tsdb: improve initTime busy-wait loop

- Add runtime.Gosched() to yield CPU during the busy-wait loop
- Reduce timeout from 500ms to 100ms (actual wait should be microseconds)
- Add warning log when timeout occurs to aid debugging

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

tsdb/head_append.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"log/slog"
 	"math"
+	"runtime"
 	"time"
 
 	"github.com/prometheus/prometheus/model/exemplar"
@@ -120,12 +121,18 @@ func (h *Head) initTime(t int64) {
 	if !h.minTime.CompareAndSwap(math.MaxInt64, t) {
 		// Concurrent appends that are initializing.
 		// Wait until h.maxTime is swapped to avoid minTime/maxTime races.
-		antiDeadlockTimeout := time.After(500 * time.Millisecond)
+		// This should complete in microseconds under normal operation.
+		antiDeadlockTimeout := time.After(100 * time.Millisecond)
 		for h.maxTime.Load() == math.MinInt64 {
 			select {
 			case <-antiDeadlockTimeout:
+				// This should never happen in normal operation.
+				// If it does, there may be a bug or the system is severely overloaded.
+				h.logger.Warn("initTime timeout waiting for maxTime initialization")
 				return
 			default:
+				// Yield to allow the initializing goroutine to complete.
+				runtime.Gosched()
 			}
 		}
 		return