Fix metric timeseries gaps on nodes with many cores

On nodes with >300 cores, one interval's burst of per-hwthread metrics
overran the fixed 200-slot channels. With blocking sends at every hop,
sink back-pressure propagated to the collectors, the collection round
exceeded the interval, and time.Ticker silently dropped the missed
ticks - whole intervals were skipped without any log message.

- multiChanTicker: deliver ticks non-blockingly and warn when a
  consumer misses a tick instead of stalling all consumers; guard the
  channel list with a mutex (data race with AddChannel)
- collectorManager: run the collection round detached from the tick
  loop, skip-and-warn when a round is still running, log per-collector
  and per-round durations at debug level, close serial collectors on
  shutdown
- metricRouter: buffer the interval timestamp channel and drain it
  before stamping, so metrics never carry the previous interval's
  timestamp; warn when the collector input channel is full at tick time
- main: scale the inter-manager channels to max(200, 24*NumCPU),
  overridable with the new optional channel_buffer_size config option
- add first unit tests for ticker, collector manager and router

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkg/multiChanTicker/README.md

@@ -30,8 +30,8 @@ Afterwards, you can add channels:
 
 ```golang
 t := MultiChanTicker(duration)
-c1 := make(chan time.Time)
-c2 := make(chan time.Time)
+c1 := make(chan time.Time, 1)
+c2 := make(chan time.Time, 1)
 t.AddChannel(c1)
 t.AddChannel(c2)
 
@@ -46,3 +46,5 @@ for {
 ```
 
 The result should be the same `time.Time` output in both channels, notified "simultaneously".
+
+Ticks are delivered with a non-blocking send: a consumer that has not yet read the previous tick does not stall the ticker (which would silently drop `time.Ticker` fires for all consumers); instead, the tick for that consumer is skipped and a warning is logged. Register buffered channels (capacity 1) so a consumer that is briefly busy at tick time does not lose the tick.

pkg/multiChanTicker/multiChanTicker.go

@@ -8,6 +8,8 @@
 package multiChanTicker
 
 import (
+	"fmt"
+	"sync"
 	"time"
 
 	cclog "github.com/ClusterCockpit/cc-lib/v2/ccLogger"
@@ -15,6 +17,7 @@ import (
 
 type multiChanTicker struct {
 	ticker   *time.Ticker
+	mutex    sync.Mutex // protects channels, which is appended to while the tick goroutine iterates it
 	channels []chan time.Time
 	done     chan bool
 }
@@ -40,21 +43,30 @@ func (t *multiChanTicker) Init(duration time.Duration) {
 				return
 			case ts := <-t.ticker.C:
 				cclog.ComponentDebug("MultiChanTicker", "Tick", ts)
-				for _, c := range t.channels {
+				t.mutex.Lock()
+				for i, c := range t.channels {
+					// Non-blocking send: a consumer that has not yet read the
+					// previous tick must not stall the ticker, otherwise
+					// time.Ticker silently drops fires for ALL consumers
 					select {
-					case <-t.done:
-						done()
-						return
 					case c <- ts:
+					default:
+						cclog.ComponentWarn("MultiChanTicker", fmt.Sprintf("consumer %d did not read previous tick, dropping tick %v", i, ts))
 					}
 				}
+				t.mutex.Unlock()
 			}
 		}
 	}()
 }
 
 func (t *multiChanTicker) AddChannel(channel chan time.Time) {
+	if cap(channel) == 0 {
+		cclog.ComponentWarn("MultiChanTicker", "unbuffered channel registered, ticks may be dropped if the consumer is not ready")
+	}
+	t.mutex.Lock()
 	t.channels = append(t.channels, channel)
+	t.mutex.Unlock()
 }
 
 func (t *multiChanTicker) Close() {

pkg/multiChanTicker/multiChanTicker_test.go

@@ -0,0 +1,51 @@
+package multiChanTicker
+
+import (
+	"testing"
+	"time"
+)
+
+// A consumer that never reads its channel must not stall the ticker
+// or starve the other consumers
+func TestStalledConsumerDoesNotStarveOthers(t *testing.T) {
+	stalled := make(chan time.Time, 1) // never read
+	fast := make(chan time.Time, 1)
+
+	ticker := NewTicker(10 * time.Millisecond)
+	defer ticker.Close()
+	ticker.AddChannel(stalled)
+	ticker.AddChannel(fast)
+
+	received := 0
+	deadline := time.After(5 * time.Second)
+	for received < 5 {
+		select {
+		case <-fast:
+			received++
+		case <-deadline:
+			t.Fatalf("received only %d ticks while another consumer stalled", received)
+		}
+	}
+}
+
+// Close() must return promptly even if no consumer reads its channel
+func TestCloseWithStalledConsumer(t *testing.T) {
+	stalled := make(chan time.Time, 1) // never read
+
+	ticker := NewTicker(10 * time.Millisecond)
+	ticker.AddChannel(stalled)
+
+	// Let some ticks fire and be dropped
+	time.Sleep(50 * time.Millisecond)
+
+	closed := make(chan struct{})
+	go func() {
+		ticker.Close()
+		close(closed)
+	}()
+	select {
+	case <-closed:
+	case <-time.After(5 * time.Second):
+		t.Fatal("Close() blocked with a stalled consumer")
+	}
+}