Move gzip internal reader creation into ReadToEnd

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

@@ -5,10 +5,8 @@ package reader // import "github.com/open-telemetry/opentelemetry-collector-cont
 
 import (
 	"bufio"
-	"compress/gzip"
 	"errors"
 	"fmt"
-	"io"
 	"os"
 	"time"
 
@@ -76,6 +74,7 @@ func (f *Factory) NewReaderFromMetadata(file *os.File, m *Metadata) (r *Reader,
 		decoder:           decode.New(f.Encoding),
 		lineSplitFunc:     f.SplitFunc,
 		deleteAtEOF:       f.DeleteAtEOF,
+		compression:       f.Compression,
 	}
 	r.set.Logger = r.set.Logger.With(zap.String("path", r.fileName))
 
@@ -123,30 +122,5 @@ func (f *Factory) NewReaderFromMetadata(file *os.File, m *Metadata) (r *Reader,
 		r.FileAttributes[k] = v
 	}
 
-	switch f.Compression {
-	case "gzip":
-		info, err := r.file.Stat()
-		if err != nil {
-			return nil, fmt.Errorf("gzip stat: %w", err)
-		}
-
-		// use a gzip Reader with an underlying SectionReader to pick up at the last
-		// offset of a gzip compressed file
-		gzipReader, err := gzip.NewReader(io.NewSectionReader(file, r.Offset, info.Size()))
-		if err != nil {
-			if !errors.Is(err, io.EOF) {
-				return nil, fmt.Errorf("new gzip: %w", err)
-			}
-		} else {
-			r.reader = gzipReader
-		}
-		r.deferFunc = func() {
-			// set the offset of the reader to the end of the file to ensure the offset is not set to the
-			// position of the scanner, which operates on the uncompressed data
-			r.Offset = info.Size()
-		}
-	default:
-		r.reader = file
-	}
 	return r, nil
 }

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

@@ -5,6 +5,7 @@ package reader // import "github.com/open-telemetry/opentelemetry-collector-cont
 
 import (
 	"bufio"
+	"compress/gzip"
 	"context"
 	"errors"
 	"io"
@@ -41,20 +42,46 @@ type Reader struct {
 	maxLogSize             int
 	lineSplitFunc          bufio.SplitFunc
 	splitFunc              bufio.SplitFunc
-	deferFunc              func()
 	decoder                *decode.Decoder
 	headerReader           *header.Reader
 	processFunc            emit.Callback
 	emitFunc               emit.Callback
 	deleteAtEOF            bool
 	needsUpdateFingerprint bool
+	compression            string
 }
 
 // ReadToEnd will read until the end of the file
 func (r *Reader) ReadToEnd(ctx context.Context) {
-	if r.reader == nil {
-		// Nothing to do. This can happen if it was already determined that we're at EOF.
-		return
+	switch r.compression {
+	case "gzip":
+		// We need to create a gzip reader each time ReadToEnd is called because the underlying
+		// SectionReader can only read a fixed window (from previous offset to EOF).
+		info, err := r.file.Stat()
+		if err != nil {
+			r.set.Logger.Error("Failed to stat", zap.Error(err))
+			return
+		}
+		currentEOF := info.Size()
+
+		// use a gzip Reader with an underlying SectionReader to pick up at the last
+		// offset of a gzip compressed file
+		gzipReader, err := gzip.NewReader(io.NewSectionReader(r.file, r.Offset, currentEOF))
+		if err != nil {
+			if !errors.Is(err, io.EOF) {
+				r.set.Logger.Error("Failed to create gzip reader", zap.Error(err))
+			}
+			return
+		} else {
+			r.reader = gzipReader
+		}
+		// Offset tracking in an uncompressed file is based on the length of emitted tokens, but in this case
+		// we need to set the offset to the end of the file.
+		defer func() {
+			r.Offset = currentEOF
+		}()
+	default:
+		r.reader = r.file
 	}
 
 	if _, err := r.file.Seek(r.Offset, 0); err != nil {
@@ -66,9 +93,6 @@ func (r *Reader) ReadToEnd(ctx context.Context) {
 		if r.needsUpdateFingerprint {
 			r.updateFingerprint()
 		}
-		if r.deferFunc != nil {
-			r.deferFunc()
-		}
 	}()
 
 	s := scanner.New(r, r.maxLogSize, r.initialBufferSize, r.Offset, r.splitFunc)