Merge pull request #16342 from markdroth/pf_keep_unselected_subchannels

Change pick_first to not unref unselected subchannels.

11 files changed, 126 insertions, 224 deletions

diff --git a/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc b/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc
index 9120abfa3c..602d6e92f9 100644
--- a/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc
+++ b/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc
@@ -126,7 +126,6 @@ class PickFirst : public LoadBalancingPolicy {
   void ShutdownLocked() override;
 
   void StartPickingLocked();
-  void DestroyUnselectedSubchannelsLocked();
   void UpdateChildRefsLocked();
 
   // All our subchannels.
@@ -250,14 +249,9 @@ void PickFirst::CancelMatchingPicksLocked(uint32_t initial_metadata_flags_mask,
 
 void PickFirst::StartPickingLocked() {
   started_picking_ = true;
-  if (subchannel_list_ != nullptr) {
-    for (size_t i = 0; i < subchannel_list_->num_subchannels(); ++i) {
-      if (subchannel_list_->subchannel(i)->subchannel() != nullptr) {
-        subchannel_list_->subchannel(i)
-            ->CheckConnectivityStateAndStartWatchingLocked();
-        break;
-      }
-    }
+  if (subchannel_list_ != nullptr && subchannel_list_->num_subchannels() > 0) {
+    subchannel_list_->subchannel(0)
+        ->CheckConnectivityStateAndStartWatchingLocked();
   }
 }
 
@@ -294,15 +288,6 @@ bool PickFirst::PickLocked(PickState* pick, grpc_error** error) {
   return false;
 }
 
-void PickFirst::DestroyUnselectedSubchannelsLocked() {
-  for (size_t i = 0; i < subchannel_list_->num_subchannels(); ++i) {
-    PickFirstSubchannelData* sd = subchannel_list_->subchannel(i);
-    if (selected_ != sd) {
-      sd->UnrefSubchannelLocked("selected_different_subchannel");
-    }
-  }
-}
-
 grpc_connectivity_state PickFirst::CheckConnectivityLocked(grpc_error** error) {
   return grpc_connectivity_state_get(&state_tracker_, error);
 }
@@ -419,7 +404,6 @@ void PickFirst::UpdateLocked(const grpc_channel_args& args) {
         if (sd->CheckConnectivityStateLocked(&error) == GRPC_CHANNEL_READY) {
           selected_ = sd;
           subchannel_list_ = std::move(subchannel_list);
-          DestroyUnselectedSubchannelsLocked();
           sd->StartConnectivityWatchLocked();
           // If there was a previously pending update (which may or may
           // not have contained the currently selected subchannel), drop
@@ -504,7 +488,6 @@ void PickFirst::PickFirstSubchannelData::ProcessConnectivityChangeLocked(
         p->TryReresolutionLocked(&grpc_lb_pick_first_trace, GRPC_ERROR_NONE);
         // In transient failure. Rely on re-resolution to recover.
         p->selected_ = nullptr;
-        UnrefSubchannelLocked("pf_selected_shutdown");
         StopConnectivityWatchLocked();
       } else {
         grpc_connectivity_state_set(&p->state_tracker_, connectivity_state,
@@ -535,11 +518,9 @@ void PickFirst::PickFirstSubchannelData::ProcessConnectivityChangeLocked(
     case GRPC_CHANNEL_TRANSIENT_FAILURE: {
       StopConnectivityWatchLocked();
       PickFirstSubchannelData* sd = this;
-      do {
-        size_t next_index =
-            (sd->Index() + 1) % subchannel_list()->num_subchannels();
-        sd = subchannel_list()->subchannel(next_index);
-      } while (sd->subchannel() == nullptr);
+      size_t next_index =
+          (sd->Index() + 1) % subchannel_list()->num_subchannels();
+      sd = subchannel_list()->subchannel(next_index);
       // Case 1: Only set state to TRANSIENT_FAILURE if we've tried
       // all subchannels.
       if (sd->Index() == 0 && subchannel_list() == p->subchannel_list_.get()) {
@@ -600,8 +581,6 @@ void PickFirst::PickFirstSubchannelData::ProcessUnselectedReadyLocked() {
   if (grpc_lb_pick_first_trace.enabled()) {
     gpr_log(GPR_INFO, "Pick First %p selected subchannel %p", p, subchannel());
   }
-  // Drop all other subchannels, since we are now connected.
-  p->DestroyUnselectedSubchannelsLocked();
   // Update any calls that were waiting for a pick.
   PickState* pick;
   while ((pick = p->pending_picks_)) {
diff --git a/src/core/ext/filters/client_channel/lb_policy/subchannel_list.h b/src/core/ext/filters/client_channel/lb_policy/subchannel_list.h
index 0fa2f04e73..91ddaec8b8 100644
--- a/src/core/ext/filters/client_channel/lb_policy/subchannel_list.h
+++ b/src/core/ext/filters/client_channel/lb_policy/subchannel_list.h
@@ -102,11 +102,6 @@ class SubchannelData {
     return pending_connectivity_state_unsafe_;
   }
 
