pikpak: fix truncated single part uploads reported as ok when source ends early

If the source supplied fewer bytes than its declared size, the single
part upload path accepted the short body and stored a truncated file
recorded with the declared size, reporting a successful upload. The
multipart path already checks for this.

Count the bytes actually read and fail the upload if they do not match
the declared size, which cancels the partially created file.

This was found by the FsPutShortEOF integration test.

backend/pikpak/pikpak.go

@@ -58,6 +58,7 @@ import (
 	"github.com/rclone/rclone/lib/oauthutil"
 	"github.com/rclone/rclone/lib/pacer"
 	"github.com/rclone/rclone/lib/random"
+	"github.com/rclone/rclone/lib/readers"
 	"github.com/rclone/rclone/lib/rest"
 	"golang.org/x/oauth2"
 )
@@ -1446,10 +1447,11 @@ func (f *Fs) uploadByResumable(ctx context.Context, in io.Reader, name string, s
 	if err != nil {
 		return fmt.Errorf("failed to create upload client: %w", err)
 	}
+	counter := readers.NewCountingReader(in)
 	req := &s3.PutObjectInput{
 		Bucket: &p.Bucket,
 		Key:    &p.Key,
-		Body:   io.NopCloser(in),
+		Body:   io.NopCloser(counter),
 	}
 	// Apply upload options
 	for _, option := range options {
@@ -1477,7 +1479,14 @@ func (f *Fs) uploadByResumable(ctx context.Context, in io.Reader, name string, s
 		_, err = client.PutObject(ctx, req, s3opts...)
 		return f.shouldRetry(ctx, nil, err)
 	})
-	return
+	if err != nil {
+		return err
+	}
+	// Check for truncation of input
+	if size >= 0 && int64(counter.BytesRead()) != size {
+		return fmt.Errorf("expected %d bytes in input, but got %d: %w", size, counter.BytesRead(), io.ErrUnexpectedEOF)
+	}
+	return nil
 }
 
 func (f *Fs) upload(ctx context.Context, in io.Reader, leaf, dirID, gcid string, size int64, options ...fs.OpenOption) (info *api.File, err error) {