Expose clients maxwait time in SHOW CLIENTS response via admin (#639)

* Expose clients maxwait time in SHOW CLIENTS response via PgCat admin
Displays the maxwait via maxwait_seconds and maxwait_us columns for each client that can be used to track down the wait time per client in a case where the overall pool stats shows waiting time. The maxwait_us, similar to the pool stats setup, is configured to display as a remainder alongside the maxwait_seconds.

* Use maxwait instead of maxwait_seconds to match pools column name

---------

Co-authored-by: Calvin Hughes <9379992+calvinhughes@users.noreply.github.com>

Author: calvinhughes

src/admin.rs

@@ -690,13 +690,16 @@ where
         ("query_count", DataType::Numeric),
         ("error_count", DataType::Numeric),
         ("age_seconds", DataType::Numeric),
+        ("maxwait", DataType::Numeric),
+        ("maxwait_us", DataType::Numeric),
     ];
 
     let new_map = get_client_stats();
     let mut res = BytesMut::new();
     res.put(row_description(&columns));
 
     for (_, client) in new_map {
+        let max_wait = client.max_wait_time.load(Ordering::Relaxed);
         let row = vec![
             format!("{:#010X}", client.client_id()),
             client.pool_name(),
@@ -710,6 +713,8 @@ where
                 .duration_since(client.connect_time())
                 .as_secs()
                 .to_string(),
+            (max_wait / 1_000_000).to_string(),
+            (max_wait % 1_000_000).to_string(),
         ];
 
         res.put(data_row(&row));

tests/ruby/stats_spec.rb

@@ -329,6 +329,40 @@
       admin_conn.close
       connections.map(&:close)
     end
+
+    context "when client has waited for a server" do
+      let(:processes) { Helpers::Pgcat.single_instance_setup("sharded_db", 2) }
+
+      it "shows correct maxwait" do
+        threads = []
+        connections = Array.new(3) { |i| PG::connect("#{pgcat_conn_str}?application_name=app#{i}") }
+        connections.each do |c|
+          threads << Thread.new { c.async_exec("SELECT pg_sleep(1.5)") rescue nil }
+        end
+
+        sleep(2.5) # Allow time for stats to update
+        admin_conn = PG::connect(processes.pgcat.admin_connection_string)
+        results = admin_conn.async_exec("SHOW CLIENTS")
+
+        normal_client_results = results.reject { |r| r["database"] == "pgcat" }
+
+        non_waiting_clients = normal_client_results.select { |c| c["maxwait"] == "0" }
+        waiting_clients = normal_client_results.select { |c| c["maxwait"].to_i > 0 }
+
+        expect(non_waiting_clients.count).to eq(2)
+        non_waiting_clients.each do |client|
+          expect(client["maxwait_us"].to_i).to be_between(0, 50_000)
+        end
+
+        expect(waiting_clients.count).to eq(1)
+        waiting_clients.each do |client|
+          expect(client["maxwait_us"].to_i).to be_within(200_000).of(500_000)
+        end
+
+        admin_conn.close
+        connections.map(&:close)
+      end
+    end
   end