libvirt/src/util/virlockspace.h

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Introduce an internal API for handling file based lockspaces The previously introduced virFile{Lock,Unlock} APIs provide a way to acquire/release fcntl() locks on individual files. For unknown reason though, the POSIX spec says that fcntl() locks are released when *any* file handle referring to the same path is closed. In the following sequence threadA: fd1 = open("foo") threadB: fd2 = open("foo") threadA: virFileLock(fd1) threadB: virFileLock(fd2) threadB: close(fd2) you'd expect threadA to come out holding a lock on 'foo', and indeed it does hold a lock for a very short time. Unfortunately when threadB does close(fd2) this releases the lock associated with fd1. For the current libvirt use case for virFileLock - pidfiles - this doesn't matter since the lock is acquired at startup while single threaded an never released until exit. To provide a more generally useful API though, it is necessary to introduce a slightly higher level abstraction, which is to be referred to as a "lockspace". This is to be provided by a virLockSpacePtr object in src/util/virlockspace.{c,h}. The core idea is that the lockspace keeps track of what files are already open+locked. This means that when a 2nd thread comes along and tries to acquire a lock, it doesn't end up opening and closing a new FD. The lockspace just checks the current list of held locks and immediately returns VIR_ERR_RESOURCE_BUSY. NB, the API as it stands is designed on the basis that the files being locked are not being otherwise opened and used by the application code. One approach to using this API is to acquire locks based on a hash of the filepath. eg to lock /var/lib/libvirt/images/foo.img the application might do virLockSpacePtr lockspace = virLockSpaceNew("/var/lib/libvirt/imagelocks"); lockname = md5sum("/var/lib/libvirt/images/foo.img"); virLockSpaceAcquireLock(lockspace, lockname); NB, in this example, the caller should ensure that the path is canonicalized before calculating the checksum. It is also possible to do locks directly on resources by using a NULL lockspace directory and then using the file path as the lock name eg virLockSpacePtr lockspace = virLockSpaceNew(NULL); virLockSpaceAcquireLock(lockspace, "/var/lib/libvirt/images/foo.img"); This is only safe to do though if no other part of the process will be opening the files. This will be the case when this code is used inside the soon-to-be-reposted virlockd daemon Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
2012-08-02 16:02:40 +00:00
/*
* virlockspace.h: simple file based lockspaces
*
* Copyright (C) 2012 Red Hat, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; If not, see
* <http://www.gnu.org/licenses/>.
*
*/
#ifndef LIBVIRT_VIRLOCKSPACE_H
# define LIBVIRT_VIRLOCKSPACE_H
Introduce an internal API for handling file based lockspaces The previously introduced virFile{Lock,Unlock} APIs provide a way to acquire/release fcntl() locks on individual files. For unknown reason though, the POSIX spec says that fcntl() locks are released when *any* file handle referring to the same path is closed. In the following sequence threadA: fd1 = open("foo") threadB: fd2 = open("foo") threadA: virFileLock(fd1) threadB: virFileLock(fd2) threadB: close(fd2) you'd expect threadA to come out holding a lock on 'foo', and indeed it does hold a lock for a very short time. Unfortunately when threadB does close(fd2) this releases the lock associated with fd1. For the current libvirt use case for virFileLock - pidfiles - this doesn't matter since the lock is acquired at startup while single threaded an never released until exit. To provide a more generally useful API though, it is necessary to introduce a slightly higher level abstraction, which is to be referred to as a "lockspace". This is to be provided by a virLockSpacePtr object in src/util/virlockspace.{c,h}. The core idea is that the lockspace keeps track of what files are already open+locked. This means that when a 2nd thread comes along and tries to acquire a lock, it doesn't end up opening and closing a new FD. The lockspace just checks the current list of held locks and immediately returns VIR_ERR_RESOURCE_BUSY. NB, the API as it stands is designed on the basis that the files being locked are not being otherwise opened and used by the application code. One approach to using this API is to acquire locks based on a hash of the filepath. eg to lock /var/lib/libvirt/images/foo.img the application might do virLockSpacePtr lockspace = virLockSpaceNew("/var/lib/libvirt/imagelocks"); lockname = md5sum("/var/lib/libvirt/images/foo.img"); virLockSpaceAcquireLock(lockspace, lockname); NB, in this example, the caller should ensure that the path is canonicalized before calculating the checksum. It is also possible to do locks directly on resources by using a NULL lockspace directory and then using the file path as the lock name eg virLockSpacePtr lockspace = virLockSpaceNew(NULL); virLockSpaceAcquireLock(lockspace, "/var/lib/libvirt/images/foo.img"); This is only safe to do though if no other part of the process will be opening the files. This will be the case when this code is used inside the soon-to-be-reposted virlockd daemon Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
2012-08-02 16:02:40 +00:00
# include "internal.h"
2012-12-12 17:53:50 +00:00