-  // Unrefs the subchannel.  May be used if an individual subchannel is
-  // no longer needed even though the subchannel list as a whole is not
-  // being unreffed.
-  virtual void UnrefSubchannelLocked(const char* reason);
-
   // Resets the connection backoff.
   // TODO(roth): This method should go away when we move the backoff
   // code out of the subchannel and into the LB policies.
@@ -154,6 +149,10 @@ class SubchannelData {
       grpc_connectivity_state connectivity_state,
       grpc_error* error) GRPC_ABSTRACT;
 
+  // Unrefs the subchannel.  May be overridden by subclasses that need
+  // to perform extra cleanup when unreffing the subchannel.
+  virtual void UnrefSubchannelLocked(const char* reason);
+
  private:
   // Updates connected_subchannel_ based on pending_connectivity_state_unsafe_.
   // Returns true if the connectivity state should be reported.
diff --git a/src/core/ext/filters/client_channel/resolver.h b/src/core/ext/filters/client_channel/resolver.h
index 48f2e89095..e9acbb7c41 100644
--- a/src/core/ext/filters/client_channel/resolver.h
+++ b/src/core/ext/filters/client_channel/resolver.h
@@ -81,18 +81,7 @@ class Resolver : public InternallyRefCountedWithTracing<Resolver> {
   ///
   /// If this causes new data to become available, then the currently
   /// pending call to \a NextLocked() will return the new result.
-  ///
-  /// Note: Currently, all resolvers are required to return a new result
-  /// shortly after this method is called.  For pull-based mechanisms, if
-  /// the implementation decides to delay querying the name service, it
-  /// should immediately return a new copy of the previously returned
-  /// result (and it can then return the updated data later, when it
-  /// actually does query the name service).  For push-based mechanisms,
-  /// the implementation should immediately return a new copy of the
-  /// last-seen result.
-  /// TODO(roth): Remove this requirement once we fix pick_first to not
-  /// throw away unselected subchannels.
-  virtual void RequestReresolutionLocked() GRPC_ABSTRACT;
+  virtual void RequestReresolutionLocked() {}
 
   /// Resets the re-resolution backoff, if any.
   /// This needs to be implemented only by pull-based implementations;
diff --git a/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.cc b/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.cc
index f2bb5f3c71..dfa52867d8 100644
--- a/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.cc
+++ b/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.cc
@@ -373,13 +373,7 @@ void AresDnsResolver::OnResolvedLocked(void* arg, grpc_error* error) {
 void AresDnsResolver::MaybeStartResolvingLocked() {
   // If there is an existing timer, the time it fires is the earliest time we
   // can start the next resolution.
-  if (have_next_resolution_timer_) {
-    // TODO(dgq): remove the following two lines once Pick First stops
-    // discarding subchannels after selecting.
-    ++resolved_version_;
-    MaybeFinishNextLocked();
-    return;
-  }
+  if (have_next_resolution_timer_) return;
   if (last_resolution_timestamp_ >= 0) {
     const grpc_millis earliest_next_resolution =
         last_resolution_timestamp_ + min_time_between_resolutions_;
@@ -401,10 +395,6 @@ void AresDnsResolver::MaybeStartResolvingLocked() {
       self.release();
       grpc_timer_init(&next_resolution_timer_, ms_until_next_resolution,
                       &on_next_resolution_);
-      // TODO(dgq): remove the following two lines once Pick First stops
-      // discarding subchannels after selecting.
-      ++resolved_version_;
-      MaybeFinishNextLocked();
       return;
     }
   }
diff --git a/src/core/ext/filters/client_channel/resolver/dns/native/dns_resolver.cc b/src/core/ext/filters/client_channel/resolver/dns/native/dns_resolver.cc
index 282caf215c..65ff1ec1a5 100644
--- a/src/core/ext/filters/client_channel/resolver/dns/native/dns_resolver.cc
+++ b/src/core/ext/filters/client_channel/resolver/dns/native/dns_resolver.cc
@@ -247,13 +247,7 @@ void NativeDnsResolver::OnResolvedLocked(void* arg, grpc_error* error) {
 void NativeDnsResolver::MaybeStartResolvingLocked() {
   // If there is an existing timer, the time it fires is the earliest time we
   // can start the next resolution.
-  if (have_next_resolution_timer_) {
-    // TODO(dgq): remove the following two lines once Pick First stops
-    // discarding subchannels after selecting.
-    ++resolved_version_;
-    MaybeFinishNextLocked();
-    return;
-  }
+  if (have_next_resolution_timer_) return;
   if (last_resolution_timestamp_ >= 0) {
     const grpc_millis earliest_next_resolution =
         last_resolution_timestamp_ + min_time_between_resolutions_;
@@ -275,10 +269,6 @@ void NativeDnsResolver::MaybeStartResolvingLocked() {
       self.release();
       grpc_timer_init(&next_resolution_timer_, ms_until_next_resolution,
                       &on_next_resolution_);
-      // TODO(dgq): remove the following two lines once Pick First stops
-      // discarding subchannels after selecting.
-      ++resolved_version_;
-      MaybeFinishNextLocked();
       return;
     }
   }
diff --git a/src/core/ext/filters/client_channel/resolver/fake/fake_resolver.cc b/src/core/ext/filters/client_channel/resolver/fake/fake_resolver.cc
index 99a33f2277..144ac24a56 100644
--- a/src/core/ext/filters/client_channel/resolver/fake/fake_resolver.cc
+++ b/src/core/ext/filters/client_channel/resolver/fake/fake_resolver.cc
@@ -73,11 +73,6 @@ class FakeResolver : public Resolver {
   // Results to use for the pretended re-resolution in
   // RequestReresolutionLocked().
   grpc_channel_args* reresolution_results_ = nullptr;
-  // TODO(juanlishen): This can go away once pick_first is changed to not throw
-  // away its subchannels, since that will eliminate its dependence on
-  // channel_saw_error_locked() causing an immediate resolver return.
-  // A copy of the most-recently used resolution results.
-  grpc_channel_args* last_used_results_ = nullptr;
   // pending next completion, or NULL
   grpc_closure* next_completion_ = nullptr;
   // target result address for next completion
@@ -96,7 +91,6 @@ FakeResolver::FakeResolver(const ResolverArgs& args) : Resolver(args.combiner) {
 FakeResolver::~FakeResolver() {
   grpc_channel_args_destroy(next_results_);
   grpc_channel_args_destroy(reresolution_results_);
-  grpc_channel_args_destroy(last_used_results_);
   grpc_channel_args_destroy(channel_args_);
 }
 
