MM-43716: enforce size limits in slack importer (#20144)

Automatic Merge

utils/file.go

@@ -4,6 +4,7 @@
 package utils
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -112,3 +113,23 @@ func CopyDir(src string, dst string) (err error) {
 
 	return
 }
+
+var SizeLimitExceeded = errors.New("Size limit exceeded")
+
+type LimitedReaderWithError struct {
+	limitedReader *io.LimitedReader
+}
+
+func NewLimitedReaderWithError(reader io.Reader, maxBytes int64) *LimitedReaderWithError {
+	return &LimitedReaderWithError{
+		limitedReader: &io.LimitedReader{R: reader, N: maxBytes + 1},
+	}
+}
+
+func (l *LimitedReaderWithError) Read(p []byte) (int, error) {
+	n, err := l.limitedReader.Read(p)
+	if l.limitedReader.N <= 0 && err == io.EOF {
+		return n, SizeLimitExceeded
+	}
+	return n, err
+}