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/sftp/sftp.go

@@ -15,7 +15,6 @@ 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"
@@ -35,7 +34,6 @@ type SFTP struct {
 
 	posixRename bool
 
-	sem sema.Semaphore
 	layout.Layout
 	Config
 	backend.Modes
@@ -140,11 +138,7 @@ func Open(ctx context.Context, cfg Config) (*SFTP, error) {
 }
 
 func open(ctx context.Context, sftp *SFTP, cfg Config) (*SFTP, error) {
-	sem, err := sema.New(cfg.Connections)
-	if err != nil {
-		return nil, err
-	}
-
+	var err error
 	sftp.Layout, err = layout.ParseLayout(ctx, sftp, cfg.Layout, defaultLayout, cfg.Path)
 	if err != nil {
 		return nil, err
@@ -158,7 +152,6 @@ func open(ctx context.Context, sftp *SFTP, cfg Config) (*SFTP, error) {
 
 	sftp.Config = cfg
 	sftp.p = cfg.Path
-	sftp.sem = sem
 	sftp.Modes = m
 	return sftp, nil
 }
@@ -308,17 +301,10 @@ func (r *SFTP) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader
 		return err
 	}
 
-	if err := h.Valid(); err != nil {
-		return backoff.Permanent(err)
-	}
-
 	filename := r.Filename(h)
 	tmpFilename := filename + "-restic-temp-" + tempSuffix()
 	dirname := r.Dirname(h)
 
-	r.sem.GetToken()
-	defer r.sem.ReleaseToken()
-
 	// create new file
 	f, err := r.c.OpenFile(tmpFilename, os.O_CREATE|os.O_EXCL|os.O_WRONLY)
 
@@ -414,52 +400,27 @@ func (r *SFTP) Load(ctx context.Context, h restic.Handle, length int, offset int
 	return backend.DefaultLoad(ctx, h, length, offset, r.openReader, fn)
 }
 
-// wrapReader wraps an io.ReadCloser to run an additional function on Close.
-type wrapReader struct {
-	io.ReadCloser
-	io.WriterTo
-	f func()
-}
-
-func (wr *wrapReader) Close() error {
-	err := wr.ReadCloser.Close()
-	wr.f()
-	return err
-}
-
 func (r *SFTP) openReader(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
-	r.sem.GetToken()
 	f, err := r.c.Open(r.Filename(h))
 	if err != nil {
-		r.sem.ReleaseToken()
 		return nil, err
 	}
 
 	if offset > 0 {
 		_, err = f.Seek(offset, 0)
 		if err != nil {
-			r.sem.ReleaseToken()
 			_ = f.Close()
 			return nil, err
 		}
 	}
 
-	// use custom close wrapper to also provide WriteTo() on the wrapper
-	rd := &wrapReader{
-		ReadCloser: f,
-		WriterTo:   f,
-		f: func() {
-			r.sem.ReleaseToken()
-		},
-	}
-
 	if length > 0 {
 		// unlimited reads usually use io.Copy which needs WriteTo support at the underlying reader
 		// limited reads are usually combined with io.ReadFull which reads all required bytes into a buffer in one go
-		return backend.LimitReadCloser(rd, int64(length)), nil
+		return backend.LimitReadCloser(f, int64(length)), nil
 	}
 
-	return rd, nil
+	return f, nil
 }
 
 // Stat returns information about a blob.
@@ -468,13 +429,6 @@ func (r *SFTP) Stat(ctx context.Context, h restic.Handle) (restic.FileInfo, erro
 		return restic.FileInfo{}, err
 	}
 
-	if err := h.Valid(); err != nil {
-		return restic.FileInfo{}, backoff.Permanent(err)
-	}
-
-	r.sem.GetToken()
-	defer r.sem.ReleaseToken()
-
 	fi, err := r.c.Lstat(r.Filename(h))
 	if err != nil {
 		return restic.FileInfo{}, errors.Wrap(err, "Lstat")
@@ -489,9 +443,6 @@ func (r *SFTP) Remove(ctx context.Context, h restic.Handle) error {
 		return err
 	}
 
-	r.sem.GetToken()
-	defer r.sem.ReleaseToken()
-
 	return r.c.Remove(r.Filename(h))
 }
 
@@ -501,9 +452,7 @@ func (r *SFTP) List(ctx context.Context, t restic.FileType, fn func(restic.FileI
 	basedir, subdirs := r.Basedir(t)
 	walker := r.c.Walk(basedir)
 	for {
-		r.sem.GetToken()
 		ok := walker.Step()
-		r.sem.ReleaseToken()
 		if !ok {
 			break
 		}