Fix issue when exclude_older_than is enabled without `ordering_crit…

…eria` configured (open-telemetry#32683)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->

Fix issue when `exclude_older_than` is enabled without
`ordering_criteria` configured

**Link to tracking Issue:** <Issue number if applicable> open-telemetry#32681

**Testing:** <Describe what testing was performed and which tests were
added.>

**Documentation:** <Describe the documentation added.>

.chloggen/fix-stanza-matcher.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: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: pkg/stanza
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Fix issue when `exclude_older_than` is enabled without `ordering_criteria` configured"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [32681]
+
+# (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/sort.go

@@ -132,6 +132,16 @@ func SortTemporal(regexKey string, ascending bool, layout string, location strin
 	)
 }
 
+type TopNOption int
+
+//nolint:unparam
+func (t TopNOption) apply(items []*item) ([]*item, error) {
+	if len(items) <= int(t) {
+		return items, nil
+	}
+	return items[:t], nil
+}
+
 type mtimeSortOption struct{}
 
 type mtimeItem struct {

pkg/stanza/fileconsumer/matcher/matcher.go

@@ -91,7 +91,6 @@ func New(c Criteria) (*Matcher, error) {
 	if c.OrderingCriteria.TopN == 0 {
 		c.OrderingCriteria.TopN = defaultOrderingCriteriaTopN
 	}
-	m.topN = c.OrderingCriteria.TopN
 
 	var regex *regexp.Regexp
 	if orderingCriteriaNeedsRegex(c.OrderingCriteria.SortBy) {
@@ -138,6 +137,8 @@ func New(c Criteria) (*Matcher, error) {
 		}
 	}
 
+	m.filterOpts = append(m.filterOpts, filter.TopNOption(c.OrderingCriteria.TopN))
+
 	return m, nil
 }
 
@@ -156,7 +157,6 @@ type Matcher struct {
 	include    []string
 	exclude    []string
 	regex      *regexp.Regexp
-	topN       int
 	filterOpts []filter.Option
 }
 
@@ -178,10 +178,5 @@ func (m Matcher) MatchFiles() ([]string, error) {
 	if len(result) == 0 {
 		return result, errors.Join(err, errs)
 	}
-
-	if len(result) <= m.topN {
-		return result, errors.Join(err, errs)
-	}
-
-	return result[:m.topN], errors.Join(err, errs)
+	return result, errs
 }