[pkg/stanza] Fix issue where batching caused incorrect starting point (…

…open-telemetry#27773)

.chloggen/pkg-stanza-fileconsumer-reader-from-end-batches.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: filelogreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix issue where batching of files could result in ignoring start_at setting.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [27773]
+
+# (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: []

pkg/stanza/fileconsumer/file.go

@@ -118,6 +118,9 @@ func (m *Manager) poll(ctx context.Context) {
 		matches = matches[m.maxBatchFiles:]
 	}
 	m.consume(ctx, matches)
+
+	// Any new files that appear should be consumed entirely
+	m.readerFactory.FromBeginning = true
 }
 
 func (m *Manager) consume(ctx context.Context, paths []string) {
@@ -145,9 +148,6 @@ func (m *Manager) consume(ctx context.Context, paths []string) {
 	}
 	wg.Wait()
 
-	// Any new files that appear should be consumed entirely
-	m.readerFactory.FromBeginning = true
-
 	m.roller.roll(ctx, readers)
 	m.saveCurrent(readers)
 	m.syncLastPollFiles(ctx)

pkg/stanza/fileconsumer/file_test.go

@@ -1098,6 +1098,56 @@ func TestFileBatching(t *testing.T) {
 	}
 }
 
+func TestFileBatchingRespectsStartAtEnd(t *testing.T) {
+	t.Parallel()
+
+	initFiles := 10
+	moreFiles := 10
+	maxConcurrentFiles := 2
+
+	tempDir := t.TempDir()
+	cfg := NewConfig().includeDir(tempDir)
+	cfg.StartAt = "end"
+	cfg.MaxConcurrentFiles = maxConcurrentFiles
+
+	operator, emitChan := buildTestManager(t, cfg)
+	operator.persister = testutil.NewMockPersister("test")
+
+	temps := make([]*os.File, 0, initFiles+moreFiles)
+	for i := 0; i < initFiles; i++ {
+		temps = append(temps, openTemp(t, tempDir))
+	}
+
+	// Write one log to each file
+	for i, temp := range temps {
+		message := fmt.Sprintf("file %d: %s", i, "written before start")
+		_, err := temp.WriteString(message + "\n")
+		require.NoError(t, err)
+	}
+
+	// Poll and expect no logs
+	operator.poll(context.Background())
+	expectNoTokens(t, emitChan)
+
+	// Create some more files
+	for i := 0; i < moreFiles; i++ {
+		temps = append(temps, openTemp(t, tempDir))
+	}
+
+	// Write a log to each file
+	expectedTokens := make([][]byte, 0, initFiles+moreFiles)
+	for i, temp := range temps {
+		message := fmt.Sprintf("file %d: %s", i, "written after start")
+		_, err := temp.WriteString(message + "\n")
+		require.NoError(t, err)
+		expectedTokens = append(expectedTokens, []byte(message))
+	}
+
+	// Poll again and expect one line from each file.
+	operator.poll(context.Background())
+	waitForTokens(t, emitChan, expectedTokens)
+}
+
 func TestFileReader_FingerprintUpdated(t *testing.T) {
 	t.Parallel()