Add an API+CLI to inject metadata for bootable OSTree commits

I was doing some rpm-ostree work and I wanted to compare two
OSTree commits to see if the kernel has changed.  I think
this should be a lot more natural.

Add `ostree commit --bootable` which calls into a new generic
library API `ostree_commit_metadata_for_bootable()` that
discovers the kernel version and injects it as an `ostree.linux`
metadata key.  And for extra clarity, add an `ostree.bootable`
key.

It's interesting because the "core" OSTree layer is all about
generic files, but this is adding special APIs around bootable
OSTree commits (as opposed to e.g. flatpak as well as
things like rpm-ostree's pkgcache refs).

Eventually, I'd like to ensure everyone is using this and
hard require this metadata key for the `ostree admin deploy`
flow - mainly to prevent accidents.

src/libostree/libostree-devel.sym

@@ -22,6 +22,11 @@
    - uncomment the include in Makefile-libostree.am
 */
 
+LIBOSTREE_2021.1 {
+global:
+  ostree_commit_metadata_for_bootable;
+} LIBOSTREE_2020.8;
+
 /* Stub section for the stable release *after* this development one; don't
  * edit this other than to update the year.  This is just a copy/paste
  * source.  Replace $LASTSTABLE with the last stable version, and $NEWVERSION

src/libostree/ostree-repo-os.c

@@ -0,0 +1,83 @@
+/*
+ * SPDX-License-Identifier: LGPL-2.0+
+ *
+ * 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 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, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#include "config.h"
+
+#include <sys/types.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <gio/gfiledescriptorbased.h>
+#include <gio/gunixinputstream.h>
+#include "libglnx.h"
+#include "ostree.h"
+#include "ostree-core-private.h"
+#include "ostree-repo-os.h"
+#include "otutil.h"
+
+/**
+ * ostree_commit_metadata_for_bootable:
+ * @root: Root filesystem to be committed
+ * @dict: Dictionary to update
+ *
+ * Update provided @dict with standard metadata for bootable OSTree commits.
+ * Since: 2021.1
+ */
+_OSTREE_PUBLIC
+gboolean
+ostree_commit_metadata_for_bootable (GFile *root, GVariantDict *dict, GCancellable *cancellable, GError **error)
+{
+  g_autoptr(GFile) modules = g_file_resolve_relative_path (root, "usr/lib/modules");
+  g_autoptr(GFileEnumerator) dir_enum 
+    = g_file_enumerate_children (modules, OSTREE_GIO_FAST_QUERYINFO,
+                                 G_FILE_QUERY_INFO_NOFOLLOW_SYMLINKS,
+                                 cancellable, error);
+  if (!dir_enum)
+    return glnx_prefix_error (error, "Opening usr/lib/modules");
+
+  g_autofree char *linux_release = NULL;
+  while (TRUE)
+    {
+      GFileInfo *child_info;
+      GFile *child_path;
+      if (!g_file_enumerator_iterate (dir_enum, &child_info, &child_path,
+                                      cancellable, error))
+        return FALSE;
+      if (child_info == NULL)
+        break;
+      if (g_file_info_get_file_type (child_info) != G_FILE_TYPE_DIRECTORY)
+        continue;
+    
+      g_autoptr(GFile) kernel_path = g_file_resolve_relative_path (child_path, "vmlinuz");
+      if (!g_file_query_exists (kernel_path, NULL))
+        continue;
+
+      if (linux_release != NULL)
+        return glnx_throw (error, "Multiple kernels found in /usr/lib/modules");
+
+      linux_release = g_strdup (g_file_info_get_name (child_info));
+    }
+
+  if (linux_release)
+    {
+      g_variant_dict_insert (dict, OSTREE_METADATA_KEY_BOOTABLE, "b", TRUE);
+      g_variant_dict_insert (dict, OSTREE_METADATA_KEY_LINUX, "s", linux_release);
+      return TRUE;
+    }
+  return glnx_throw (error, "No kernel found in /usr/lib/modules");
+}

