serve: fix --auth-proxy stopping VFS cache uploads after 5 minutes

With --auth-proxy the backend of each user is shut down once it has
been unused for 5 minutes. With --vfs-cache-mode writes or full, files
are uploaded to the backend after the transfer which wrote them has
finished, which didn't count as a use. So an upload which hadn't
finished 5 minutes after the user's last command (or disconnection for
serve sftp) was stopped, leaving the file in the VFS cache but not on
the backend.

The backend is now kept until it has no files open for write and
nothing in the VFS cache waiting to be uploaded.

This was introduced in v1.75.1 by

f425f8d46 serve: refactor VFS and proxy handling into Provider

cmd/serve/proxy/proxy.go

@@ -233,6 +233,13 @@ func New(ctx context.Context, opt *Options, vfsOpt *vfscommon.Options) *Proxy {
 			entry.vfs.Shutdown()
 		}
 	})
+	// Shutting down a VFS stops it writing, so keep one which still
+	// has data to write, e.g. files in the VFS cache waiting to be
+	// uploaded after the transfer which wrote them has finished.
+	p.vfsCache.SetCanExpire(func(value any) bool {
+		entry, ok := value.(cacheEntry)
+		return !ok || entry.vfs == nil || !entry.vfs.Busy()
+	})
 	return p
 }
 

cmd/serve/proxy/proxy_test.go

@@ -475,3 +475,29 @@ func TestCallAccessKey(t *testing.T) {
 	_, _, err = p.CallAccessKey("AKID", remoteAddr, false)
 	require.ErrorContains(t, err, "revoked")
 }
+
+// TestBusyVFSNotExpired checks a VFS which still has data to write
+// isn't expired from the cache, and so shut down, until it is written.
+func TestBusyVFSNotExpired(t *testing.T) {
+	opt := Opt
+	opt.AuthProxy = "go run ../servetest/proxy_code.go " + t.TempDir()
+	p := New(context.Background(), &opt, &vfscommon.Opt)
+	defer p.Shutdown()
+	p.vfsCache.SetExpireDuration(time.Millisecond).SetExpireInterval(10 * time.Millisecond)
+
+	VFS, _, err := p.Call("user", "pass", false, "192.0.2.1:1024")
+	require.NoError(t, err)
+	fh, err := VFS.Create("file.txt")
+	require.NoError(t, err)
+
+	assert.Never(t, func() bool {
+		return p.vfsCache.Entries() == 0
+	}, 250*time.Millisecond, 10*time.Millisecond, "VFS expired with a file open for write")
+	assert.Equal(t, int32(1), VFS.Stats()["inUse"], "VFS shut down with a file open for write")
+
+	require.NoError(t, fh.Close())
+	assert.Eventually(t, func() bool {
+		return p.vfsCache.Entries() == 0
+	}, 10*time.Second, 10*time.Millisecond, "VFS not expired once it had nothing to write")
+	assert.Equal(t, int32(0), VFS.Stats()["inUse"], "VFS not shut down once it had nothing to write")
+}