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/sema/backend.go

@@ -0,0 +1,87 @@
+package sema
+
+import (
+	"context"
+	"io"
+
+	"github.com/cenkalti/backoff/v4"
+	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/restic"
+)
+
+// make sure that SemaphoreBackend implements restic.Backend
+var _ restic.Backend = &SemaphoreBackend{}
+
+// SemaphoreBackend limits the number of concurrent operations.
+type SemaphoreBackend struct {
+	restic.Backend
+	sem semaphore
+}
+
+// New creates a backend that limits the concurrent operations on the underlying backend
+func New(be restic.Backend) *SemaphoreBackend {
+	sem, err := newSemaphore(be.Connections())
+	if err != nil {
+		panic(err)
+	}
+
+	return &SemaphoreBackend{
+		Backend: be,
+		sem:     sem,
+	}
+}
+
+// Save adds new Data to the backend.
+func (be *SemaphoreBackend) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader) error {
+	if err := h.Valid(); err != nil {
+		return backoff.Permanent(err)
+	}
+
+	be.sem.GetToken()
+	defer be.sem.ReleaseToken()
+
+	return be.Backend.Save(ctx, h, rd)
+}
+
+// Load runs fn with a reader that yields the contents of the file at h at the
+// given offset.
+func (be *SemaphoreBackend) Load(ctx context.Context, h restic.Handle, length int, offset int64, fn func(rd io.Reader) error) error {
+	if err := h.Valid(); err != nil {
+		return backoff.Permanent(err)
+	}
+	if offset < 0 {
+		return backoff.Permanent(errors.New("offset is negative"))
+	}
+	if length < 0 {
+		return backoff.Permanent(errors.Errorf("invalid length %d", length))
+	}
+
+	be.sem.GetToken()
+	defer be.sem.ReleaseToken()
+
+	return be.Backend.Load(ctx, h, length, offset, fn)
+}
+
+// Stat returns information about a file in the backend.
+func (be *SemaphoreBackend) Stat(ctx context.Context, h restic.Handle) (restic.FileInfo, error) {
+	if err := h.Valid(); err != nil {
+		return restic.FileInfo{}, backoff.Permanent(err)
+	}
+
+	be.sem.GetToken()
+	defer be.sem.ReleaseToken()
+
+	return be.Backend.Stat(ctx, h)
+}
+
+// Remove deletes a file from the backend.
+func (be *SemaphoreBackend) Remove(ctx context.Context, h restic.Handle) error {
+	if err := h.Valid(); err != nil {
+		return backoff.Permanent(err)
+	}
+
+	be.sem.GetToken()
+	defer be.sem.ReleaseToken()
+
+	return be.Backend.Remove(ctx, h)
+}