src/libostree/ostree-repo-os.h

@@ -0,0 +1,47 @@
+/*
+ * SPDX-License-Identifier: LGPL-2.0+
+ *
+ * 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 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, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#pragma once
+
+#include <sys/stat.h>
+#include <gio/gio.h>
+#include <ostree-types.h>
+
+G_BEGIN_DECLS
+
+/** 
+ * OSTREE_METADATA_KEY_BOOTABLE:
+ *
+ * GVariant type `b`: Set if this commit is intended to be bootable
+ * Since: 2021.1
+ */
+#define OSTREE_METADATA_KEY_BOOTABLE "ostree.bootable"
+/** 
+ * OSTREE_METADATA_KEY_LINUX:
+ *
+ * GVariant type `s`: Contains the Linux kernel release (i.e. `uname -r`)
+ * Since: 2021.1
+ */
+#define OSTREE_METADATA_KEY_LINUX "ostree.linux"
+
+_OSTREE_PUBLIC
+gboolean
+ostree_commit_metadata_for_bootable (GFile *root, GVariantDict *dict, GCancellable *cancellable, GError **error);
+
+G_END_DECLS

src/libostree/ostree.h

@@ -24,6 +24,7 @@
 #include <ostree-async-progress.h>
 #include <ostree-core.h>
 #include <ostree-repo.h>
+#include <ostree-repo-os.h>
 #include <ostree-mutable-tree.h>
 #include <ostree-remote.h>
 #include <ostree-repo-file.h>

src/ostree/ot-builtin-commit.c

@@ -35,6 +35,7 @@
 
 static char *opt_subject;
 static char *opt_body;
+static char *opt_bootable;
 static char *opt_body_file;
 static gboolean opt_editor;
 static char *opt_parent;
@@ -112,6 +113,7 @@ static GOptionEntry options[] = {
   { "owner-uid", 0, 0, G_OPTION_ARG_INT, &opt_owner_uid, "Set file ownership user id", "UID" },
   { "owner-gid", 0, 0, G_OPTION_ARG_INT, &opt_owner_gid, "Set file ownership group id", "GID" },
   { "canonical-permissions", 0, 0, G_OPTION_ARG_NONE, &opt_canonical_permissions, "Canonicalize permissions in the same way bare-user does for hardlinked files", NULL },
+  { "bootable", 0, 0, G_OPTION_ARG_NONE, &opt_bootable, "Flag this commit as a bootable OSTree (e.g. contains a Linux kernel)", NULL },
   { "mode-ro-executables", 0, 0, G_OPTION_ARG_NONE, &opt_ro_executables, "Ensure executable files are not writable", NULL },
   { "no-xattrs", 0, 0, G_OPTION_ARG_NONE, &opt_no_xattrs, "Do not import extended attributes", NULL },
   { "selinux-policy", 0, 0, G_OPTION_ARG_FILENAME, &opt_selinux_policy, "Set SELinux labels based on policy in root filesystem PATH (may be /)", "PATH" },
@@ -501,7 +503,7 @@ ostree_builtin_commit (int argc, char **argv, OstreeCommandInvocation *invocatio
       goto out;
     }
 
-  if (opt_metadata_strings || opt_metadata_variants || opt_metadata_keep)
+  if (opt_metadata_strings || opt_metadata_variants || opt_metadata_keep || opt_bootable)
     {
       g_autoptr(GVariantBuilder) builder =
         g_variant_builder_new (G_VARIANT_TYPE ("a{sv}"));
@@ -841,6 +843,17 @@ ostree_builtin_commit (int argc, char **argv, OstreeCommandInvocation *invocatio
           fill_bindings (repo, old_metadata, &metadata);
         }
 
+      if (opt_bootable)
+        {
+          g_autoptr(GVariant) old_metadata = g_steal_pointer (&metadata);
+          g_auto(GVariantDict) bootmeta;
+          g_variant_dict_init (&bootmeta, old_metadata);
+          if (!ostree_commit_metadata_for_bootable (root, &bootmeta, cancellable, error))
+            goto out;
+
+          metadata = g_variant_ref_sink (g_variant_dict_end (&bootmeta));
+        }
+
       if (!opt_timestamp)
         {
           if (!ostree_repo_write_commit (repo, parent, opt_subject, commit_body, metadata,