fixup! daemon: Make "diff" methods safer

Also hold the writer lock when pulling remote "/usr/share/rpm" for
package diffs.

src/daemon/rpmostreed-sysroot.c

@@ -62,8 +62,9 @@ struct _RpmostreedSysroot {
 
   /* The OS interface's various diff methods can run concurrently with
    * transactions, which is safe except when the transaction is writing
-   * new deployments to disk.  Protect these critical sections with the
-   * writer lock, and have the diff methods hold reader locks. */
+   * new deployments to disk or downloading RPM package details.  The
+   * writer lock protects these critical sections so the diff methods
+   * (holding reader locks) can run safely. */
   GRWLock method_rw_lock;
 
   GFileMonitor *monitor;

src/daemon/rpmostreed-transaction-types.c

@@ -95,6 +95,31 @@ safe_sysroot_upgrader_deploy (OstreeSysrootUpgrader *upgrader,
   return success;
 }
 
+static gboolean
+safe_sysroot_upgrader_pull_package_diff (OstreeSysrootUpgrader *upgrader,
+                                         OstreeAsyncProgress *progress,
+                                         gboolean *out_changed,
+                                         GCancellable *cancellable,
+                                         GError **error)
+{
+  RpmostreedSysroot *global_sysroot = rpmostreed_sysroot_get ();
+  gboolean success;
+
+  rpmostreed_sysroot_writer_lock (global_sysroot);
+
+  success = ostree_sysroot_upgrader_pull_one_dir (upgrader,
+                                                  "/usr/share/rpm",
+                                                  0, 0,
+                                                  progress,
+                                                  out_changed,
+                                                  cancellable,
+                                                  error);
+
+  rpmostreed_sysroot_writer_unlock (global_sysroot);
+
+  return success;
+}
+
 static gboolean
 safe_sysroot_write_deployments (OstreeSysroot *sysroot,
                                 GPtrArray *deployments,
@@ -192,9 +217,9 @@ package_diff_transaction_execute (RpmostreedTransaction *transaction,
   progress = ostree_async_progress_new ();
   rpmostreed_transaction_connect_download_progress (transaction, progress);
   rpmostreed_transaction_connect_signature_progress (transaction, repo);
-  if (!ostree_sysroot_upgrader_pull_one_dir (upgrader, "/usr/share/rpm",
-                                             0, 0, progress, &changed,
-                                             cancellable, error))
+
+  if (!safe_sysroot_upgrader_pull_package_diff (upgrader, progress, &changed,
+                                                cancellable, error))
     goto out;
 
   rpmostree_transaction_emit_progress_end (RPMOSTREE_TRANSACTION (transaction));