@@ -109,17 +103,11 @@ void FakeResolver::NextLocked(grpc_channel_args** target_result,
 }
 
 void FakeResolver::RequestReresolutionLocked() {
-  // A resolution must have been returned before an error is seen.
-  GPR_ASSERT(last_used_results_ != nullptr);
-  grpc_channel_args_destroy(next_results_);
   if (reresolution_results_ != nullptr) {
+    grpc_channel_args_destroy(next_results_);
     next_results_ = grpc_channel_args_copy(reresolution_results_);
-  } else {
-    // If reresolution_results is unavailable, re-resolve with the most-recently
-    // used results to avoid a no-op re-resolution.
-    next_results_ = grpc_channel_args_copy(last_used_results_);
+    MaybeFinishNextLocked();
   }
-  MaybeFinishNextLocked();
 }
 
 void FakeResolver::MaybeFinishNextLocked() {
@@ -161,8 +149,6 @@ void FakeResolverResponseGenerator::SetResponseLocked(void* arg,
   FakeResolver* resolver = closure_arg->generator->resolver_;
   grpc_channel_args_destroy(resolver->next_results_);
   resolver->next_results_ = closure_arg->response;
-  grpc_channel_args_destroy(resolver->last_used_results_);
-  resolver->last_used_results_ = grpc_channel_args_copy(closure_arg->response);
   resolver->MaybeFinishNextLocked();
   Delete(closure_arg);
 }
diff --git a/src/core/ext/filters/client_channel/resolver/fake/fake_resolver.h b/src/core/ext/filters/client_channel/resolver/fake/fake_resolver.h
index e5175f9b7b..708eaf1147 100644
--- a/src/core/ext/filters/client_channel/resolver/fake/fake_resolver.h
+++ b/src/core/ext/filters/client_channel/resolver/fake/fake_resolver.h
@@ -53,7 +53,8 @@ class FakeResolverResponseGenerator
   // The new re-resolution response replaces any previous re-resolution
   // response that may have been set by a previous call.
   // If the re-resolution response is set to NULL, then the fake
-  // resolver will return the last value set via \a SetResponse().
+  // resolver will not return anything when \a RequestReresolutionLocked()
+  // is called.
   void SetReresolutionResponse(grpc_channel_args* response);
 
   // Tells the resolver to return a transient failure (signalled by
diff --git a/src/core/ext/filters/client_channel/resolver/sockaddr/sockaddr_resolver.cc b/src/core/ext/filters/client_channel/resolver/sockaddr/sockaddr_resolver.cc
index f74ac5aebe..801734764b 100644
--- a/src/core/ext/filters/client_channel/resolver/sockaddr/sockaddr_resolver.cc
+++ b/src/core/ext/filters/client_channel/resolver/sockaddr/sockaddr_resolver.cc
@@ -50,8 +50,6 @@ class SockaddrResolver : public Resolver {
   void NextLocked(grpc_channel_args** result,
                   grpc_closure* on_complete) override;
 
-  void RequestReresolutionLocked() override;
-
   void ShutdownLocked() override;
 
  private:
@@ -90,11 +88,6 @@ void SockaddrResolver::NextLocked(grpc_channel_args** target_result,
   MaybeFinishNextLocked();
 }
 
