[filebeat][gcs] - Simplified state checkpoint calculation (#40937)

* Simplified state checkpoint calculation. The existing state checkpoint calculation logic was convoluted and depended on unreliable flag logic, then it has been simplified so that if errors occur they are become simpler to debug.

CHANGELOG-developer.next.asciidoc

@@ -205,6 +205,7 @@ The list below covers the major changes between 7.0.0-rc2 and main only.
 - Add a configuration option for TCP/UDP network type. {issue}40407[40407] {pull}40623[40623]
 - Added debug logging to parquet reader in x-pack/libbeat/reader. {pull}40651[40651]
 - Added filebeat debug histograms for s3 object size and events per processed s3 object. {pull}40775[40775]
+- Simplified GCS input state checkpoint calculation logic. {issue}40878[40878] {pull}40937[40937] 
 - Simplified Azure Blob Storage input state checkpoint calculation logic. {issue}40674[40674] {pull}40936[40936]
 
 ==== Deprecated

x-pack/filebeat/input/gcs/scheduler.go

@@ -7,6 +7,8 @@ package gcs
 import (
 	"context"
 	"fmt"
+	"slices"
+	"sort"
 	"strings"
 	"sync"
 
@@ -181,41 +183,19 @@ func (s *scheduler) fetchObjectPager(ctx context.Context, pageSize int) *iterato
 // moveToLastSeenJob, moves to the latest job position past the last seen job
 // Jobs are stored in lexicographical order always, hence the latest position can be found either on the basis of job name or timestamp
 func (s *scheduler) moveToLastSeenJob(jobs []*job) []*job {
-	var latestJobs []*job
-	jobsToReturn := make([]*job, 0)
-	counter := 0
-	flag := false
-	ignore := false
-
-	for _, job := range jobs {
-		switch {
-		case job.Timestamp().After(s.state.checkpoint().LatestEntryTime):
-			latestJobs = append(latestJobs, job)
-		case job.Name() == s.state.checkpoint().ObjectName:
-			flag = true
-		case job.Name() > s.state.checkpoint().ObjectName:
-			flag = true
-			counter--
-		case job.Name() <= s.state.checkpoint().ObjectName && (!ignore):
-			ignore = true
-		}
-		counter++
-	}
-
-	if flag && (counter < len(jobs)-1) {
-		jobsToReturn = jobs[counter+1:]
-	} else if !flag && !ignore {
-		jobsToReturn = jobs
-	}
+	cp := s.state.checkpoint()
+	jobs = slices.DeleteFunc(jobs, func(j *job) bool {
+		return !(j.Timestamp().After(cp.LatestEntryTime) || j.Name() > cp.ObjectName)
+	})
 
-	// in a senario where there are some jobs which have a later time stamp
+	// In a scenario where there are some jobs which have a greater timestamp
 	// but lesser lexicographic order and some jobs have greater lexicographic order
-	// than the current checkpoint object name, then we append the latest jobs
-	if len(jobsToReturn) != len(jobs) && len(latestJobs) > 0 {
-		jobsToReturn = append(latestJobs, jobsToReturn...)
-	}
-
-	return jobsToReturn
+	// than the current checkpoint blob name, we then sort around the pivot checkpoint
+	// timestamp.
+	sort.SliceStable(jobs, func(i, _ int) bool {
+		return jobs[i].Timestamp().After(cp.LatestEntryTime)
+	})
+	return jobs
 }
 
 func (s *scheduler) addFailedJobs(ctx context.Context, jobs []*job) []*job {