Improved performance of distributed read endpoint (#42810)

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
remove if NA -->
**Related issue:** Resolves #42808 

This is another hot path optimization recommended by Claude Code. I QA'd
it with a local osquery perf run.

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.

## Testing

- [x] QA'd all new/changed functionality manually

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Refactor**
* Enhanced performance of the distributed read endpoint by optimizing
lock contention management during jitter table access operations. This
change reduces latency and improves system responsiveness when handling
distributed read requests, particularly benefiting high-concurrency
scenarios. The optimization maintains all existing functionality while
providing better performance characteristics for read-heavy workloads.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

server/service/osquery.go

@@ -849,15 +849,22 @@ func (svc *Service) hostRequiresConditionalAccessMicrosoftIngestion(ctx context.
 }
 
 func (svc *Service) shouldUpdate(lastUpdated time.Time, interval time.Duration, hostID uint) bool {
-	svc.jitterMu.Lock()
-	defer svc.jitterMu.Unlock()
+	svc.jitterMu.RLock()
+	jh := svc.jitterH[interval]
+	svc.jitterMu.RUnlock()
 
-	if svc.jitterH[interval] == nil {
-		svc.jitterH[interval] = newJitterHashTable(int(int64(svc.config.Osquery.MaxJitterPercent) * int64(interval.Minutes()) / 100.0))
-		svc.logger.DebugContext(context.TODO(), "jitter table created", "bucketCount", svc.jitterH[interval].bucketCount)
+	if jh == nil {
+		svc.jitterMu.Lock()
+		// Double-check after acquiring write lock.
+		if svc.jitterH[interval] == nil {
+			svc.jitterH[interval] = newJitterHashTable(int(int64(svc.config.Osquery.MaxJitterPercent) * int64(interval.Minutes()) / 100.0))
+			svc.logger.DebugContext(context.TODO(), "jitter table created", "bucketCount", svc.jitterH[interval].bucketCount)
+		}
+		jh = svc.jitterH[interval]
+		svc.jitterMu.Unlock()
 	}
 
-	jitter := svc.jitterH[interval].jitterForHost(hostID)
+	jitter := jh.jitterForHost(hostID)
 	cutoff := svc.clock.Now().Add(-(interval + jitter))
 	return lastUpdated.Before(cutoff)
 }

server/service/osquery_test.go

@@ -1253,7 +1253,7 @@ func TestHostDetailQueries(t *testing.T) {
 		logger:   slog.New(slog.DiscardHandler),
 		config:   config.TestConfig(),
 		ds:       ds,
-		jitterMu: new(sync.Mutex),
+		jitterMu: new(sync.RWMutex),
 		jitterH:  make(map[time.Duration]*jitterHashTable),
 	}
 
@@ -2296,7 +2296,7 @@ func TestMDMQueries(t *testing.T) {
 		logger:   slog.New(slog.DiscardHandler),
 		config:   config.TestConfig(),
 		ds:       ds,
-		jitterMu: new(sync.Mutex),
+		jitterMu: new(sync.RWMutex),
 		jitterH:  make(map[time.Duration]*jitterHashTable),
 	}
 

server/service/service.go

@@ -48,7 +48,7 @@ type Service struct {
 
 	authz *authz.Authorizer
 
-	jitterMu *sync.Mutex
+	jitterMu *sync.RWMutex
 	jitterH  map[time.Duration]*jitterHashTable
 
 	geoIP fleet.GeoIP
@@ -172,7 +172,7 @@ func NewService(
 		failingPolicySet:  failingPolicySet,
 		authz:             authorizer,
 		jitterH:           make(map[time.Duration]*jitterHashTable),
-		jitterMu:          new(sync.Mutex),
+		jitterMu:          new(sync.RWMutex),
 		geoIP:             geoIP,
 		enrollHostLimiter: enrollHostLimiter,
 		depStorage:        depStorage,