-void SockaddrResolver::RequestReresolutionLocked() {
-  published_ = false;
-  MaybeFinishNextLocked();
-}
-
 void SockaddrResolver::ShutdownLocked() {
   if (next_completion_ != nullptr) {
     *target_result_ = nullptr;
diff --git a/test/core/client_channel/resolvers/dns_resolver_cooldown_test.cc b/test/core/client_channel/resolvers/dns_resolver_cooldown_test.cc
index b1f3a1c08a..1a7db40f59 100644
--- a/test/core/client_channel/resolvers/dns_resolver_cooldown_test.cc
+++ b/test/core/client_channel/resolvers/dns_resolver_cooldown_test.cc
@@ -28,12 +28,16 @@
 #include "src/core/lib/iomgr/sockaddr_utils.h"
 #include "test/core/util/test_config.h"
 
+constexpr int kMinResolutionPeriodMs = 1000;
+// Provide some slack when checking intervals, to allow for test timing issues.
+constexpr int kMinResolutionPeriodForCheckMs = 900;
+
 extern grpc_address_resolver_vtable* grpc_resolve_address_impl;
 static grpc_address_resolver_vtable* default_resolve_address;
 
 static grpc_combiner* g_combiner;
 
-grpc_ares_request* (*g_default_dns_lookup_ares_locked)(
+static grpc_ares_request* (*g_default_dns_lookup_ares_locked)(
     const char* dns_server, const char* name, const char* default_port,
     grpc_pollset_set* interested_parties, grpc_closure* on_done,
     grpc_lb_addresses** addrs, bool check_grpclb, char** service_config_json,
@@ -43,7 +47,7 @@ grpc_ares_request* (*g_default_dns_lookup_ares_locked)(
 // times a system-level resolution has happened.
 static int g_resolution_count;
 
-struct iomgr_args {
+static struct iomgr_args {
   gpr_event ev;
   gpr_atm done_atm;
   gpr_mu* mu;
@@ -61,6 +65,16 @@ static void test_resolve_address_impl(const char* name,
   default_resolve_address->resolve_address(
       name, default_port, g_iomgr_args.pollset_set, on_done, addrs);
   ++g_resolution_count;
+  static grpc_millis last_resolution_time = 0;
+  if (last_resolution_time == 0) {
+    last_resolution_time =
+        grpc_timespec_to_millis_round_up(gpr_now(GPR_CLOCK_MONOTONIC));
+  } else {
+    grpc_millis now =
+        grpc_timespec_to_millis_round_up(gpr_now(GPR_CLOCK_MONOTONIC));
+    GPR_ASSERT(now - last_resolution_time >= kMinResolutionPeriodForCheckMs);
+    last_resolution_time = now;
+  }
 }
 
 static grpc_error* test_blocking_resolve_address_impl(
@@ -73,7 +87,7 @@ static grpc_error* test_blocking_resolve_address_impl(
 static grpc_address_resolver_vtable test_resolver = {
     test_resolve_address_impl, test_blocking_resolve_address_impl};
 
-grpc_ares_request* test_dns_lookup_ares_locked(
+static grpc_ares_request* test_dns_lookup_ares_locked(
     const char* dns_server, const char* name, const char* default_port,
     grpc_pollset_set* interested_parties, grpc_closure* on_done,
     grpc_lb_addresses** addrs, bool check_grpclb, char** service_config_json,
@@ -82,6 +96,16 @@ grpc_ares_request* test_dns_lookup_ares_locked(
       dns_server, name, default_port, g_iomgr_args.pollset_set, on_done, addrs,
       check_grpclb, service_config_json, combiner);
   ++g_resolution_count;
+  static grpc_millis last_resolution_time = 0;
+  if (last_resolution_time == 0) {
+    last_resolution_time =
+        grpc_timespec_to_millis_round_up(gpr_now(GPR_CLOCK_MONOTONIC));
+  } else {
+    grpc_millis now =
+        grpc_timespec_to_millis_round_up(gpr_now(GPR_CLOCK_MONOTONIC));
+    GPR_ASSERT(now - last_resolution_time >= kMinResolutionPeriodForCheckMs);
+    last_resolution_time = now;
+  }
   return result;
 }
 
@@ -91,7 +115,7 @@ static gpr_timespec test_deadline(void) {
 
 static void do_nothing(void* arg, grpc_error* error) {}
 
-void iomgr_args_init(iomgr_args* args) {
+static void iomgr_args_init(iomgr_args* args) {
   gpr_event_init(&args->ev);
   args->pollset = static_cast<grpc_pollset*>(gpr_zalloc(grpc_pollset_size()));
   grpc_pollset_init(args->pollset, &args->mu);
@@ -100,7 +124,7 @@ void iomgr_args_init(iomgr_args* args) {
   gpr_atm_rel_store(&args->done_atm, 0);
 }
 
-void iomgr_args_finish(iomgr_args* args) {
+static void iomgr_args_finish(iomgr_args* args) {
   GPR_ASSERT(gpr_event_wait(&args->ev, test_deadline()));
   grpc_pollset_set_del_pollset(args->pollset_set, args->pollset);
   grpc_pollset_set_destroy(args->pollset_set);
@@ -146,29 +170,19 @@ struct OnResolutionCallbackArg {
   const char* uri_str = nullptr;
   grpc_core::OrphanablePtr<grpc_core::Resolver> resolver;
   grpc_channel_args* result = nullptr;
-  grpc_millis delay_before_second_resolution = 0;
 };
 
