smb: improve multithreaded upload performance using multiple connections

In the current design, OpenWriterAt provides the interface for random-access
writes, and openChunkWriterFromOpenWriterAt wraps this interface to enable
parallel chunk uploads using multiple goroutines. A global connection pool is
already in place to manage SMB connections across files.

However, currently only one connection is used per file, which makes multiple
goroutines compete for the connection during multithreaded writes.

This changes create separate connections for each goroutine, which allows true
parallelism by giving each goroutine its own SMB connection

Signed-off-by: sudipto baral <sudiptobaral.me@gmail.com>

backend/smb/filepool.go

@@ -0,0 +1,99 @@
+package smb
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"sync"
+
+	"github.com/cloudsoda/go-smb2"
+	"golang.org/x/sync/errgroup"
+)
+
+// FsInterface defines the methods that filePool needs from Fs
+type FsInterface interface {
+	getConnection(ctx context.Context, share string) (*conn, error)
+	putConnection(pc **conn, err error)
+	removeSession()
+}
+
+type file struct {
+	*smb2.File
+	c *conn
+}
+
+type filePool struct {
+	ctx   context.Context
+	fs    FsInterface
+	share string
+	path  string
+
+	mu   sync.Mutex
+	pool []*file
+}
+
+func newFilePool(ctx context.Context, fs FsInterface, share, path string) *filePool {
+	return &filePool{
+		ctx:   ctx,
+		fs:    fs,
+		share: share,
+		path:  path,
+	}
+}
+
+func (p *filePool) get() (*file, error) {
+	p.mu.Lock()
+	if len(p.pool) > 0 {
+		f := p.pool[len(p.pool)-1]
+		p.pool = p.pool[:len(p.pool)-1]
+		p.mu.Unlock()
+		return f, nil
+	}
+	p.mu.Unlock()
+
+	c, err := p.fs.getConnection(p.ctx, p.share)
+	if err != nil {
+		return nil, err
+	}
+
+	fl, err := c.smbShare.OpenFile(p.path, os.O_WRONLY, 0o644)
+	if err != nil {
+		p.fs.putConnection(&c, err)
+		return nil, fmt.Errorf("failed to open: %w", err)
+	}
+
+	return &file{File: fl, c: c}, nil
+}
+
+func (p *filePool) put(f *file, err error) {
+	if f == nil {
+		return
+	}
+
+	if err != nil {
+		_ = f.Close()
+		p.fs.putConnection(&f.c, err)
+		return
+	}
+
+	p.mu.Lock()
+	p.pool = append(p.pool, f)
+	p.mu.Unlock()
+}
+
+func (p *filePool) drain() error {
+	p.mu.Lock()
+	files := p.pool
+	p.pool = nil
+	p.mu.Unlock()
+
+	g, _ := errgroup.WithContext(p.ctx)
+	for _, f := range files {
+		g.Go(func() error {
+			err := f.Close()
+			p.fs.putConnection(&f.c, err)
+			return err
+		})
+	}
+	return g.Wait()
+}