bytes, strings: add Reader.Size methods
As noted on recently on golang-nuts, there's currently no way to know
the total size of a strings.Reader or bytes.Reader when using ReadAt
on them. Most callers resort to wrapping it in an io.SectionReader to
retain that information.
The SizeReaderAt abstraction (an io.ReaderAt with a Size() int64
method) has proven useful as a way of expressing a concurrency-safe
read-only number of bytes.
As one example, see http://talks.golang.org/2013/oscon-dl.slide#49 and
the rest of that presentation for its use in dl.google.com.
SizeReaderAt is also used in the open source google-api-go-client, and
within Google's internal codebase, where it exists in a public package
created in 2013 with the package comment: "These may migrate to the
standard library after we have enough experience with their feel."
I'm still as happy with the SizeReaderAt abstraction and its
composabilty as I was in 2013, so I'd like to make these two Readers
also be SizeReaderAts.
Fixes #9667
Change-Id: Ie6f145ada419dd116280472d8c029f046d5edf70
Reviewed-on: https://go-review.googlesource.com/3199
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/src/strings/reader.go b/src/strings/reader.go
index 82df974..7a872fb 100644
--- a/src/strings/reader.go
+++ b/src/strings/reader.go
@@ -28,6 +28,12 @@
return int(int64(len(r.s)) - r.i)
}
+// Size returns the original length of the underlying string.
+// Size is the number of bytes available for reading via ReadAt.
+// The returned value is always the same and is not affected by calls
+// to any other method.
+func (r *Reader) Size() int64 { return int64(len(r.s)) }
+
func (r *Reader) Read(b []byte) (n int, err error) {
if len(b) == 0 {
return 0, nil
