[chore][pkg/stanza] refactor: remove function juggling (#36108)

Separates header processing functions from content processing functions
to improve code clarity.
This is a follow-up from
#35870 (comment).

pkg/stanza/fileconsumer/internal/reader/factory.go

@@ -74,7 +74,6 @@ func (f *Factory) NewReaderFromMetadata(file *os.File, m *Metadata) (r *Reader,
 		initialBufferSize:    f.InitialBufferSize,
 		maxLogSize:           f.MaxLogSize,
 		decoder:              decode.New(f.Encoding),
-		lineSplitFunc:        f.SplitFunc,
 		deleteAtEOF:          f.DeleteAtEOF,
 		includeFileRecordNum: f.IncludeFileRecordNumber,
 		compression:          f.Compression,
@@ -103,18 +102,14 @@ func (f *Factory) NewReaderFromMetadata(file *os.File, m *Metadata) (r *Reader,
 	}
 
 	flushFunc := m.FlushState.Func(f.SplitFunc, f.FlushTimeout)
-	r.lineSplitFunc = trim.WithFunc(trim.ToLength(flushFunc, f.MaxLogSize), f.TrimFunc)
+	r.contentSplitFunc = trim.WithFunc(trim.ToLength(flushFunc, f.MaxLogSize), f.TrimFunc)
 	r.emitFunc = f.EmitFunc
-	if f.HeaderConfig == nil || m.HeaderFinalized {
-		r.splitFunc = r.lineSplitFunc
-		r.processFunc = r.emitFunc
-	} else {
+	if f.HeaderConfig != nil && !m.HeaderFinalized {
+		r.headerSplitFunc = f.HeaderConfig.SplitFunc
 		r.headerReader, err = header.NewReader(f.TelemetrySettings, *f.HeaderConfig)
 		if err != nil {
 			return nil, err
 		}
-		r.splitFunc = f.HeaderConfig.SplitFunc
-		r.processFunc = r.headerReader.Process
 	}
 
 	attributes, err := f.Attributes.Resolve(file)

pkg/stanza/fileconsumer/internal/reader/reader.go

@@ -42,11 +42,10 @@ type Reader struct {
 	fingerprintSize        int
 	initialBufferSize      int
 	maxLogSize             int
-	lineSplitFunc          bufio.SplitFunc
-	splitFunc              bufio.SplitFunc
+	headerSplitFunc        bufio.SplitFunc
+	contentSplitFunc       bufio.SplitFunc
 	decoder                *decode.Decoder
 	headerReader           *header.Reader
-	processFunc            emit.Callback
 	emitFunc               emit.Callback
 	deleteAtEOF            bool
 	needsUpdateFingerprint bool
@@ -116,7 +115,7 @@ func (r *Reader) ReadToEnd(ctx context.Context) {
 }
 
 func (r *Reader) readHeader(ctx context.Context) (doneReadingFile bool) {
-	s := scanner.New(r, r.maxLogSize, r.initialBufferSize, r.Offset, r.splitFunc)
+	s := scanner.New(r, r.maxLogSize, r.initialBufferSize, r.Offset, r.headerSplitFunc)
 
 	// Read the tokens from the file until no more header tokens are found or the end of file is reached.
 	for {
@@ -167,10 +166,6 @@ func (r *Reader) readHeader(ctx context.Context) (doneReadingFile bool) {
 	r.HeaderFinalized = true
 	r.initialBufferSize = scanner.DefaultBufferSize
 
-	// Switch to the normal split and process functions.
-	r.splitFunc = r.lineSplitFunc
-	r.processFunc = r.emitFunc
-
 	// Reset position in file to r.Offest after the header scanner might have moved it past a content token.
 	if _, err := r.file.Seek(r.Offset, 0); err != nil {
 		r.set.Logger.Error("failed to seek post-header", zap.Error(err))
@@ -182,7 +177,7 @@ func (r *Reader) readHeader(ctx context.Context) (doneReadingFile bool) {
 
 func (r *Reader) readContents(ctx context.Context) {
 	// Create the scanner to read the contents of the file.
-	s := scanner.New(r, r.maxLogSize, r.initialBufferSize, r.Offset, r.splitFunc)
+	s := scanner.New(r, r.maxLogSize, r.initialBufferSize, r.Offset, r.contentSplitFunc)
 
 	// Iterate over the contents of the file.
 	for {
@@ -214,7 +209,7 @@ func (r *Reader) readContents(ctx context.Context) {
 			r.FileAttributes[attrs.LogFileRecordNumber] = r.RecordNum
 		}
 
-		err = r.processFunc(ctx, token, r.FileAttributes)
+		err = r.emitFunc(ctx, token, r.FileAttributes)
 		if err != nil {
 			r.set.Logger.Error("failed to process token", zap.Error(err))
 		}