operations: reopen downloads on error when using check --download and cat

Before this change, some parts of operations called the Open method on
objects directly, and some called NewReOpen to make an object which
can re-open itself on errors.

This adds a new function operations.Open which should be called
instead of fs.Object.Open to open a reliable stream of data and
changes all call sites to use that.

This means `rclone check --download` and `rclone cat` will re-open
files on failures.

See: https://forum.rclone.org/t/does-rclone-support-retries-for-check-when-using-download-flag/38641

fs/operations/check.go

@@ -335,7 +335,7 @@ func CheckIdenticalDownload(ctx context.Context, dst, src fs.Object) (differ boo
 
 // Does the work for CheckIdenticalDownload
 func checkIdenticalDownload(ctx context.Context, dst, src fs.Object) (differ bool, err error) {
-	in1, err := dst.Open(ctx)
+	in1, err := Open(ctx, dst)
 	if err != nil {
 		return true, fmt.Errorf("failed to open %q: %w", dst, err)
 	}
@@ -345,7 +345,7 @@ func checkIdenticalDownload(ctx context.Context, dst, src fs.Object) (differ boo
 	}()
 	in1 = tr1.Account(ctx, in1).WithBuffer() // account and buffer the transfer
 
-	in2, err := src.Open(ctx)
+	in2, err := Open(ctx, src)
 	if err != nil {
 		return true, fmt.Errorf("failed to open %q: %w", src, err)
 	}
@@ -483,7 +483,7 @@ func (c *checkMarch) checkSum(ctx context.Context, obj fs.Object, download bool,
 			<-c.tokens // get the token back to free up a slot
 			c.wg.Done()
 		}()
-		if in, err = obj.Open(ctx); err != nil {
+		if in, err = Open(ctx, obj); err != nil {
 			return
 		}
 		tr := accounting.Stats(ctx).NewTransfer(obj)
@@ -538,7 +538,7 @@ type HashSums map[string]string
 
 // ParseSumFile parses a hash SUM file and returns hashes as a map
 func ParseSumFile(ctx context.Context, sumFile fs.Object) (HashSums, error) {
-	rd, err := sumFile.Open(ctx)
+	rd, err := Open(ctx, sumFile)
 	if err != nil {
 		return nil, err
 	}

fs/operations/multithread.go

@@ -62,7 +62,6 @@ type multiThreadCopyState struct {
 
 // Copy a single stream into place
 func (mc *multiThreadCopyState) copyStream(ctx context.Context, stream int) (err error) {
-	ci := fs.GetConfig(ctx)
 	defer func() {
 		if err != nil {
 			fs.Debugf(mc.src, "multi-thread copy: stream %d/%d failed: %v", stream+1, mc.streams, err)
@@ -79,7 +78,7 @@ func (mc *multiThreadCopyState) copyStream(ctx context.Context, stream int) (err
 
 	fs.Debugf(mc.src, "multi-thread copy: stream %d/%d (%d-%d) size %v starting", stream+1, mc.streams, start, end, fs.SizeSuffix(end-start))
 
-	rc, err := NewReOpen(ctx, mc.src, ci.LowLevelRetries, &fs.RangeOption{Start: start, End: end - 1})
+	rc, err := Open(ctx, mc.src, &fs.RangeOption{Start: start, End: end - 1})
 	if err != nil {
 		return fmt.Errorf("multipart copy: failed to open source: %w", err)
 	}

fs/operations/operations.go

@@ -421,7 +421,7 @@ func Copy(ctx context.Context, f fs.Fs, dst fs.Object, remote string, src fs.Obj
 				for _, option := range ci.DownloadHeaders {
 					options = append(options, option)
 				}
-				in0, err = NewReOpen(ctx, src, ci.LowLevelRetries, options...)
+				in0, err = Open(ctx, src, options...)
 				if err != nil {
 					err = fmt.Errorf("failed to open source object: %w", err)
 				} else {
@@ -1026,7 +1026,7 @@ func hashSum(ctx context.Context, ht hash.Type, base64Encoded bool, downloadFlag
 		for _, option := range fs.GetConfig(ctx).DownloadHeaders {
 			options = append(options, option)
 		}
-		in, err := NewReOpen(ctx, o, fs.GetConfig(ctx).LowLevelRetries, options...)
+		in, err := Open(ctx, o, options...)
 		if err != nil {
 			return "ERROR", fmt.Errorf("failed to open file %v: %w", o, err)
 		}
@@ -1326,7 +1326,7 @@ func Cat(ctx context.Context, f fs.Fs, w io.Writer, offset, count int64, sep []b
 		for _, option := range ci.DownloadHeaders {
 			options = append(options, option)
 		}
-		in, err := o.Open(ctx, options...)
+		in, err := Open(ctx, o, options...)
 		if err != nil {
 			err = fs.CountError(err)
 			fs.Errorf(o, "Failed to open: %v", err)

fs/operations/reopen.go

@@ -29,12 +29,14 @@ var (
 	errorTooManyTries = errors.New("failed to reopen: too many retries")
 )
 
-// NewReOpen makes a handle which will reopen itself and seek to where it was on errors
+// NewReOpen makes a handle which will reopen itself and seek to where
+// it was on errors up to maxTries times.
 //
-// If hashOption is set this will be applied when reading from the start.
+// If an fs.HashesOption is set this will be applied when reading from
+// the start.
 //
-// If rangeOption is set then this will applied when reading from the
+// If an fs.RangeOption is set then this will applied when reading from
-// start, and updated on retries.
+// the start, and updated on retries.
 func NewReOpen(ctx context.Context, src fs.Object, maxTries int, options ...fs.OpenOption) (rc io.ReadCloser, err error) {
 	h := &ReOpen{
 		ctx:      ctx,
@@ -51,6 +53,24 @@ func NewReOpen(ctx context.Context, src fs.Object, maxTries int, options ...fs.O
 	return h, nil
 }
 
+// Open makes a handle which will reopen itself and seek to where it
+// was on errors.
+//
+// If an fs.HashesOption is set this will be applied when reading from
+// the start.
+//
+// If an fs.RangeOption is set then this will applied when reading from
+// the start, and updated on retries.
+//
+// It will obey LowLevelRetries in the ctx as the maximum number of
+// tries.
+//
+// Use this instead of calling the Open method on fs.Objects
+func Open(ctx context.Context, src fs.Object, options ...fs.OpenOption) (rc io.ReadCloser, err error) {
+	maxTries := fs.GetConfig(ctx).LowLevelRetries
+	return NewReOpen(ctx, src, maxTries, options...)
+}
+
 // open the underlying handle - call with lock held
 //
 // we don't retry here as the Open() call will itself have low level retries