[filelogreceiver]: Add ability to sort by mtime (open-telemetry#28850)

**Description:** <Describe what has changed.>
* Adds a new `mtime` sort type, which will sort files by their modified
time
* Add a feature gate for `mtime` sort type

An optional follow-up performance improvement may be made here, to have
the finder return fs.DirEntry directly to query the mtime without making
an extra call to os.Stat for each file.

**Link to tracking Issue:** open-telemetry#27812

**Testing:**
* Added unit tests for new functionality

**Documentation:** 
* Added new `mode` parameter to filelogreceiver docs

.chloggen/filelogreceiver_sort-by-mtime-rework.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: filelogreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add the ability to order files by mtime, to only read the most recently modified files
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [27812]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

pkg/stanza/fileconsumer/matcher/internal/filter/filter.go

@@ -11,7 +11,7 @@ import (
 )
 
 type Option interface {
-	// Returned error is for explanitory purposes only.
+	// Returned error is for explanatory purposes only.
 	// All options will be called regardless of error.
 	apply([]*item) ([]*item, error)
 }
@@ -49,6 +49,12 @@ type item struct {
 }
 
 func newItem(value string, regex *regexp.Regexp) (*item, error) {
+	if regex == nil {
+		return &item{
+			value: value,
+		}, nil
+	}
+
 	match := regex.FindStringSubmatch(value)
 	if match == nil {
 		return nil, fmt.Errorf("'%s' does not match regex", value)

pkg/stanza/fileconsumer/matcher/internal/filter/sort.go

@@ -5,6 +5,7 @@ package filter // import "github.com/open-telemetry/opentelemetry-collector-cont
 
 import (
 	"fmt"
+	"os"
 	"sort"
 	"strconv"
 	"time"
@@ -18,24 +19,24 @@ type parseFunc func(string) (any, error)
 
 type compareFunc func(a, b any) bool
 
-type sortOption struct {
+type regexSortOption struct {
 	regexKey string
 	parseFunc
 	compareFunc
 }
 
-func newSortOption(regexKey string, parseFunc parseFunc, compareFunc compareFunc) (Option, error) {
+func newRegexSortOption(regexKey string, parseFunc parseFunc, compareFunc compareFunc) (Option, error) {
 	if regexKey == "" {
 		return nil, fmt.Errorf("regex key must be specified")
 	}
-	return sortOption{
+	return regexSortOption{
 		regexKey:    regexKey,
 		parseFunc:   parseFunc,
 		compareFunc: compareFunc,
 	}, nil
 }
 
-func (o sortOption) apply(items []*item) ([]*item, error) {
+func (o regexSortOption) apply(items []*item) ([]*item, error) {
 	// Special case where sort.Slice will not run the 'less' func.
 	// We still need to ensure it parses in order to ensure the file should be included.
 	if len(items) == 1 {
@@ -80,7 +81,7 @@ func (o sortOption) apply(items []*item) ([]*item, error) {
 }
 
 func SortNumeric(regexKey string, ascending bool) (Option, error) {
-	return newSortOption(regexKey,
+	return newRegexSortOption(regexKey,
 		func(s string) (any, error) {
 			return strconv.Atoi(s)
 		},
@@ -94,7 +95,7 @@ func SortNumeric(regexKey string, ascending bool) (Option, error) {
 }
 
 func SortAlphabetical(regexKey string, ascending bool) (Option, error) {
-	return newSortOption(regexKey,
+	return newRegexSortOption(regexKey,
 		func(s string) (any, error) {
 			return s, nil
 		},
@@ -118,7 +119,7 @@ func SortTemporal(regexKey string, ascending bool, layout string, location strin
 	if err != nil {
 		return nil, fmt.Errorf("load location %s: %w", loc, err)
 	}
-	return newSortOption(regexKey,
+	return newRegexSortOption(regexKey,
 		func(s string) (any, error) {
 			return timeutils.ParseStrptime(layout, s, loc)
 		},
@@ -130,3 +131,46 @@ func SortTemporal(regexKey string, ascending bool, layout string, location strin
 		},
 	)
 }
+
+type mtimeSortOption struct{}
+
+type mtimeItem struct {
+	mtime time.Time
+	path  string
+	item  *item
+}
+
+func (m mtimeSortOption) apply(items []*item) ([]*item, error) {
+	mtimeItems := make([]mtimeItem, 0, len(items))
+	var errs error
+	for _, item := range items {
+		path := item.value
+		fi, err := os.Stat(path)
+		if err != nil {
+			errs = multierr.Append(errs, err)
+			continue
+		}
+
+		mtimeItems = append(mtimeItems, mtimeItem{
+			mtime: fi.ModTime(),
+			path:  path,
+			item:  item,
+		})
+	}
+
+	sort.SliceStable(mtimeItems, func(i, j int) bool {
+		// This checks if item i > j, in order to reverse the sort (most recently modified file is first in the list)
+		return mtimeItems[i].mtime.After(mtimeItems[j].mtime)
+	})
+
+	filteredValues := make([]*item, 0, len(items))
+	for _, mtimeItem := range mtimeItems {
+		filteredValues = append(filteredValues, mtimeItem.item)
+	}
+
+	return filteredValues, errs
+}
+
+func SortMtime() Option {
+	return mtimeSortOption{}
+}

