diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 8a7d13801318d..06b51551238a9 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,13 @@
+*   Properly synchronize `Mysql2Adapter#active?` and `TrilogyAdapter#active?`
+
+    As well as `disconnect!` and `verify!`.
+
+    This generally isn't a big problem as connections must not be shared between
+    threads, but is required when running transactional tests or system tests
+    and could lead to a SEGV.
+
+    *Jean Boussier*
+
 *   Support `:source_location` tag option for query log tags
 
     ```ruby
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index e435f582e516f..af23562dba8b5 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -718,13 +718,14 @@ def reconnect!(restore_transactions: false)
         end
       end
 
-
       # Disconnects from the database if already connected. Otherwise, this
       # method does nothing.
       def disconnect!
-        clear_cache!(new_connection: true)
-        reset_transaction
-        @raw_connection_dirty = false
+        @lock.synchronize do
+          clear_cache!(new_connection: true)
+          reset_transaction
+          @raw_connection_dirty = false
+        end
       end
 
       # Immediately forget this connection ever existed. Unlike disconnect!,
@@ -780,19 +781,17 @@ def requires_reloading?
       # is no longer active, then this method will reconnect to the database.
       def verify!
         unless active?
-          if @unconfigured_connection
-            @lock.synchronize do
-              if @unconfigured_connection
-                @raw_connection = @unconfigured_connection
-                @unconfigured_connection = nil
-                configure_connection
-                @verified = true
-                return
-              end
+          @lock.synchronize do
+            if @unconfigured_connection
+              @raw_connection = @unconfigured_connection
+              @unconfigured_connection = nil
+              configure_connection
+              @verified = true
+              return
             end
-          end
 
-          reconnect!(restore_transactions: true)
+            reconnect!(restore_transactions: true)
+          end
         end
 
         @verified = true
diff --git a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
index ce325ac032da7..2a85a24864958 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
@@ -113,7 +113,7 @@ def connected?
       end
 
       def active?
-        !!@raw_connection&.ping
+        connected? && @lock.synchronize { @raw_connection&.ping } || false
       end
 
       alias :reset! :reconnect!
@@ -121,15 +121,19 @@ def active?
       # Disconnects from the database if already connected.
       # Otherwise, this method does nothing.
       def disconnect!
-        super
-        @raw_connection&.close
-        @raw_connection = nil
+        @lock.synchronize do
+          super
+          @raw_connection&.close
+          @raw_connection = nil
+        end
       end
 
       def discard! # :nodoc:
-        super
-        @raw_connection&.automatic_close = false
-        @raw_connection = nil
+        @lock.synchronize do
+          super
+          @raw_connection&.automatic_close = false
+          @raw_connection = nil
+        end
       end
 
       private
@@ -144,9 +148,11 @@ def connect
         end
 
         def reconnect
-          @raw_connection&.close
-          @raw_connection = nil
-          connect
+          @lock.synchronize do
+            @raw_connection&.close
+            @raw_connection = nil
+            connect
+          end
         end
 
         def configure_connection
diff --git a/activerecord/lib/active_record/connection_adapters/trilogy_adapter.rb b/activerecord/lib/active_record/connection_adapters/trilogy_adapter.rb
index 46d782a3b3a67..3eebc83cdeb12 100644
--- a/activerecord/lib/active_record/connection_adapters/trilogy_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/trilogy_adapter.rb
@@ -117,7 +117,7 @@ def connected?
       end
 
       def active?
-        connection&.ping || false
+        connected? && @lock.synchronize { @raw_connection&.ping } || false
       rescue ::Trilogy::Error
         false
       end
@@ -125,18 +125,18 @@ def active?
       alias reset! reconnect!
 
       def disconnect!
-        super
-        unless connection.nil?
-          connection.close
-          self.connection = nil
+        @lock.synchronize do
+          super
+          @raw_connection&.close
+          @raw_connection = nil
         end
       end
 
       def discard!
-        super
-        unless connection.nil?
-          connection.discard!
-          self.connection = nil
+        @lock.synchronize do
+          super
+          @raw_connection&.discard!
+          @raw_connection = nil
         end
       end
 
