Add sysroot.bootloader repo config key

The sysroot.bootloader key is used to configure the bootloader
that OSTree uses when deploying a sysroot. Having this key
allows specifying behavior not to use the default bootloader
backend code, which is preferable when creating a first
deployment from the sysroot (ostreedev#1774).

As of now, the key can take the values "auto" or "none". If
the key is not given, the value defaults to "auto".

"auto" causes _ostree_sysroot_query_bootloader() to be used
when writing a new deployment, which is the original behavior
that dynamically detects which bootloader to use.

"none" avoids querying the bootloader dynamically. The BLS
config fragments are still written to
sysroot/boot/loader/entries for use by higher-level software.

More values can be supported in future to specify a single
bootloader, different behavior for the bootloader code, or
a list of bootloaders to try.

Resolves: ostreedev#1774

src/libostree/ostree-repo-private.h

@@ -169,6 +169,7 @@ struct OstreeRepo {
   guint64 payload_link_threshold;
   gint fs_support_reflink; /* The underlying filesystem has support for ioctl (FICLONE..) */
   gchar **repo_finders;
+  gchar *bootloader; /* Configure which bootloader to use. */
 
   OstreeRepo *parent_repo;
 };

src/libostree/ostree-repo.c

@@ -3113,6 +3113,36 @@ reload_remote_config (OstreeRepo          *self,
   return TRUE;
 }
 
+static gboolean
+reload_sysroot_config (OstreeRepo          *self,
+                       GCancellable        *cancellable,
+                       GError             **error)
+{
+  { g_autofree char *bootloader = NULL;
+
+    if (!ot_keyfile_get_value_with_default (self->config, "sysroot",
+                                            "bootloader", "auto",
+                                            &bootloader, error))
+      return FALSE;
+
+    if (bootloader == NULL)
+      return glnx_throw (error, "Invalid empty bootloader configuration");
+
+    /* Can later add support for specifying a single bootloader e.g.
+     * "grub", "sd-boot", "rpi" here too. See:
+     * https://github.com/ostreedev/ostree/issues/1719
+     * https://github.com/ostreedev/ostree/issues/1801
+     */
+    if (strcmp (bootloader, "auto") != 0 &&
+        strcmp (bootloader, "none") != 0)
+      return glnx_throw (error, "Invalid configured bootloader '%s'", bootloader);
+
+    self->bootloader = g_steal_pointer (&bootloader);
+  }
+
+  return TRUE;
+}
+
 /**
  * ostree_repo_reload_config:
  * @self: repo
@@ -3131,6 +3161,8 @@ ostree_repo_reload_config (OstreeRepo          *self,
     return FALSE;
   if (!reload_remote_config (self, cancellable, error))
     return FALSE;
+  if (!reload_sysroot_config (self, cancellable, error))
+    return FALSE;
   return TRUE;
 }
 
@@ -6060,3 +6092,21 @@ ostree_repo_get_default_repo_finders (OstreeRepo *self)
 
   return (const gchar * const *)self->repo_finders;
 }
+
+/**
+ * ostree_repo_get_bootloader:
+ * @self: an #OstreeRepo
+ * 
+ * Get the bootloader configured. See the documentation for the
+ * "sysroot.bootloader" config key.
+ * 
+ * Returns: bootloader configuration for the sysroot
+ * Since: 2019.3
+ */
+const gchar *
+ostree_repo_get_bootloader (OstreeRepo   *self)
+{
+  g_return_val_if_fail (OSTREE_IS_REPO (self), NULL);
+
+  return self->bootloader;
+}

src/libostree/ostree-repo.h

@@ -115,6 +115,9 @@ gboolean      ostree_repo_set_collection_id (OstreeRepo   *self,
 _OSTREE_PUBLIC
 const gchar * const * ostree_repo_get_default_repo_finders (OstreeRepo   *self);
 
+_OSTREE_PUBLIC
+const gchar * ostree_repo_get_bootloader (OstreeRepo   *self);
+
 _OSTREE_PUBLIC
 GFile *       ostree_repo_get_path (OstreeRepo  *self);
 

src/libostree/ostree-sysroot-deploy.c

@@ -2077,6 +2077,7 @@ write_deployments_bootswap (OstreeSysroot     *self,
                                cancellable, error))
     return FALSE;
 
+  // TODO: move this to ostree_sysroot_write_deployments_with_options?
   /* Need the repo to try and extract the versions for deployments.
    * But this is a "nice-to-have" for the bootloader UI, so failure
    * here is not fatal to the whole operation.  We just gracefully
@@ -2342,9 +2343,23 @@ ostree_sysroot_write_deployments_with_options (OstreeSysroot     *self,
             return glnx_throw_errno_prefix (error, "Remounting /boot read-write");
         }
 
-      if (!_ostree_sysroot_query_bootloader (self, &bootloader, cancellable, error))
-        return FALSE;
-      bootloader_is_atomic = bootloader != NULL && _ostree_bootloader_is_atomic (bootloader);
+      g_autoptr(OstreeRepo) repo = NULL;
+      if (!ostree_sysroot_get_repo (self, &repo, cancellable, error))
+        {
+          return glnx_throw (error, "Getting repo config");
+        }
+
+      g_autofree const char *bootloader_config =
+        ostree_repo_get_bootloader (repo);
+
+      g_debug ("Using bootloader configuration: %s", bootloader_config);
+
+      if (g_strcmp0 (bootloader_config, "auto") == 0)
+        {
+          if (!_ostree_sysroot_query_bootloader (self, &bootloader, cancellable, error))
+            return FALSE;
+          bootloader_is_atomic = bootloader != NULL && _ostree_bootloader_is_atomic (bootloader);
+        }
 
       /* Note equivalent of try/finally here */
       gboolean success = write_deployments_bootswap (self, new_deployments, opts, bootloader,