github.com/meulengracht/snapd@v0.0.0-20210719210640-8bde69bcc84e/overlord/snapstate/backend/locking.go (about)

     1  // -*- Mode: Go; indent-tabs-mode: t -*-
     2  
     3  /*
     4   * Copyright (C) 2020 Canonical Ltd
     5   *
     6   * This program is free software: you can redistribute it and/or modify
     7   * it under the terms of the GNU General Public License version 3 as
     8   * published by the Free Software Foundation.
     9   *
    10   * This program is distributed in the hope that it will be useful,
    11   * but WITHOUT ANY WARRANTY; without even the implied warranty of
    12   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    13   * GNU General Public License for more details.
    14   *
    15   * You should have received a copy of the GNU General Public License
    16   * along with this program.  If not, see <http://www.gnu.org/licenses/>.
    17   *
    18   */
    19  package backend
    20  
    21  import (
    22  	"github.com/snapcore/snapd/cmd/snaplock"
    23  	"github.com/snapcore/snapd/cmd/snaplock/runinhibit"
    24  	"github.com/snapcore/snapd/osutil"
    25  	"github.com/snapcore/snapd/snap"
    26  )
    27  
    28  func (b Backend) RunInhibitSnapForUnlink(info *snap.Info, hint runinhibit.Hint, decision func() error) (lock *osutil.FileLock, err error) {
    29  	// A process may be created after the soft refresh done upon
    30  	// the request to refresh a snap. If such process is alive by
    31  	// the time this code is reached the refresh process is stopped.
    32  
    33  	// Grab per-snap lock to prevent new processes from starting. This is
    34  	// sufficient to perform the check, even though individual processes
    35  	// may fork or exit, we will have per-security-tag information about
    36  	// what is running.
    37  	lock, err = snaplock.OpenLock(info.InstanceName())
    38  	if err != nil {
    39  		return nil, err
    40  	}
    41  	// Keep a copy of lock, so that we can close it in the function below.
    42  	// The regular lock variable is assigned to by return, due to the named
    43  	// return values.
    44  	lockToClose := lock
    45  	defer func() {
    46  		// If we have a lock but we are returning an error then unlock the lock
    47  		// by closing it.
    48  		if lockToClose != nil && err != nil {
    49  			lockToClose.Close()
    50  		}
    51  	}()
    52  	if err := lock.Lock(); err != nil {
    53  		return nil, err
    54  	}
    55  	//
    56  	if err := decision(); err != nil {
    57  		return nil, err
    58  	}
    59  	// Decision function did not fail so we can, while we still hold the snap
    60  	// lock, install the snap run inhibition hint, returning the snap lock to
    61  	// the caller.
    62  	//
    63  	// XXX: should we move this logic to the place that calls the "soft"
    64  	// check instead? Doing so would somewhat change the semantic of soft
    65  	// and hard checks, as it would effectively make hard check a no-op,
    66  	// but it might provide a nicer user experience.
    67  	if err := runinhibit.LockWithHint(info.InstanceName(), hint); err != nil {
    68  		return nil, err
    69  	}
    70  	return lock, nil
    71  }
    72  
    73  // WithSnapLock executes given action with the snap lock held.
    74  //
    75  // The lock is also used by snap-confine during pre-snap mount namespace
    76  // initialization. Holding it allows to ensure mutual exclusion during the
    77  // process of preparing a new snap app or hook processes. It does not prevent
    78  // existing application or hook processes from forking.
    79  //
    80  // Note that this is not a method of the Backend type, so that it can be
    81  // invoked from doInstall, which does not have access to a backend object.
    82  func WithSnapLock(info *snap.Info, action func() error) error {
    83  	lock, err := snaplock.OpenLock(info.InstanceName())
    84  	if err != nil {
    85  		return err
    86  	}
    87  	// Closing the lock also unlocks it, if locked.
    88  	defer lock.Close()
    89  	if err := lock.Lock(); err != nil {
    90  		return err
    91  	}
    92  	return action()
    93  }