-// Counter for the number of times a resolution notification callback has been
-// invoked.
-static int g_on_resolution_invocations_count;
-
 // Set to true by the last callback in the resolution chain.
-bool g_all_callbacks_invoked;
+static bool g_all_callbacks_invoked;
 
-void on_fourth_resolution(void* arg, grpc_error* error) {
+static void on_second_resolution(void* arg, grpc_error* error) {
   OnResolutionCallbackArg* cb_arg = static_cast<OnResolutionCallbackArg*>(arg);
   grpc_channel_args_destroy(cb_arg->result);
   GPR_ASSERT(error == GRPC_ERROR_NONE);
-  ++g_on_resolution_invocations_count;
-  gpr_log(GPR_INFO,
-          "4th: g_on_resolution_invocations_count: %d, g_resolution_count: %d",
-          g_on_resolution_invocations_count, g_resolution_count);
-  // In this case we expect to have incurred in another system-level resolution
-  // because on_third_resolution slept for longer than the min resolution
-  // period.
-  GPR_ASSERT(g_on_resolution_invocations_count == 4);
-  GPR_ASSERT(g_resolution_count == 3);
+  gpr_log(GPR_INFO, "2nd: g_resolution_count: %d", g_resolution_count);
+  // The resolution callback was not invoked until new data was
+  // available, which was delayed until after the cooldown period.
+  GPR_ASSERT(g_resolution_count == 2);
   cb_arg->resolver.reset();
   gpr_atm_rel_store(&g_iomgr_args.done_atm, 1);
   gpr_mu_lock(g_iomgr_args.mu);
@@ -179,67 +193,13 @@ void on_fourth_resolution(void* arg, grpc_error* error) {
   g_all_callbacks_invoked = true;
 }
 
-void on_third_resolution(void* arg, grpc_error* error) {
-  OnResolutionCallbackArg* cb_arg = static_cast<OnResolutionCallbackArg*>(arg);
-  grpc_channel_args_destroy(cb_arg->result);
-  GPR_ASSERT(error == GRPC_ERROR_NONE);
-  ++g_on_resolution_invocations_count;
-  gpr_log(GPR_INFO,
-          "3rd: g_on_resolution_invocations_count: %d, g_resolution_count: %d",
-          g_on_resolution_invocations_count, g_resolution_count);
-  // The timer set because of the previous re-resolution request fires, so a new
-  // system-level resolution happened.
-  GPR_ASSERT(g_on_resolution_invocations_count == 3);
-  GPR_ASSERT(g_resolution_count == 2);
-  grpc_core::ExecCtx::Get()->TestOnlySetNow(
-      cb_arg->delay_before_second_resolution * 2);
-  cb_arg->resolver->NextLocked(
-      &cb_arg->result,
-      GRPC_CLOSURE_CREATE(on_fourth_resolution, arg,
-                          grpc_combiner_scheduler(g_combiner)));
-  cb_arg->resolver->RequestReresolutionLocked();
-  gpr_mu_lock(g_iomgr_args.mu);
-  GRPC_LOG_IF_ERROR("pollset_kick",
-                    grpc_pollset_kick(g_iomgr_args.pollset, nullptr));
-  gpr_mu_unlock(g_iomgr_args.mu);
-}
-
-void on_second_resolution(void* arg, grpc_error* error) {
-  OnResolutionCallbackArg* cb_arg = static_cast<OnResolutionCallbackArg*>(arg);
-  grpc_channel_args_destroy(cb_arg->result);
-  GPR_ASSERT(error == GRPC_ERROR_NONE);
-  ++g_on_resolution_invocations_count;
-  gpr_log(GPR_INFO,
-          "2nd: g_on_resolution_invocations_count: %d, g_resolution_count: %d",
-          g_on_resolution_invocations_count, g_resolution_count);
-  // The resolution request for which this function is the callback happened
-  // before the min resolution period. Therefore, no new system-level
-  // resolutions happened, as indicated by g_resolution_count. But a resolution
-  // timer was set to fire when the cooldown finishes.
-  GPR_ASSERT(g_on_resolution_invocations_count == 2);
-  GPR_ASSERT(g_resolution_count == 1);
-  // Register a new callback to capture the timer firing.
-  cb_arg->resolver->NextLocked(
-      &cb_arg->result,
-      GRPC_CLOSURE_CREATE(on_third_resolution, arg,
-                          grpc_combiner_scheduler(g_combiner)));
-  gpr_mu_lock(g_iomgr_args.mu);
-  GRPC_LOG_IF_ERROR("pollset_kick",
-                    grpc_pollset_kick(g_iomgr_args.pollset, nullptr));
-  gpr_mu_unlock(g_iomgr_args.mu);
-}
-
-void on_first_resolution(void* arg, grpc_error* error) {
+static void on_first_resolution(void* arg, grpc_error* error) {
   OnResolutionCallbackArg* cb_arg = static_cast<OnResolutionCallbackArg*>(arg);
   grpc_channel_args_destroy(cb_arg->result);
   GPR_ASSERT(error == GRPC_ERROR_NONE);
-  ++g_on_resolution_invocations_count;
-  gpr_log(GPR_INFO,
-          "1st: g_on_resolution_invocations_count: %d, g_resolution_count: %d",
-          g_on_resolution_invocations_count, g_resolution_count);
+  gpr_log(GPR_INFO, "1st: g_resolution_count: %d", g_resolution_count);
   // There's one initial system-level resolution and one invocation of a
   // notification callback (the current function).
-  GPR_ASSERT(g_on_resolution_invocations_count == 1);
   GPR_ASSERT(g_resolution_count == 1);
   cb_arg->resolver->NextLocked(
       &cb_arg->result,
@@ -265,9 +225,7 @@ static void start_test_under_combiner(void* arg, grpc_error* error) {
   grpc_core::ResolverArgs args;
   args.uri = uri;
   args.combiner = g_combiner;
-  g_on_resolution_invocations_count = 0;
   g_resolution_count = 0;
-  constexpr int kMinResolutionPeriodMs = 1000;
 