# include "virjson.h"
Introduce an internal API for handling file based lockspaces The previously introduced virFile{Lock,Unlock} APIs provide a way to acquire/release fcntl() locks on individual files. For unknown reason though, the POSIX spec says that fcntl() locks are released when *any* file handle referring to the same path is closed. In the following sequence threadA: fd1 = open("foo") threadB: fd2 = open("foo") threadA: virFileLock(fd1) threadB: virFileLock(fd2) threadB: close(fd2) you'd expect threadA to come out holding a lock on 'foo', and indeed it does hold a lock for a very short time. Unfortunately when threadB does close(fd2) this releases the lock associated with fd1. For the current libvirt use case for virFileLock - pidfiles - this doesn't matter since the lock is acquired at startup while single threaded an never released until exit. To provide a more generally useful API though, it is necessary to introduce a slightly higher level abstraction, which is to be referred to as a "lockspace". This is to be provided by a virLockSpacePtr object in src/util/virlockspace.{c,h}. The core idea is that the lockspace keeps track of what files are already open+locked. This means that when a 2nd thread comes along and tries to acquire a lock, it doesn't end up opening and closing a new FD. The lockspace just checks the current list of held locks and immediately returns VIR_ERR_RESOURCE_BUSY. NB, the API as it stands is designed on the basis that the files being locked are not being otherwise opened and used by the application code. One approach to using this API is to acquire locks based on a hash of the filepath. eg to lock /var/lib/libvirt/images/foo.img the application might do virLockSpacePtr lockspace = virLockSpaceNew("/var/lib/libvirt/imagelocks"); lockname = md5sum("/var/lib/libvirt/images/foo.img"); virLockSpaceAcquireLock(lockspace, lockname); NB, in this example, the caller should ensure that the path is canonicalized before calculating the checksum. It is also possible to do locks directly on resources by using a NULL lockspace directory and then using the file path as the lock name eg virLockSpacePtr lockspace = virLockSpaceNew(NULL); virLockSpaceAcquireLock(lockspace, "/var/lib/libvirt/images/foo.img"); This is only safe to do though if no other part of the process will be opening the files. This will be the case when this code is used inside the soon-to-be-reposted virlockd daemon Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
2012-08-02 16:02:40 +00:00
typedef struct _virLockSpace virLockSpace;
typedef virLockSpace *virLockSpacePtr;
virLockSpacePtr virLockSpaceNew(const char *directory);
virLockSpacePtr virLockSpaceNewPostExecRestart(virJSONValuePtr object);
virJSONValuePtr virLockSpacePreExecRestart(virLockSpacePtr lockspace);
Introduce an internal API for handling file based lockspaces The previously introduced virFile{Lock,Unlock} APIs provide a way to acquire/release fcntl() locks on individual files. For unknown reason though, the POSIX spec says that fcntl() locks are released when *any* file handle referring to the same path is closed. In the following sequence threadA: fd1 = open("foo") threadB: fd2 = open("foo") threadA: virFileLock(fd1) threadB: virFileLock(fd2) threadB: close(fd2) you'd expect threadA to come out holding a lock on 'foo', and indeed it does hold a lock for a very short time. Unfortunately when threadB does close(fd2) this releases the lock associated with fd1. For the current libvirt use case for virFileLock - pidfiles - this doesn't matter since the lock is acquired at startup while single threaded an never released until exit. To provide a more generally useful API though, it is necessary to introduce a slightly higher level abstraction, which is to be referred to as a "lockspace". This is to be provided by a virLockSpacePtr object in src/util/virlockspace.{c,h}. The core idea is that the lockspace keeps track of what files are already open+locked. This means that when a 2nd thread comes along and tries to acquire a lock, it doesn't end up opening and closing a new FD. The lockspace just checks the current list of held locks and immediately returns VIR_ERR_RESOURCE_BUSY. NB, the API as it stands is designed on the basis that the files being locked are not being otherwise opened and used by the application code. One approach to using this API is to acquire locks based on a hash of the filepath. eg to lock /var/lib/libvirt/images/foo.img the application might do virLockSpacePtr lockspace = virLockSpaceNew("/var/lib/libvirt/imagelocks"); lockname = md5sum("/var/lib/libvirt/images/foo.img"); virLockSpaceAcquireLock(lockspace, lockname); NB, in this example, the caller should ensure that the path is canonicalized before calculating the checksum. It is also possible to do locks directly on resources by using a NULL lockspace directory and then using the file path as the lock name eg virLockSpacePtr lockspace = virLockSpaceNew(NULL); virLockSpaceAcquireLock(lockspace, "/var/lib/libvirt/images/foo.img"); This is only safe to do though if no other part of the process will be opening the files. This will be the case when this code is used inside the soon-to-be-reposted virlockd daemon Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
2012-08-02 16:02:40 +00:00
void virLockSpaceFree(virLockSpacePtr lockspace);
const char *virLockSpaceGetDirectory(virLockSpacePtr lockspace);
int virLockSpaceCreateResource(virLockSpacePtr lockspace,
const char *resname);
int virLockSpaceDeleteResource(virLockSpacePtr lockspace,
const char *resname);
typedef enum {
VIR_LOCK_SPACE_ACQUIRE_SHARED = (1 << 0),
VIR_LOCK_SPACE_ACQUIRE_AUTOCREATE = (1 << 1),
} virLockSpaceAcquireFlags;
int virLockSpaceAcquireResource(virLockSpacePtr lockspace,
const char *resname,
pid_t owner,
unsigned int flags);
int virLockSpaceReleaseResource(virLockSpacePtr lockspace,
const char *resname,
pid_t owner);
int virLockSpaceReleaseResourcesForOwner(virLockSpacePtr lockspace,
pid_t owner);
#endif /* LIBVIRT_VIRLOCKSPACE_H */