lockfile: New code to lock files, massaged from systemd

Will be used by OSTree to lock the sysroot against concurrent
manipulation.  Taken from current systemd git master, tweaked to
GLibify.

Makefile-libglnx.am

@@ -30,6 +30,8 @@ libglnx_la_SOURCES = \
 	$(libglnx_srcpath)/glnx-dirfd.c \
 	$(libglnx_srcpath)/glnx-fdio.h \
 	$(libglnx_srcpath)/glnx-fdio.c \
+	$(libglnx_srcpath)/glnx-lockfile.h \
+	$(libglnx_srcpath)/glnx-lockfile.c \
 	$(libglnx_srcpath)/glnx-libcontainer.h \
 	$(libglnx_srcpath)/glnx-libcontainer.c \
 	$(libglnx_srcpath)/glnx-xattrs.h \

glnx-lockfile.c

@@ -0,0 +1,193 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+  Now copied into libglnx:
+    - Use GError
+
+  Copyright 2010 Lennart Poettering
+  Copyright 2015 Colin Walters <walters@verbum.org>
+
+  systemd 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.
+
+  systemd 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 systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include "config.h"
+
+#include <stdlib.h>
+#include <stdbool.h>
+#include <errno.h>
+#include <string.h>
+#include <stdio.h>
+#include <limits.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/file.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+#include "glnx-lockfile.h"
+#include "glnx-errors.h"
+#include "glnx-fdio.h"
+#include "glnx-backport-autocleanups.h"
+#include "glnx-local-alloc.h"
+
+#define newa(t, n) ((t*) alloca(sizeof(t)*(n)))
+
+/**
+ * glnx_make_lock_file:
+ * @dfd: Directory file descriptor (if not `AT_FDCWD`, must have lifetime `>=` @out_lock)
+ * @p: Path
+ * @operation: one of `LOCK_SH`, `LOCK_EX`, `LOCK_UN`, as passed to flock()
+ * @out_lock: (out) (caller allocates): Return location for lock
+ * @error: Error
+ *
+ * Block until a lock file named @p (relative to @dfd) can be created,
+ * using the flags in @operation, returning the lock data in the
+ * caller-allocated location @out_lock.
+ *
+ * This API wraps new-style process locking if available, otherwise
+ * falls back to BSD locks.
+ */
+gboolean
+glnx_make_lock_file(int dfd, const char *p, int operation, GLnxLockFile *out_lock, GError **error) {
+        gboolean ret = FALSE;
+        glnx_fd_close int fd = -1;
+        g_autofree char *t = NULL;
+        int r;
+
+        /*
+         * We use UNPOSIX locks if they are available. They have nice
+         * semantics, and are mostly compatible with NFS. However,
+         * they are only available on new kernels. When we detect we
+         * are running on an older kernel, then we fall back to good
+         * old BSD locks. They also have nice semantics, but are
+         * slightly problematic on NFS, where they are upgraded to
+         * POSIX locks, even though locally they are orthogonal to
+         * POSIX locks.
+         */
+
+        t = g_strdup(p);
+
+        for (;;) {
+#ifdef F_OFD_SETLK
+                struct flock fl = {
+                        .l_type = (operation & ~LOCK_NB) == LOCK_EX ? F_WRLCK : F_RDLCK,
+                        .l_whence = SEEK_SET,
+                };
+#endif
+                struct stat st;
+
+                fd = openat(dfd, p, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
+                if (fd < 0) {
+                        glnx_set_error_from_errno(error);
+                        goto out;
+                }
+
+                /* Unfortunately, new locks are not in RHEL 7.1 glibc */
+#ifdef F_OFD_SETLK
+                r = fcntl(fd, (operation & LOCK_NB) ? F_OFD_SETLK : F_OFD_SETLKW, &fl);
+#else
+                r = -1;
+                errno = EINVAL;
+#endif
+                if (r < 0) {
+
+                        /* If the kernel is too old, use good old BSD locks */
+                        if (errno == EINVAL)
+                                r = flock(fd, operation);
+
+                        if (r < 0) {
+                                glnx_set_error_from_errno(error);
+                                goto out;
+                        }
+                }
+
+                /* If we acquired the lock, let's check if the file
+                 * still exists in the file system. If not, then the
+                 * previous exclusive owner removed it and then closed
+                 * it. In such a case our acquired lock is worthless,
+                 * hence try again. */
+
+                r = fstat(fd, &st);
+                if (r < 0) {
+                        glnx_set_error_from_errno(error);
+                        goto out;
+                }
+                if (st.st_nlink > 0)
+                        break;
+
+                (void) close(fd);
+                fd = -1;
+        }
+
+        /* Note that if this is not AT_FDCWD, the caller takes responsibility
+         * for the fd's lifetime being >= that of the lock.
+         */
+        out_lock->dfd = dfd;
+        out_lock->path = t;
+        out_lock->fd = fd;
+        out_lock->operation = operation;
+
+        fd = -1;
+        t = NULL;
+
+        ret = TRUE;
+ out:
+        return ret;
+}
+
+void glnx_release_lock_file(GLnxLockFile *f) {
+        int r;
+
+        if (!f)
+                return;
+
+        if (f->path) {
+
+                /* If we are the exclusive owner we can safely delete
+                 * the lock file itself. If we are not the exclusive
+                 * owner, we can try becoming it. */
+
+                if (f->fd >= 0 &&
+                    (f->operation & ~LOCK_NB) == LOCK_SH) {
+#ifdef F_OFD_SETLK
+                        static const struct flock fl = {
+                                .l_type = F_WRLCK,
+                                .l_whence = SEEK_SET,
+                        };
+
+                        r = fcntl(f->fd, F_OFD_SETLK, &fl);
+#else
+                        r = -1;
+                        errno = EINVAL;
+#endif
+                        if (r < 0 && errno == EINVAL)
+                                r = flock(f->fd, LOCK_EX|LOCK_NB);
+
+                        if (r >= 0)
+                                f->operation = LOCK_EX|LOCK_NB;
+                }
+
+                if ((f->operation & ~LOCK_NB) == LOCK_EX) {
+                        (void) unlinkat(f->dfd, f->path, 0);
+                }
+
+                g_free(f->path);
+                f->path = NULL;
+        }
+
+        (void) close (f->fd);
+        f->fd = -1;
+        f->operation = 0;
+}

glnx-lockfile.h

@@ -0,0 +1,41 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+#pragma once
+
+/***
+  This file is part of systemd.
+
+  Copyright 2011 Lennart Poettering
+  Copyright 2015 Colin Walters <walters@verbum.org>
+
+  systemd 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.
+
+  systemd 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 systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include "config.h"
+
+#include "glnx-backport-autoptr.h"
+
+typedef struct GLnxLockFile {
+        int dfd;
+        char *path;
+        int fd;
+        int operation;
+} GLnxLockFile;
+
+gboolean glnx_make_lock_file(int dfd, const char *p, int operation, GLnxLockFile *ret, GError **error);
+void glnx_release_lock_file(GLnxLockFile *f);
+
+G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(GLnxLockFile, glnx_release_lock_file)
+
+#define GLNX_LOCK_FILE_INIT { .fd = -1, .dfd = AT_FDCWD, .path = NULL }

libglnx.h

@@ -26,6 +26,7 @@ G_BEGIN_DECLS
 
 #include <glnx-local-alloc.h>
 #include <glnx-backport-autocleanups.h>
+#include <glnx-lockfile.h>
 #include <glnx-errors.h>
 #include <glnx-dirfd.h>
 #include <glnx-shutil.h>