   grpc_arg cooldown_arg;
   cooldown_arg.key =
@@ -280,7 +238,6 @@ static void start_test_under_combiner(void* arg, grpc_error* error) {
   res_cb_arg->resolver = factory->CreateResolver(args);
   grpc_channel_args_destroy(cooldown_channel_args);
   GPR_ASSERT(res_cb_arg->resolver != nullptr);
-  res_cb_arg->delay_before_second_resolution = kMinResolutionPeriodMs;
   // First resolution, would incur in system-level resolution.
   res_cb_arg->resolver->NextLocked(
       &res_cb_arg->result,
diff --git a/test/core/client_channel/resolvers/fake_resolver_test.cc b/test/core/client_channel/resolvers/fake_resolver_test.cc
index 14caa3ea5d..f6696bf127 100644
--- a/test/core/client_channel/resolvers/fake_resolver_test.cc
+++ b/test/core/client_channel/resolvers/fake_resolver_test.cc
@@ -124,8 +124,8 @@ static void test_fake_resolver() {
       build_fake_resolver(combiner, response_generator.get());
   GPR_ASSERT(resolver.get() != nullptr);
   // Test 1: normal resolution.
-  // next_results != NULL, reresolution_results == NULL, last_used_results ==
-  // NULL. Expected response is next_results.
+  // next_results != NULL, reresolution_results == NULL.
+  // Expected response is next_results.
   grpc_channel_args* results = create_new_resolver_result();
   on_resolution_arg on_res_arg = create_on_resolution_arg(results);
   grpc_closure* on_resolution = GRPC_CLOSURE_CREATE(
@@ -137,10 +137,9 @@ static void test_fake_resolver() {
   GPR_ASSERT(gpr_event_wait(&on_res_arg.ev,
                             grpc_timeout_seconds_to_deadline(5)) != nullptr);
   // Test 2: update resolution.
-  // next_results != NULL, reresolution_results == NULL, last_used_results !=
-  // NULL. Expected response is next_results.
+  // next_results != NULL, reresolution_results == NULL.
+  // Expected response is next_results.
   results = create_new_resolver_result();
-  grpc_channel_args* last_used_results = grpc_channel_args_copy(results);
   on_res_arg = create_on_resolution_arg(results);
   on_resolution = GRPC_CLOSURE_CREATE(on_resolution_cb, &on_res_arg,
                                       grpc_combiner_scheduler(combiner));
@@ -150,21 +149,9 @@ static void test_fake_resolver() {
   grpc_core::ExecCtx::Get()->Flush();
   GPR_ASSERT(gpr_event_wait(&on_res_arg.ev,
                             grpc_timeout_seconds_to_deadline(5)) != nullptr);
-  // Test 3: fallback re-resolution.
-  // next_results == NULL, reresolution_results == NULL, last_used_results !=
-  // NULL. Expected response is last_used_results.
-  on_res_arg = create_on_resolution_arg(last_used_results);
-  on_resolution = GRPC_CLOSURE_CREATE(on_resolution_cb, &on_res_arg,
-                                      grpc_combiner_scheduler(combiner));
-  resolver->NextLocked(&on_res_arg.resolver_result, on_resolution);
-  // Trigger a re-resolution.
-  resolver->RequestReresolutionLocked();
-  grpc_core::ExecCtx::Get()->Flush();
-  GPR_ASSERT(gpr_event_wait(&on_res_arg.ev,
-                            grpc_timeout_seconds_to_deadline(5)) != nullptr);
-  // Test 4: normal re-resolution.
-  // next_results == NULL, reresolution_results != NULL, last_used_results !=
-  // NULL. Expected response is reresolution_results.
+  // Test 3: normal re-resolution.
+  // next_results == NULL, reresolution_results != NULL.
+  // Expected response is reresolution_results.
   grpc_channel_args* reresolution_results = create_new_resolver_result();
   on_res_arg =
       create_on_resolution_arg(grpc_channel_args_copy(reresolution_results));
@@ -180,9 +167,9 @@ static void test_fake_resolver() {
   grpc_core::ExecCtx::Get()->Flush();
   GPR_ASSERT(gpr_event_wait(&on_res_arg.ev,
                             grpc_timeout_seconds_to_deadline(5)) != nullptr);
-  // Test 5: repeat re-resolution.
-  // next_results == NULL, reresolution_results != NULL, last_used_results !=
-  // NULL. Expected response is reresolution_results.
+  // Test 4: repeat re-resolution.
+  // next_results == NULL, reresolution_results != NULL.
+  // Expected response is reresolution_results.
   on_res_arg = create_on_resolution_arg(reresolution_results);
   on_resolution = GRPC_CLOSURE_CREATE(on_resolution_cb, &on_res_arg,
                                       grpc_combiner_scheduler(combiner));
@@ -192,11 +179,10 @@ static void test_fake_resolver() {
   grpc_core::ExecCtx::Get()->Flush();
   GPR_ASSERT(gpr_event_wait(&on_res_arg.ev,
                             grpc_timeout_seconds_to_deadline(5)) != nullptr);
-  // Test 6: normal resolution.
-  // next_results != NULL, reresolution_results != NULL, last_used_results !=
-  // NULL. Expected response is next_results.
+  // Test 5: normal resolution.
+  // next_results != NULL, reresolution_results != NULL.
+  // Expected response is next_results.
   results = create_new_resolver_result();
-  last_used_results = grpc_channel_args_copy(results);
   on_res_arg = create_on_resolution_arg(results);
   on_resolution = GRPC_CLOSURE_CREATE(on_resolution_cb, &on_res_arg,
                                       grpc_combiner_scheduler(combiner));
@@ -206,23 +192,7 @@ static void test_fake_resolver() {
   grpc_core::ExecCtx::Get()->Flush();
   GPR_ASSERT(gpr_event_wait(&on_res_arg.ev,
                             grpc_timeout_seconds_to_deadline(5)) != nullptr);
-  // Test 7: fallback re-resolution.
-  // next_results == NULL, reresolution_results == NULL, last_used_results !=
-  // NULL. Expected response is last_used_results.
-  on_res_arg = create_on_resolution_arg(last_used_results);
-  on_resolution = GRPC_CLOSURE_CREATE(on_resolution_cb, &on_res_arg,
-                                      grpc_combiner_scheduler(combiner));
-  resolver->NextLocked(&on_res_arg.resolver_result, on_resolution);
-  // Reset reresolution_results.
-  response_generator->SetReresolutionResponse(nullptr);
-  // Flush here to guarantee that reresolution_results has been reset.
-  grpc_core::ExecCtx::Get()->Flush();
-  // Trigger a re-resolution.
-  resolver->RequestReresolutionLocked();
-  grpc_core::ExecCtx::Get()->Flush();
-  GPR_ASSERT(gpr_event_wait(&on_res_arg.ev,
-                            grpc_timeout_seconds_to_deadline(5)) != nullptr);
-  // Test 8: no-op.
+  // Test 6: no-op.
   // Requesting a new resolution without setting the response shouldn't trigger
   // the resolution callback.
   memset(&on_res_arg, 0, sizeof(on_res_arg));
diff --git a/test/cpp/end2end/client_lb_end2end_test.cc b/test/cpp/end2end/client_lb_end2end_test.cc
index 68219c16dc..e5d6132012 100644
--- a/test/cpp/end2end/client_lb_end2end_test.cc
+++ b/test/cpp/end2end/client_lb_end2end_test.cc
@@ -129,12 +129,23 @@ class ClientLbEnd2endTest : public ::testing::Test {
     }
   }
 
