious contact keeps the status at Connecting, returns without
rescheduling. From then on the chain is dead and onDisconnected() will
not restart it, so a node that goes on to lose every contact stays
disconnected for good. Only an explicit addBootstrap() or a
connectivityChanged() can revive it, and a node whose network simply
went away gets neither.

Clearing the handle on the early return restores the meaning
onDisconnected() relies on.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/dht.cpp

@@ -2270,8 +2270,16 @@ Dht::onDisconnected()
 void
 Dht::bootstrap()
 {
-    if (dht4.status != NodeStatus::Disconnected || dht6.status != NodeStatus::Disconnected)
+    if (dht4.status != NodeStatus::Disconnected || dht6.status != NodeStatus::Disconnected) {
+        // The job that led here has already run, and the scheduler does not
+        // clear the handle to a job it has executed. onDisconnected() reads a
+        // set handle as "a bootstrap is already scheduled" and does nothing,
+        // so leaving it set here means the node never tries to rejoin the
+        // network once it has lost it. Clearing it is what says that no
+        // bootstrap is pending any more.
+        bootstrapJob.reset();
         return;
+    }
     if (logger_)
         logger_->debug("[{}] Bootstraping", myid.to_view());
     bootstrap_pending = false;