pkg/stanza/fileconsumer/matcher/internal/filter/sort_test.go

@@ -5,8 +5,11 @@ package filter
 
 import (
 	"fmt"
+	"os"
+	"path/filepath"
 	"regexp"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -169,3 +172,71 @@ func TestSort(t *testing.T) {
 		}
 	}
 }
+
+func TestMTimeFilter(t *testing.T) {
+	epoch := time.Unix(0, 0)
+	cases := []struct {
+		name        string
+		files       []string
+		fileMTimes  []time.Time
+		expectedErr string
+		expect      []string
+	}{
+		{
+			name:       "No files",
+			files:      []string{},
+			fileMTimes: []time.Time{},
+			expect:     []string{},
+		},
+		{
+			name:       "Single file",
+			files:      []string{"a.log"},
+			fileMTimes: []time.Time{epoch},
+			expect:     []string{"a.log"},
+		},
+		{
+			name:       "Multiple files",
+			files:      []string{"a.log", "b.log"},
+			fileMTimes: []time.Time{epoch, epoch.Add(time.Hour)},
+			expect:     []string{"b.log", "a.log"},
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			tmpDir := t.TempDir()
+			items := []*item{}
+			// Create files with specified mtime
+			for i, file := range tc.files {
+				mtime := tc.fileMTimes[i]
+				fullPath := filepath.Join(tmpDir, file)
+
+				f, err := os.Create(fullPath)
+				require.NoError(t, err)
+				require.NoError(t, f.Close())
+				require.NoError(t, os.Chtimes(fullPath, epoch, mtime))
+
+				it, err := newItem(fullPath, nil)
+				require.NoError(t, err)
+
+				items = append(items, it)
+			}
+
+			f := SortMtime()
+			result, err := f.apply(items)
+			if tc.expectedErr != "" {
+				require.EqualError(t, err, tc.expectedErr)
+			} else {
+				require.NoError(t, err)
+			}
+
+			relativeResult := []string{}
+			for _, r := range result {
+				rel, err := filepath.Rel(tmpDir, r.value)
+				require.NoError(t, err)
+				relativeResult = append(relativeResult, rel)
+			}
+
+			require.Equal(t, tc.expect, relativeResult)
+		})
+	}
+}

pkg/stanza/fileconsumer/matcher/matcher.go

@@ -8,6 +8,8 @@ import (
 	"fmt"
 	"regexp"
 
+	"go.opentelemetry.io/collector/featuregate"
+
 	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/stanza/fileconsumer/matcher/internal/filter"
 	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/stanza/fileconsumer/matcher/internal/finder"
 )
@@ -16,12 +18,20 @@ const (
 	sortTypeNumeric      = "numeric"
 	sortTypeTimestamp    = "timestamp"
 	sortTypeAlphabetical = "alphabetical"
+	sortTypeMtime        = "mtime"
 )
 
 const (
 	defaultOrderingCriteriaTopN = 1
 )
 
+var mtimeSortTypeFeatureGate = featuregate.GlobalRegistry().MustRegister(
+	"filelog.mtimeSortType",
+	featuregate.StageAlpha,
+	featuregate.WithRegisterDescription("When enabled, allows usage of `ordering_criteria.mode` = `mtime`."),
+	featuregate.WithRegisterReferenceURL("https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/27812"),
+)
+
 type Criteria struct {
 	Include          []string         `mapstructure:"include,omitempty"`
 	Exclude          []string         `mapstructure:"exclude,omitempty"`
@@ -63,10 +73,6 @@ func New(c Criteria) (*Matcher, error) {
 		}, nil
 	}
 
-	if c.OrderingCriteria.Regex == "" {
-		return nil, fmt.Errorf("'regex' must be specified when 'sort_by' is specified")
-	}
-
 	if c.OrderingCriteria.TopN < 0 {
 		return nil, fmt.Errorf("'top_n' must be a positive integer")
 	}
@@ -75,9 +81,17 @@ func New(c Criteria) (*Matcher, error) {
 		c.OrderingCriteria.TopN = defaultOrderingCriteriaTopN
 	}
 
-	regex, err := regexp.Compile(c.OrderingCriteria.Regex)
-	if err != nil {
-		return nil, fmt.Errorf("compile regex: %w", err)
+	var regex *regexp.Regexp
+	if orderingCriteriaNeedsRegex(c.OrderingCriteria.SortBy) {
+		if c.OrderingCriteria.Regex == "" {
+			return nil, fmt.Errorf("'regex' must be specified when 'sort_by' is specified")
+		}
+
+		var err error
+		regex, err = regexp.Compile(c.OrderingCriteria.Regex)
+		if err != nil {
+			return nil, fmt.Errorf("compile regex: %w", err)
+		}
 	}
 
 	var filterOpts []filter.Option
@@ -101,6 +115,11 @@ func New(c Criteria) (*Matcher, error) {
 				return nil, fmt.Errorf("timestamp sort: %w", err)
 			}
 			filterOpts = append(filterOpts, f)
+		case sortTypeMtime:
+			if !mtimeSortTypeFeatureGate.IsEnabled() {
+				return nil, fmt.Errorf("the %q feature gate must be enabled to use %q sort type", mtimeSortTypeFeatureGate.ID(), sortTypeMtime)
+			}
+			filterOpts = append(filterOpts, filter.SortMtime())
 		default:
 			return nil, fmt.Errorf("'sort_type' must be specified")
 		}
@@ -115,6 +134,17 @@ func New(c Criteria) (*Matcher, error) {
 	}, nil
 }
 
+// orderingCriteriaNeedsRegex returns true if any of the sort options require a regex to be set.
+func orderingCriteriaNeedsRegex(sorts []Sort) bool {
+	for _, s := range sorts {
+		switch s.SortType {
+		case sortTypeNumeric, sortTypeAlphabetical, sortTypeTimestamp:
+			return true
+		}
+	}
+	return false
+}
+
 type Matcher struct {
 	include    []string
 	exclude    []string