Add mount functionality to 'Directory'

This is needed for 'chroot' runners that must mount the special
filesystems '/proc' and '/sys' in the input root.
The necessary syscalls are not yet merged into the 'sys' package,
so this includes a patch while we work to upstream the code.

BUILD.bazel

@@ -1,6 +1,6 @@
 load("@bazel_gazelle//:def.bzl", "gazelle")
 load("@com_github_bazelbuild_buildtools//buildifier:def.bzl", "buildifier")
-load("@npm//:defs.bzl", "npm_link_all_packages", "npm_link_targets")
+load("@npm//:defs.bzl", "npm_link_all_packages")
 
 # gazelle:prefix github.com/buildbarn/bb-storage
 # gazelle:resolve proto build/bazel/remote/execution/v2/remote_execution.proto @com_github_bazelbuild_remote_apis//build/bazel/remote/execution/v2:remote_execution_proto

go_dependencies.bzl

@@ -1569,6 +1569,7 @@ def go_dependencies():
         name = "org_golang_x_sys",
         importpath = "golang.org/x/sys",
         sum = "h1:SqMFp9UcQJZa+pmYuAKjd9xq1f0j5rLcDIk0mj4qAsA=",
+        patches = ["//:patches/org_golang_x_sys/unix_add_Fsconfig_syscall_on_linux.patch"],
         version = "v0.10.0",
     )
     go_repository(

pkg/filesystem/directory.go

@@ -112,6 +112,12 @@ type Directory interface {
 	// Function that base types may use to implement calls that
 	// require double dispatching, such as hardlinking and renaming.
 	Apply(arg interface{}) error
+
+	// Mount and Unmount.
+	// This uses `Mountat` functionality, but not `Unmountat`.
+	// see https://github.com/buildbarn/bb-remote-execution/issues/115 for information.
+	Mount(mountpoint path.Component, source string, fstype string) error
+	Unmount(mountpoint path.Component) error
 }
 
 // DirectoryCloser is a Directory handle that can be released.

pkg/filesystem/local_directory_darwin.go

@@ -23,3 +23,7 @@ func (d *localDirectory) Mknod(name path.Component, perm os.FileMode, deviceNumb
 func clonefileImpl(oldFD int, oldName string, newFD int, newName string) error {
 	return unix.Clonefileat(oldFD, oldName, newFD, newName, unix.CLONE_NOFOLLOW)
 }
+
+func (d *localDirectory) Mount(mountpoint path.Component, source string, fstype string) error {
+	return status.Error(codes.Unimplemented, "Mount is not supported")
+}

pkg/filesystem/local_directory_freebsd.go

@@ -24,3 +24,7 @@ func (d *localDirectory) Mknod(name path.Component, perm os.FileMode, deviceNumb
 func clonefileImpl(oldFD int, oldName string, newFD int, newName string) error {
 	return status.Error(codes.Unimplemented, "Clonefile is only supported on Darwin")
 }
+
+func (d *localDirectory) Mount(mountpoint path.Component, source string, fstype string) error {
+	return status.Error(codes.Unimplemented, "Mount is not supported")
+}

pkg/filesystem/local_directory_linux.go

@@ -8,6 +8,7 @@ import (
 	"runtime"
 
 	"github.com/buildbarn/bb-storage/pkg/filesystem/path"
+	"github.com/buildbarn/bb-storage/pkg/util"
 
 	"golang.org/x/sys/unix"
 	"google.golang.org/grpc/codes"
@@ -34,3 +35,38 @@ func (d *localDirectory) Mknod(name path.Component, perm os.FileMode, deviceNumb
 func clonefileImpl(oldFD int, oldName string, newFD int, newName string) error {
 	return status.Error(codes.Unimplemented, "Clonefile is only supported on Darwin")
 }
+
+func (d *localDirectory) Mount(mountpoint path.Component, source string, fstype string) error {
+	// NB: `Fsmount` creates a file descriptor to the mount object,
+	// that can be used to move it again.
+	// We do not use it,
+	// unmount will fail with `EBUSY`
+	// if it is left open.
+	mountname := mountpoint.String()
+	fd, err := unix.Fsopen(fstype, unix.FSOPEN_CLOEXEC)
+	if err != nil {
+		return util.StatusWrapf(err, "Fsopen '%s'", fstype)
+	}
+
+	err = unix.Fsconfig(fd, unix.FSCONFIG_SET_STRING, "source", source, 0)
+	if err != nil {
+		return util.StatusWrapf(err, "Fsconfig source '%s'", source)
+	}
+
+	err = unix.Fsconfig(fd, unix.FSCONFIG_CMD_CREATE, "", "", 0)
+	if err != nil {
+		return util.StatusWrap(err, "Fsconfig create")
+	}
+
+	mfd, err := unix.Fsmount(fd, unix.FSMOUNT_CLOEXEC, unix.MS_NOEXEC)
+	defer unix.Close(mfd)
+	if err != nil {
+		return util.StatusWrap(err, "Fsmount")
+	}
+	err = unix.MoveMount(mfd, "", d.fd, mountname, unix.MOVE_MOUNT_F_EMPTY_PATH)
+	if err != nil {
+		return util.StatusWrapf(err, "Movemount mountname '%s' in file descriptor %d", mountname, d.fd)
+	}
+
+	return nil
+}

pkg/filesystem/local_directory_unix.go

@@ -268,7 +268,7 @@ func (d *localDirectory) Remove(name path.Component) error {
 
 var workingDirectoryLock sync.Mutex
 
-func (d *localDirectory) unmount(name path.Component) error {
+func (d *localDirectory) Unmount(name path.Component) error {
 	defer runtime.KeepAlive(d)
 
 	// POSIX systems provide no umountat() system call that permits
@@ -302,7 +302,7 @@ func (d *localDirectory) removeAllChildren(parentDeviceNumber rawDeviceNumber) e
 		// unmount until the remaining directory is on the same
 		// file system.
 		for parentDeviceNumber != childDeviceNumber {
-			if err := d.unmount(component); err != nil {
+			if err := d.Unmount(component); err != nil {
 				return err
 			}
 			fileType, childDeviceNumber, _, err = d.lstat(component)

pkg/filesystem/local_directory_windows.go

@@ -978,3 +978,11 @@ func (d *localDirectory) Apply(arg interface{}) error {
 		return syscall.EXDEV
 	}
 }
+
+func (d *localDirectory) Mount(mountpoint path.Component, source string, fstype string) error {
+	return status.Error(codes.Unimplemented, "Mount is not supported")
+}
+
+func (d *localDirectory) Unmount(mountpoint path.Component) error {
+	return status.Error(codes.Unimplemented, "Unmount is not supported")
+}