[-] fix QueryMeasurements nil pointer dereference, fixes #766

internal/reaper/cache.go

@@ -32,7 +32,7 @@ func GetMonitoredDatabaseByUniqueName(ctx context.Context, name string) (*source
 	monitoredDbCacheLock.RLock()
 	defer monitoredDbCacheLock.RUnlock()
 	md, exists := monitoredDbCache[name]
-	if !exists || md == nil {
+	if !exists || md == nil || md.Conn == nil {
 		return nil, fmt.Errorf("database %s not found in cache", name)
 	}
 	return md, nil

internal/reaper/reaper.go

@@ -264,7 +264,6 @@ func (r *Reaper) ShutdownOldWorkers(ctx context.Context, hostsToShutDownDueToRol
 	r.CloseResourcesForRemovedMonitoredDBs(hostsToShutDownDueToRoleChange)
 }
 
-// metrics.ControlMessage notifies of shutdown + interval change
 func (r *Reaper) reapMetricMeasurements(ctx context.Context, md *sources.SourceConn, metricName string) {
 	hostState := make(map[string]map[string]string)
 	var lastUptimeS int64 = -1 // used for "server restarted" event detection

internal/sources/conn.go

@@ -276,20 +276,16 @@ func (mds SourceConns) SyncFromReader(r Reader) (newmds SourceConns, err error)
 		return nil, err
 	}
 	newmds, err = srcs.ResolveDatabases()
-	for _, newMD := range newmds {
+	for i, newMD := range newmds {
 		md := mds.GetMonitoredDatabase(newMD.Name)
 		if md == nil {
 			continue
 		}
 		if reflect.DeepEqual(md.Source, newMD.Source) {
-			// keep the existing connection if the source is the same
-			newMD.Conn = md.Conn
-			newMD.ConnConfig = md.ConnConfig
+			// replace with the existing connection if the source is the same
+			newmds[i] = md
 			continue
 		}
-		if md.Conn != nil {
-			md.Conn.Close()
-		}
 	}
 	return newmds, err
 }

internal/sources/conn_test.go

@@ -240,7 +240,6 @@ func TestMonitoredDatabases_SyncFromReader(t *testing.T) {
 	assert.NotNil(t, mdbs, "ResolveDatabases() = nil, want not nil")
 	// pretend that we have a connection
 	mdbs[0].Conn = db
-	db.ExpectClose()
 	// sync the databases and make sure they are the same
 	newmdbs, _ := mdbs.SyncFromReader(reader)
 	assert.NotNil(t, newmdbs)