-  void StartServers(size_t num_servers,
-                    std::vector<int> ports = std::vector<int>()) {
+  void CreateServers(size_t num_servers,
+                     std::vector<int> ports = std::vector<int>()) {
+    servers_.clear();
     for (size_t i = 0; i < num_servers; ++i) {
       int port = 0;
       if (ports.size() == num_servers) port = ports[i];
-      servers_.emplace_back(new ServerData(server_host_, port));
+      servers_.emplace_back(new ServerData(port));
+    }
+  }
+
+  void StartServer(size_t index) { servers_[index]->Start(server_host_); }
+
+  void StartServers(size_t num_servers,
+                    std::vector<int> ports = std::vector<int>()) {
+    CreateServers(num_servers, ports);
+    for (size_t i = 0; i < num_servers; ++i) {
+      StartServer(i);
     }
   }
 
@@ -240,20 +251,23 @@ class ClientLbEnd2endTest : public ::testing::Test {
     std::unique_ptr<std::thread> thread_;
     bool server_ready_ = false;
 
-    explicit ServerData(const grpc::string& server_host, int port = 0) {
+    explicit ServerData(int port = 0) {
       port_ = port > 0 ? port : grpc_pick_unused_port_or_die();
+    }
+
+    void Start(const grpc::string& server_host) {
       gpr_log(GPR_INFO, "starting server on port %d", port_);
       std::mutex mu;
       std::unique_lock<std::mutex> lock(mu);
       std::condition_variable cond;
       thread_.reset(new std::thread(
-          std::bind(&ServerData::Start, this, server_host, &mu, &cond)));
+          std::bind(&ServerData::Serve, this, server_host, &mu, &cond)));
       cond.wait(lock, [this] { return server_ready_; });
       server_ready_ = false;
       gpr_log(GPR_INFO, "server startup complete");
     }
 
