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/s3/s3.go

@@ -13,12 +13,10 @@ import (
 
 	"github.com/restic/restic/internal/backend"
 	"github.com/restic/restic/internal/backend/layout"
-	"github.com/restic/restic/internal/backend/sema"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/restic"
 
-	"github.com/cenkalti/backoff/v4"
 	"github.com/minio/minio-go/v7"
 	"github.com/minio/minio-go/v7/pkg/credentials"
 )
@@ -26,7 +24,6 @@ import (
 // Backend stores data on an S3 endpoint.
 type Backend struct {
 	client *minio.Client
-	sem    sema.Semaphore
 	cfg    Config
 	layout.Layout
 }
@@ -102,14 +99,8 @@ func open(ctx context.Context, cfg Config, rt http.RoundTripper) (*Backend, erro
 		return nil, errors.Wrap(err, "minio.New")
 	}
 
-	sem, err := sema.New(cfg.Connections)
-	if err != nil {
-		return nil, err
-	}
-
 	be := &Backend{
 		client: client,
-		sem:    sem,
 		cfg:    cfg,
 	}
 
@@ -271,15 +262,8 @@ func (be *Backend) Path() string {
 
 // Save stores data in the backend at the handle.
 func (be *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader) error {
-	if err := h.Valid(); err != nil {
-		return backoff.Permanent(err)
-	}
-
 	objName := be.Filename(h)
 
-	be.sem.GetToken()
-	defer be.sem.ReleaseToken()
-
 	opts := minio.PutObjectOptions{StorageClass: be.cfg.StorageClass}
 	opts.ContentType = "application/octet-stream"
 	// the only option with the high-level api is to let the library handle the checksum computation
@@ -301,6 +285,9 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindRe
 // Load runs fn with a reader that yields the contents of the file at h at the
 // given offset.
 func (be *Backend) Load(ctx context.Context, h restic.Handle, length int, offset int64, fn func(rd io.Reader) error) error {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
 	return backend.DefaultLoad(ctx, h, length, offset, be.openReader, fn)
 }
 
@@ -321,18 +308,13 @@ func (be *Backend) openReader(ctx context.Context, h restic.Handle, length int,
 		return nil, errors.Wrap(err, "SetRange")
 	}
 
-	be.sem.GetToken()
-	ctx, cancel := context.WithCancel(ctx)
-
 	coreClient := minio.Core{Client: be.client}
 	rd, _, _, err := coreClient.GetObject(ctx, be.cfg.Bucket, objName, opts)
 	if err != nil {
-		cancel()
-		be.sem.ReleaseToken()
 		return nil, err
 	}
 
-	return be.sem.ReleaseTokenOnClose(rd, cancel), err
+	return rd, err
 }
 
 // Stat returns information about a blob.
@@ -342,17 +324,14 @@ func (be *Backend) Stat(ctx context.Context, h restic.Handle) (bi restic.FileInf
 
 	opts := minio.GetObjectOptions{}
 
-	be.sem.GetToken()
 	obj, err = be.client.GetObject(ctx, be.cfg.Bucket, objName, opts)
 	if err != nil {
-		be.sem.ReleaseToken()
 		return restic.FileInfo{}, errors.Wrap(err, "client.GetObject")
 	}
 
 	// make sure that the object is closed properly.
 	defer func() {
 		e := obj.Close()
-		be.sem.ReleaseToken()
 		if err == nil {
 			err = errors.Wrap(e, "Close")
 		}
@@ -370,9 +349,7 @@ func (be *Backend) Stat(ctx context.Context, h restic.Handle) (bi restic.FileInf
 func (be *Backend) Remove(ctx context.Context, h restic.Handle) error {
 	objName := be.Filename(h)
 
-	be.sem.GetToken()
 	err := be.client.RemoveObject(ctx, be.cfg.Bucket, objName, minio.RemoveObjectOptions{})
-	be.sem.ReleaseToken()
 
 	if be.IsNotExist(err) {
 		err = nil