Add initramfs support to write extensions that produce disks

If INITRAMFS_PATH is specified and the file exists, then the produced
kernel command line will use root=UUID=$uuid_of_created_disk rather than
root=/dev/sda, which may be incorrect.

Help files have been updated to mention the new option.

This leads to an unfortunate duplication of the path to the initramfs,
in both the location field of the nested deployment and the
INITRAMFS_PATH of the disk image creation.

However, an initramfs could be produced by a chunk and put in the same
place, so it doesn't make sense to couple the rawdisk and initramfs
write extensions to remove this duplication.

Similarly, there may be multiple valid initramfs in the rootfs e.g.
extlinux loads a hypervisor, which is Linux + initramfs, and the
initramfs then boots a guest Linux system, which uses a different
initramfs.

This makes it important to explicitly let the rootfs write extensions
know which to use, or not as the case may be.

util-linux's blkid is required, since the busybox version ignores the
options to filter its output, and parsing the output is undesirable.

Because syslinux's btrfs subvolume support is limited to being able to
use a non-0 default subvolume, the initramfs has to be copied out of
the run-time rootfs subvolume and into the boot subvolume.

This pushed the required disk space of a minimal system over the 512M
threshold because we do not have the userland tooling support to be able
to do a btrfs file contents clone.

1 files changed, 54 insertions, 8 deletions

diff --git a/writeexts.py b/writeexts.py
index b4912db1..334dc15c 100644
--- a/writeexts.py
+++ b/writeexts.py
@@ -120,7 +120,8 @@ class WriteExtension(cliapp.Application):
             raise
         try:
             self.create_btrfs_system_layout(
-                temp_root, mp, version_label='factory')
+                temp_root, mp, version_label='factory',
+                disk_uuid=self.get_uuid(raw_disk))
         except BaseException, e:
             sys.stderr.write('Error creating Btrfs system layout')
             self.unmount(mp)
@@ -186,6 +187,13 @@ class WriteExtension(cliapp.Application):
         '''Create a btrfs filesystem on the disk.'''
         self.status(msg='Creating btrfs filesystem')
         cliapp.runcmd(['mkfs.btrfs', '-L', 'baserock', location])
+
+    def get_uuid(self, location):
+        '''Get the UUID of a block device's file system.'''
+        # Requires util-linux blkid; busybox one ignores options and
+        # lies by exiting successfully.
+        return cliapp.runcmd(['blkid', '-s', 'UUID', '-o', 'value',
+                              location]).strip()
         
     def mount(self, location):
         '''Mount the filesystem so it can be tweaked.
@@ -212,10 +220,12 @@ class WriteExtension(cliapp.Application):
         cliapp.runcmd(['umount', mount_point])
         os.rmdir(mount_point)
 
-    def create_btrfs_system_layout(self, temp_root, mountpoint, version_label):
+    def create_btrfs_system_layout(self, temp_root, mountpoint, version_label,
+                                   disk_uuid=None):
         '''Separate base OS versions from state using subvolumes.
 
         '''
+        initramfs = self.find_initramfs(temp_root)
         version_root = os.path.join(mountpoint, 'systems', version_label)
         state_root = os.path.join(mountpoint, 'state')
 
@@ -238,7 +248,12 @@ class WriteExtension(cliapp.Application):
         if self.bootloader_is_wanted():
             self.install_kernel(version_root, temp_root)
             self.install_syslinux_menu(mountpoint, version_root)
-            self.install_extlinux(mountpoint)
+            if initramfs is not None:
+                self.install_initramfs(initramfs, version_root)
+                self.install_extlinux(mountpoint, disk_uuid)
+            else:
+                self.install_extlinux(mountpoint)
+
 
     def create_orig(self, version_root, temp_root):
         '''Create the default "factory" system.'''
@@ -322,6 +337,29 @@ class WriteExtension(cliapp.Application):
         fstab.write()
         return state_dirs_to_create
 
+    def find_initramfs(self, temp_root):
+        '''Check whether the rootfs has an initramfs.
+
+        Uses the INITRAMFS_PATH option to locate it.
+        '''
+        if 'INITRAMFS_PATH' in os.environ:
+            initramfs = os.path.join(temp_root, os.environ['INITRAMFS_PATH'])
+            if not os.path.exists(initramfs):
+                raise morphlib.Error('INITRAMFS_PATH specified, '
+                                     'but file does not exist')
+            return initramfs
+        return None
+
+    def install_initramfs(self, initramfs_path, version_root):
+        '''Install the initramfs outside of 'orig' or 'run' subvolumes.
+
+        This is required because syslinux doesn't traverse subvolumes when
+        loading the kernel or initramfs.
+        '''
+        self.status(msg='Installing initramfs')
+        initramfs_dest = os.path.join(version_root, 'initramfs')
+        cliapp.runcmd(['cp', '-a', initramfs_path, initramfs_dest])
+
     def install_kernel(self, version_root, temp_root):
         '''Install the kernel outside of 'orig' or 'run' subvolumes'''
 
@@ -337,20 +375,28 @@ class WriteExtension(cliapp.Application):
     def get_extra_kernel_args(self):
         return os.environ.get('KERNEL_ARGS', '')
 
-    def install_extlinux(self, real_root):
+    def install_extlinux(self, real_root, disk_uuid=None):
         '''Install extlinux on the newly created disk image.'''
 
         self.status(msg='Creating extlinux.conf')
         config = os.path.join(real_root, 'extlinux.conf')
-        kernel_args = self.get_extra_kernel_args()
+        kernel_args = (
+            'rw ' # ro ought to work, but we don't test that regularly
+            'init=/sbin/init ' # default, but it doesn't hurt to be explicit
+            'rootfstype=btrfs ' # required when using initramfs, also boots
+                                # faster when specified without initramfs
+            'rootflags=subvol=systems/default/run ') # boot runtime subvol
+        kernel_args += 'root=%s ' % ('/dev/sda' if disk_uuid is None
+                                     else 'UUID=%s' % disk_uuid)
+        kernel_args += self.get_extra_kernel_args()
         with open(config, 'w') as f:
             f.write('default linux\n')
             f.write('timeout 1\n')
             f.write('label linux\n')
             f.write('kernel /systems/default/kernel\n')
-            f.write('append root=/dev/sda '
-                    'rootflags=subvol=systems/default/run '
-                    '%s init=/sbin/init rw\n' % (kernel_args))
+            if disk_uuid is not None:
+                f.write('initrd /systems/default/initramfs\n')
+            f.write('append %s\n' % kernel_args)
 
         self.status(msg='Installing extlinux')
         cliapp.runcmd(['extlinux', '--install', real_root])