backend: factor out connection limiting and parameter validation

The SemaphoreBackend now uniformly enforces the limit of concurrent
backend operations. In addition, it unifies the parameter validation.

The List() methods no longer uses a semaphore. Restic already never runs
multiple list operations in parallel.

By managing the semaphore in a wrapper backend, the sections that hold a
semaphore token grow slightly. However, the main bottleneck is IO, so
this shouldn't make much of a difference.

The key insight that enables the SemaphoreBackend is that all of the
complex semaphore handling in `openReader()` still happens within the
original call to `Load()`. Thus, getting and releasing the semaphore
tokens can be refactored to happen directly in `Load()`. This eliminates
the need for wrapping the reader in `openReader()` to release the token.

internal/backend/utils.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"io"
 
-	"github.com/cenkalti/backoff/v4"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/restic"
@@ -63,15 +62,7 @@ func LimitReadCloser(r io.ReadCloser, n int64) *LimitedReadCloser {
 func DefaultLoad(ctx context.Context, h restic.Handle, length int, offset int64,
 	openReader func(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error),
 	fn func(rd io.Reader) error) error {
-	if err := h.Valid(); err != nil {
-		return backoff.Permanent(err)
-	}
-	if offset < 0 {
-		return errors.New("offset is negative")
-	}
-	if length < 0 {
-		return errors.Errorf("invalid length %d", length)
-	}
+
 	rd, err := openReader(ctx, h, length, offset)
 	if err != nil {
 		return err