@@ -166,23 +166,15 @@ def error_number(exception)
           exception.error_code if exception.respond_to?(:error_code)
         end
 
-        def connection
-          @raw_connection
-        end
-
-        def connection=(conn)
-          @raw_connection = conn
-        end
-
         def connect
-          self.connection = self.class.new_client(@config)
+          @raw_connection = self.class.new_client(@config)
         rescue ConnectionNotEstablished => ex
           raise ex.set_pool(@pool)
         end
 
         def reconnect
-          connection&.close
-          self.connection = nil
+          @raw_connection&.close
+          @raw_connection = nil
           connect
         end
 
diff --git a/activerecord/test/cases/adapter_test.rb b/activerecord/test/cases/adapter_test.rb
index a3c30a3508c72..1cda381c2244c 100644
--- a/activerecord/test/cases/adapter_test.rb
+++ b/activerecord/test/cases/adapter_test.rb
@@ -809,6 +809,48 @@ def kill_connection_from_server(connection_id)
         end
     end
   end
+
+  class AdapterThreadSafetyTest < ActiveRecord::TestCase
+    setup do
+      @threads = []
+      @connection = ActiveRecord::Base.connection_pool.checkout
+    end
+
+    teardown do
+      @threads.each(&:kill)
+    end
+
+    test "#active? is synchronized" do
+      threads(2, 25) { @connection.select_all("SELECT 1") }
+      threads(2, 25) { @connection.verify! }
+      threads(2, 25) { @connection.disconnect! }
+
+      join
+    end
+
+    test "#verify! is synchronized" do
+      threads(2, 25) { @connection.verify! }
+      threads(2, 25) { @connection.disconnect! }
+
+      join
+    end
+
+    private
+      def join
+        @threads.shuffle.each(&:join)
+      end
+
+      def threads(count, times)
+        @threads += count.times.map do
+          Thread.new do
+            times.times do
+              yield
+              Thread.pass
+            end
+          end
+        end
+      end
+  end
 end
 
 if ActiveRecord::Base.connection.supports_advisory_locks?
diff --git a/activerecord/test/cases/adapters/trilogy/trilogy_adapter_test.rb b/activerecord/test/cases/adapters/trilogy/trilogy_adapter_test.rb
index ea210c5f613a7..2a73f7b7c8630 100644
--- a/activerecord/test/cases/adapters/trilogy/trilogy_adapter_test.rb
+++ b/activerecord/test/cases/adapters/trilogy/trilogy_adapter_test.rb
@@ -95,24 +95,24 @@ class TrilogyAdapterTest < ActiveRecord::TrilogyTestCase
   end
 
   test "#active? answers false with connection and exception" do
-    @conn.send(:connection).stub(:ping, -> { raise ::Trilogy::BaseError.new }) do
+    @conn.instance_variable_get(:@raw_connection).stub(:ping, -> { raise ::Trilogy::BaseError.new }) do
       assert_equal false, @conn.active?
     end
   end
 
   test "#reconnect answers new connection with existing connection" do
-    old_connection = @conn.send(:connection)
+    old_connection = @conn.instance_variable_get(:@raw_connection)
     @conn.reconnect!
-    connection = @conn.send(:connection)
+    connection = @conn.instance_variable_get(:@raw_connection)
 
     assert_instance_of Trilogy, connection
     assert_not_equal old_connection, connection
   end
 
   test "#reset answers new connection with existing connection" do
-    old_connection = @conn.send(:connection)
+    old_connection = @conn.instance_variable_get(:@raw_connection)
     @conn.reset!
-    connection = @conn.send(:connection)
+    connection = @conn.instance_variable_get(:@raw_connection)
 
     assert_instance_of Trilogy, connection
     assert_not_equal old_connection, connection