querier_cache: insert_querier: ignore errors to register inactive reader

Since the reader may normally dropped upon
registration, hitting an error is equivalent to having
it evicted at any time, so just log the exception
and ignore it.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

querier.cc

@@ -22,11 +22,14 @@
 #include "querier.hh"
 
 #include "schema.hh"
+#include "log.hh"
 
 #include <boost/range/adaptor/map.hpp>
 
 namespace query {
 
+logging::logger qlogger("querier_cache");
+
 enum class can_use {
     yes,
     no_schema_version_mismatch,
@@ -251,6 +254,7 @@ static void insert_querier(
     if (!irh) {
         return;
     }
+  try {
     auto cleanup_irh = defer([&] {
         sem.unregister_inactive_read(std::move(irh));
     });
@@ -282,6 +286,13 @@ static void insert_querier(
     querier_utils::set_inactive_read_handle(*it->second, std::move(irh));
     cleanup_index.cancel();
     cleanup_irh.cancel();
+  } catch (...) {
+    // It is okay to swallow the exception since
+    // we're allowed to drop the reader upon registration
+    // due to lack of resources - in which case we already
+    // drop the querier.
+    qlogger.warn("Failed to insert querier into index: {}. Ignored as if it was evicted upon registration", std::current_exception());
+  }
 }
 
 void querier_cache::insert(utils::UUID key, data_querier&& q, tracing::trace_state_ptr trace_state) {