sysroot: Add an API to lock

If a system administrator happens to type `ostree admin upgrade`
multiple times, currently that will lead to a potentially corrupted
system.

I originally attempted to do locking *internally* in `libostree`, but
that didn't work out because currently a number of the commands
perform multi-step operations that all need to be serialized.  All of
the current code in `ostree admin deploy` is an example.

Therefore, allow callers to perform locking, as most of the higher
level logic is presently implemented there.

At some point, we can revisit having internal locking, but it will be
difficult.  A more likely approach would be similar to Java's approach
with concurrency on iterators - a "fail fast" method.

src/libostree/ostree-sysroot-private.h

@@ -20,6 +20,7 @@
 
 #pragma once
 
+#include "libglnx.h"
 #include "ostree.h"
 #include "ostree-kernel-args.h"
 #include "ostree-bootloader.h"
@@ -31,6 +32,7 @@ struct OstreeSysroot {
 
   GFile *path;
   int sysroot_fd;
+  GLnxLockFile lock;
 
   gboolean loaded;
   
@@ -43,8 +45,11 @@ struct OstreeSysroot {
 
   /* Only access through ostree_sysroot_get_repo() */
   OstreeRepo *repo;
+
 };
 
+#define OSTREE_SYSROOT_LOCKFILE "ostree/lock"
+
 gboolean
 _ostree_sysroot_read_boot_loader_configs (OstreeSysroot *self,
                                           int            bootversion,

src/libostree/ostree-sysroot.c

@@ -44,6 +44,10 @@ find_booted_deployment (OstreeSysroot       *self,
  * which in particular should contain a toplevel /ostree directory.
  * Inside this directory is an #OstreeRepo in /ostree/repo, plus a set
  * of deployments in /ostree/deploy.
+ *
+ * This class is not by default safe against concurrent use by threads
+ * or external processes.  You can use ostree_sysroot_lock() to
+ * perform locking externally.
  */
 typedef struct {
   GObjectClass parent_class;
@@ -66,6 +70,8 @@ ostree_sysroot_finalize (GObject *object)
   g_clear_object (&self->sepolicy);
   g_clear_object (&self->repo);
 
+  glnx_release_lock_file (&self->lock);
+
   if (self->sysroot_fd != -1)
     (void) close (self->sysroot_fd);
 
@@ -148,6 +154,7 @@ static void
 ostree_sysroot_init (OstreeSysroot *self)
 {
   self->sysroot_fd = -1;
+  self->lock = (GLnxLockFile)GLNX_LOCK_FILE_INIT;
 }
 
 /**
@@ -1142,6 +1149,43 @@ ostree_sysroot_origin_new_from_refspec (OstreeSysroot  *sysroot,
   return ret;
 }
 
+/**
+ * ostree_sysroot_lock:
+ * @self: Self
+ * @error: Error
+ *
+ * Acquire an exclusive multi-process write lock for @self.  This call
+ * blocks until the lock has been acquired.  The lock is not
+ * reentrant.
+ *
+ * Release the lock with ostree_sysroot_unlock().  The lock will also
+ * be released if @self is deallocated.
+ */
+gboolean
+ostree_sysroot_lock (OstreeSysroot     *self,
+                     GError           **error)
+{
+  if (!ensure_sysroot_fd (self, error))
+    return FALSE;
+  return glnx_make_lock_file (self->sysroot_fd, OSTREE_SYSROOT_LOCKFILE,
+                              LOCK_EX, &self->lock, error);
+}
+
+/**
+ * ostree_sysroot_unlock:
+ * @self: Self
+ * @error: Error
+ *
+ * Clear the lock previously acquired with ostree_sysroot_lock().  It
+ * is safe to call this function if the lock has not been previously
+ * acquired.
+ */
+void
+ostree_sysroot_unlock (OstreeSysroot  *self)
+{
+  glnx_release_lock_file (&self->lock);
+}
+
 /**
  * ostree_sysroot_simple_write_deployment:
  * @sysroot: Sysroot

src/libostree/ostree-sysroot.h

@@ -62,6 +62,9 @@ char *ostree_sysroot_get_deployment_dirpath (OstreeSysroot    *self,
 
 GFile * ostree_sysroot_get_deployment_origin_path (GFile   *deployment_path);
 
+gboolean ostree_sysroot_lock (OstreeSysroot  *self, GError **error);
+void ostree_sysroot_unlock (OstreeSysroot  *self);
+
 gboolean ostree_sysroot_cleanup (OstreeSysroot       *self,
                                  GCancellable        *cancellable,
                                  GError             **error);

src/ostree/ot-admin-builtin-deploy.c

@@ -79,6 +79,9 @@ ot_admin_builtin_deploy (int argc, char **argv, GCancellable *cancellable, GErro
 
   refspec = argv[1];
 
+  if (!ostree_sysroot_lock (sysroot, error))
+    goto out;
+
   if (!ostree_sysroot_load (sysroot, cancellable, error))
     goto out;
 
@@ -172,6 +175,8 @@ ot_admin_builtin_deploy (int argc, char **argv, GCancellable *cancellable, GErro
 
   ret = TRUE;
  out:
+  if (sysroot)
+    ostree_sysroot_unlock (sysroot);
   if (origin)
     g_key_file_unref (origin);
   if (context)