File Input Cleanup - Step 1 (#155)

The codebase within the `file_input` operator is complex enough that functional changes are difficult to confidently implement. This PR is minor step towards untangling the code base.

- Open files in the `makeReaders` method
- Add `Reader.Close`
- Remove unnecessary copying of file handles

operator/builtin/input/file/file.go

@@ -136,26 +136,7 @@ func (f *InputOperator) poll(ctx context.Context) {
 		}
 	}
 
-	// Open the files first to minimize the time between listing and opening
-	files := make([]*os.File, 0, len(matches))
-	for _, path := range matches {
-		if _, ok := f.SeenPaths[path]; !ok {
-			if f.startAtBeginning {
-				f.Infow("Started watching file", "path", path)
-			} else {
-				f.Infow("Started watching file from end. To read preexisting logs, configure the argument 'start_at' to 'beginning'", "path", path)
-			}
-			f.SeenPaths[path] = struct{}{}
-		}
-		file, err := os.Open(path)
-		if err != nil {
-			f.Errorw("Failed to open file", zap.Error(err))
-			continue
-		}
-		files = append(files, file)
-	}
-
-	readers := f.makeReaders(files)
+	readers := f.makeReaders(matches)
 	f.firstCheck = false
 
 	var wg sync.WaitGroup
@@ -171,8 +152,8 @@ func (f *InputOperator) poll(ctx context.Context) {
 	wg.Wait()
 
 	// Close all files
-	for _, file := range files {
-		file.Close()
+	for _, reader := range readers {
+		reader.Close()
 	}
 
 	f.saveCurrent(readers)
@@ -208,7 +189,26 @@ func getMatches(includes, excludes []string) []string {
 // makeReaders takes a list of paths, then creates readers from each of those paths,
 // discarding any that have a duplicate fingerprint to other files that have already
 // been read this polling interval
-func (f *InputOperator) makeReaders(files []*os.File) []*Reader {
+func (f *InputOperator) makeReaders(filesPaths []string) []*Reader {
+	// Open the files first to minimize the time between listing and opening
+	files := make([]*os.File, 0, len(filesPaths))
+	for _, path := range filesPaths {
+		if _, ok := f.SeenPaths[path]; !ok {
+			if f.startAtBeginning {
+				f.Infow("Started watching file", "path", path)
+			} else {
+				f.Infow("Started watching file from end. To read preexisting logs, configure the argument 'start_at' to 'beginning'", "path", path)
+			}
+			f.SeenPaths[path] = struct{}{}
+		}
+		file, err := os.Open(path)
+		if err != nil {
+			f.Errorw("Failed to open file", zap.Error(err))
+			continue
+		}
+		files = append(files, file)
+	}
+
 	// Get fingerprints for each file
 	fps := make([]*Fingerprint, 0, len(files))
 	for _, file := range files {
@@ -220,18 +220,15 @@ func (f *InputOperator) makeReaders(files []*os.File) []*Reader {
 		fps = append(fps, fp)
 	}
 
-	// Make a copy of the files so we don't modify the original
-	filesCopy := make([]*os.File, len(files))
-	copy(filesCopy, files)
-
 	// Exclude any empty fingerprints or duplicate fingerprints to avoid doubling up on copy-truncate files
 OUTER:
 	for i := 0; i < len(fps); {
 		fp := fps[i]
 		if len(fp.FirstBytes) == 0 {
+			files[i].Close()
 			// Empty file, don't read it until we can compare its fingerprint
 			fps = append(fps[:i], fps[i+1:]...)
-			filesCopy = append(filesCopy[:i], filesCopy[i+1:]...)
+			files = append(files[:i], files[i+1:]...)
 		}
 
 		for j := 0; j < len(fps); j++ {
@@ -243,8 +240,9 @@ OUTER:
 			fp2 := fps[j]
 			if fp.StartsWith(fp2) || fp2.StartsWith(fp) {
 				// Exclude
+				files[i].Close()
 				fps = append(fps[:i], fps[i+1:]...)
-				filesCopy = append(filesCopy[:i], filesCopy[i+1:]...)
+				files = append(files[:i], files[i+1:]...)
 				continue OUTER
 			}
 		}
@@ -253,7 +251,7 @@ OUTER:
 
 	readers := make([]*Reader, 0, len(fps))
 	for i := 0; i < len(fps); i++ {
-		reader, err := f.newReader(filesCopy[i], fps[i], f.firstCheck)
+		reader, err := f.newReader(files[i], fps[i], f.firstCheck)
 		if err != nil {
 			f.Errorw("Failed to create reader", zap.Error(err))
 			continue

operator/builtin/input/file/reader.go

@@ -117,6 +117,11 @@ func (f *Reader) ReadToEnd(ctx context.Context) {
 	}
 }
 
+// Close will close the file
+func (f *Reader) Close() error {
+	return f.file.Close()
+}
+
 // Emit creates an entry with the decoded message and sends it to the next
 // operator in the pipeline
 func (f *Reader) emit(ctx context.Context, msgBuf []byte) error {