-    void Start(const grpc::string& server_host, std::mutex* mu,
+    void Serve(const grpc::string& server_host, std::mutex* mu,
                std::condition_variable* cond) {
       std::ostringstream server_address;
       server_address << server_host << ":" << port_;
@@ -601,6 +615,41 @@ TEST_F(ClientLbEnd2endTest, PickFirstReresolutionNoSelected) {
   EXPECT_EQ("pick_first", channel->GetLoadBalancingPolicyName());
 }
 
+TEST_F(ClientLbEnd2endTest, PickFirstReconnectWithoutNewResolverResult) {
+  std::vector<int> ports = {grpc_pick_unused_port_or_die()};
+  StartServers(1, ports);
+  auto channel = BuildChannel("pick_first");
+  auto stub = BuildStub(channel);
+  SetNextResolution(ports);
+  gpr_log(GPR_INFO, "****** INITIAL CONNECTION *******");
+  WaitForServer(stub, 0, DEBUG_LOCATION);
+  gpr_log(GPR_INFO, "****** STOPPING SERVER ******");
+  servers_[0]->Shutdown();
+  EXPECT_TRUE(WaitForChannelNotReady(channel.get()));
+  gpr_log(GPR_INFO, "****** RESTARTING SERVER ******");
+  StartServers(1, ports);
+  WaitForServer(stub, 0, DEBUG_LOCATION);
+}
+
+TEST_F(ClientLbEnd2endTest,
+       PickFirstReconnectWithoutNewResolverResultStartsFromTopOfList) {
+  std::vector<int> ports = {grpc_pick_unused_port_or_die(),
+                            grpc_pick_unused_port_or_die()};
+  CreateServers(2, ports);
+  StartServer(1);
+  auto channel = BuildChannel("pick_first");
+  auto stub = BuildStub(channel);
+  SetNextResolution(ports);
+  gpr_log(GPR_INFO, "****** INITIAL CONNECTION *******");
+  WaitForServer(stub, 1, DEBUG_LOCATION);
+  gpr_log(GPR_INFO, "****** STOPPING SERVER ******");
+  servers_[1]->Shutdown();
+  EXPECT_TRUE(WaitForChannelNotReady(channel.get()));
+  gpr_log(GPR_INFO, "****** STARTING BOTH SERVERS ******");
+  StartServers(2, ports);
+  WaitForServer(stub, 0, DEBUG_LOCATION);
+}
+
 TEST_F(ClientLbEnd2endTest, PickFirstCheckStateBeforeStartWatch) {
   std::vector<int> ports = {grpc_pick_unused_port_or_die()};
   StartServers(1, ports);
@@ -613,7 +662,6 @@ TEST_F(ClientLbEnd2endTest, PickFirstCheckStateBeforeStartWatch) {
   servers_[0]->Shutdown();
   // Channel 1 will receive a re-resolution containing the same server. It will
   // create a new subchannel and hold a ref to it.
-  servers_.clear();
   StartServers(1, ports);
   gpr_log(GPR_INFO, "****** SERVER RESTARTED *******");
   auto channel_2 = BuildChannel("pick_first");
@@ -632,7 +680,6 @@ TEST_F(ClientLbEnd2endTest, PickFirstCheckStateBeforeStartWatch) {
   EXPECT_TRUE(WaitForChannelNotReady(channel_2.get()));
   // Channel 2 will also receive a re-resolution containing the same server.
   // Both channels will ref the same subchannel that failed.
-  servers_.clear();
   StartServers(1, ports);
   gpr_log(GPR_INFO, "****** SERVER RESTARTED AGAIN *******");
   gpr_log(GPR_INFO, "****** CHANNEL 2 STARTING A CALL *******");
@@ -860,7 +907,7 @@ TEST_F(ClientLbEnd2endTest, RoundRobinReresolve) {
   // Kill all servers
   gpr_log(GPR_INFO, "****** ABOUT TO KILL SERVERS *******");
   for (size_t i = 0; i < servers_.size(); ++i) {
-    servers_[i]->Shutdown(false);
+    servers_[i]->Shutdown(true);
   }
   gpr_log(GPR_INFO, "****** SERVERS KILLED *******");
   gpr_log(GPR_INFO, "****** SENDING DOOMED REQUESTS *******");
@@ -921,7 +968,8 @@ TEST_F(ClientLbEnd2endTest, RoundRobinSingleReconnect) {
   // No requests have gone to the deceased server.
   EXPECT_EQ(pre_death, post_death);
   // Bring the first server back up.
-  servers_[0].reset(new ServerData(server_host_, ports[0]));
+  servers_[0].reset(new ServerData(ports[0]));
+  StartServer(0);
   // Requests should start arriving at the first server either right away (if
   // the server managed to start before the RR policy retried the subchannel) or
   // after the subchannel retry delay otherwise (RR's subchannel retried before