only report wait times from clients currently waiting to match behavior of pgbouncer (#655)

* Change maxwait to only report wait times from clients currently waiting to match behavior of pgbouncer

* Fix tests

src/stats/client.rs

@@ -38,8 +38,10 @@ pub struct ClientStats {
     /// Total time spent waiting for a connection from pool, measures in microseconds
     pub total_wait_time: Arc<AtomicU64>,
 
-    /// Maximum time spent waiting for a connection from pool, measures in microseconds
+    /// When this client started waiting.
-    pub max_wait_time: Arc<AtomicU64>,
+    /// Stored as microseconds since connect_time so it can fit in an AtomicU64 instead
+    /// of us using an "AtomicInstant"
+    pub wait_start: Arc<AtomicU64>,
 
     /// Current state of the client
     pub state: Arc<AtomicClientState>,
@@ -63,7 +65,7 @@ impl Default for ClientStats {
             username: String::new(),
             pool_name: String::new(),
             total_wait_time: Arc::new(AtomicU64::new(0)),
-            max_wait_time: Arc::new(AtomicU64::new(0)),
+            wait_start: Arc::new(AtomicU64::new(0)),
             state: Arc::new(AtomicClientState::new(ClientState::Idle)),
             transaction_count: Arc::new(AtomicU64::new(0)),
             query_count: Arc::new(AtomicU64::new(0)),
@@ -111,6 +113,11 @@ impl ClientStats {
 
     /// Reports a client is waiting for a connection
     pub fn waiting(&self) {
+        // safe to truncate, we only lose info if duration is greater than ~585,000 years
+        self.wait_start.store(
+            Instant::now().duration_since(self.connect_time).as_micros() as u64,
+            Ordering::Relaxed,
+        );
         self.state.store(ClientState::Waiting, Ordering::Relaxed);
     }
 
@@ -134,8 +141,6 @@ impl ClientStats {
     pub fn checkout_time(&self, microseconds: u64) {
         self.total_wait_time
             .fetch_add(microseconds, Ordering::Relaxed);
-        self.max_wait_time
-            .fetch_max(microseconds, Ordering::Relaxed);
     }
 
     /// Report a query executed by a client against a server

src/stats/pool.rs

@@ -4,6 +4,7 @@ use super::{ClientState, ServerState};
 use crate::{config::PoolMode, messages::DataType, pool::PoolIdentifier};
 use std::collections::HashMap;
 use std::sync::atomic::*;
+use tokio::time::Instant;
 
 use crate::pool::get_all_pools;
 
@@ -53,6 +54,7 @@ impl PoolStats {
             );
         }
 
+        let now = Instant::now();
         for client in client_map.values() {
             match map.get_mut(&PoolIdentifier {
                 db: client.pool_name(),
@@ -62,10 +64,16 @@ impl PoolStats {
                     match client.state.load(Ordering::Relaxed) {
                         ClientState::Active => pool_stats.cl_active += 1,
                         ClientState::Idle => pool_stats.cl_idle += 1,
-                        ClientState::Waiting => pool_stats.cl_waiting += 1,
+                        ClientState::Waiting => {
+                            pool_stats.cl_waiting += 1;
+                            // wait_start is measured as microseconds since connect_time
+                            // so compute wait_time as (now() - connect_time) - (wait_start - connect_time)
+                            let duration_since_connect = now.duration_since(client.connect_time());
+                            let wait_time = (duration_since_connect.as_micros() as u64)
+                                - client.wait_start.load(Ordering::Relaxed);
+                            pool_stats.maxwait = std::cmp::max(pool_stats.maxwait, wait_time);
+                        }
                     }
-                    let max_wait = client.max_wait_time.load(Ordering::Relaxed);
-                    pool_stats.maxwait = std::cmp::max(pool_stats.maxwait, max_wait);
                 }
                 None => debug!("Client from an obselete pool"),
             }

tests/ruby/stats_spec.rb

@@ -233,7 +233,7 @@ describe "Stats" do
         sleep(1.1) # Allow time for stats to update
         admin_conn = PG::connect(processes.pgcat.admin_connection_string)
         results = admin_conn.async_exec("SHOW POOLS")[0]
-        %w[cl_idle cl_cancel_req sv_idle sv_used sv_tested sv_login maxwait].each do |s|
+        %w[cl_idle cl_cancel_req sv_idle sv_used sv_tested sv_login].each do |s|
           raise StandardError, "Field #{s} was expected to be 0 but found to be #{results[s]}" if results[s] != "0"
         end
 
@@ -260,12 +260,20 @@ describe "Stats" do
           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)
+
+        # two connections waiting => they report wait time
+        sleep(1.1) # Allow time for stats to update
+        results = admin_conn.async_exec("SHOW POOLS")[0]
+        expect(results["maxwait"]).to eq("1")
+        expect(results["maxwait_us"].to_i).to be_within(200_000).of(100_000)
+
+        sleep(2.5) # Allow time for stats to update
         results = admin_conn.async_exec("SHOW POOLS")[0]
 
-        expect(results["maxwait"]).to eq("1")
+        # no connections waiting => no reported wait time
-        expect(results["maxwait_us"].to_i).to be_within(200_000).of(500_000)
+        expect(results["maxwait"]).to eq("0")
+        expect(results["maxwait_us"]).to eq("0")
         connections.map(&:close)
 
         sleep(4.5) # Allow time for stats to update