manifest_xml.py

#
# Copyright (C) 2008 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

from __future__ import print_function
import itertools
import os
import re
import sys
import xml.dom.minidom

from pyversion import is_python3
if is_python3():
  import urllib.parse
else:
  import imp
  import urlparse
  urllib = imp.new_module('urllib')
  urllib.parse = urlparse

from git_config import GitConfig
from git_refs import R_HEADS, HEAD
from project import RemoteSpec, Project, MetaProject
from error import ManifestParseError

MANIFEST_FILE_NAME = 'manifest.xml'
LOCAL_MANIFEST_NAME = 'local_manifest.xml'
LOCAL_MANIFESTS_DIR_NAME = 'local_manifests'

urllib.parse.uses_relative.extend(['ssh', 'git'])
urllib.parse.uses_netloc.extend(['ssh', 'git'])

class _Default(object):
  """Project defaults within the manifest."""

  revisionExpr = None
  destBranchExpr = None
  remote = None
  sync_j = 1
  sync_c = False
  sync_s = False

class _XmlRemote(object):
  def __init__(self,
               name,
               alias=None,
               fetch=None,
               manifestUrl=None,
               review=None):
    self.name = name
    self.fetchUrl = fetch
    self.manifestUrl = manifestUrl
    self.remoteAlias = alias
    self.reviewUrl = review
    self.resolvedFetchUrl = self._resolveFetchUrl()

  def __eq__(self, other):
    return self.__dict__ == other.__dict__

  def __ne__(self, other):
    return self.__dict__ != other.__dict__

  def _resolveFetchUrl(self):
    url = self.fetchUrl.rstrip('/')
    manifestUrl = self.manifestUrl.rstrip('/')
    p = manifestUrl.startswith('persistent-http')
    if p:
      manifestUrl = manifestUrl[len('persistent-'):]

    # urljoin will get confused if there is no scheme in the base url
    # ie, if manifestUrl is of the form <hostname:port>
    if manifestUrl.find(':') != manifestUrl.find('/') - 1:
      manifestUrl = 'gopher://' + manifestUrl
    url = urllib.parse.urljoin(manifestUrl, url)
    url = re.sub(r'^gopher://', '', url)
    if p:
      url = 'persistent-' + url
    return url

  def ToRemoteSpec(self, projectName):
    url = self.resolvedFetchUrl.rstrip('/') + '/' + projectName
    remoteName = self.name
    if self.remoteAlias:
        remoteName = self.remoteAlias
    return RemoteSpec(remoteName, url, self.reviewUrl)

class XmlManifest(object):
  """manages the repo configuration file"""

  def __init__(self, repodir):
    self.repodir = os.path.abspath(repodir)
    self.topdir = os.path.dirname(self.repodir)
    self.manifestFile = os.path.join(self.repodir, MANIFEST_FILE_NAME)
    self.globalConfig = GitConfig.ForUser()
    self.localManifestWarning = False

    self.repoProject = MetaProject(self, 'repo',
      gitdir   = os.path.join(repodir, 'repo/.git'),
      worktree = os.path.join(repodir, 'repo'))

    self.manifestProject = MetaProject(self, 'manifests',
      gitdir   = os.path.join(repodir, 'manifests.git'),
      worktree = os.path.join(repodir, 'manifests'))

    self._Unload()

  def Override(self, name):
    """Use a different manifest, just for the current instantiation.
    """
    path = os.path.join(self.manifestProject.worktree, name)
    if not os.path.isfile(path):
      raise ManifestParseError('manifest %s not found' % name)

    old = self.manifestFile
    try:
      self.manifestFile = path
      self._Unload()
      self._Load()
    finally:
      self.manifestFile = old

  def Link(self, name):
    """Update the repo metadata to use a different manifest.
    """
    self.Override(name)

    try:
      if os.path.lexists(self.manifestFile):
        os.remove(self.manifestFile)
      os.symlink('manifests/%s' % name, self.manifestFile)
    except OSError as e:
      raise ManifestParseError('cannot link manifest %s: %s' % (name, str(e)))

  def _RemoteToXml(self, r, doc, root):
    e = doc.createElement('remote')
    root.appendChild(e)
    e.setAttribute('name', r.name)
    e.setAttribute('fetch', r.fetchUrl)
    if r.remoteAlias is not None:
      e.setAttribute('alias', r.remoteAlias)
    if r.reviewUrl is not None:
      e.setAttribute('review', r.reviewUrl)

  def Save(self, fd, peg_rev=False, peg_rev_upstream=True):
    """Write the current manifest out to the given file descriptor.
    """
    mp = self.manifestProject

    groups = mp.config.GetString('manifest.groups')
    if groups:
      groups = [x for x in re.split(r'[,\s]+', groups) if x]

    doc = xml.dom.minidom.Document()
    root = doc.createElement('manifest')
    doc.appendChild(root)

    # Save out the notice.  There's a little bit of work here to give it the
    # right whitespace, which assumes that the notice is automatically indented
    # by 4 by minidom.
    if self.notice:
      notice_element = root.appendChild(doc.createElement('notice'))
      notice_lines = self.notice.splitlines()
      indented_notice = ('\n'.join(" "*4 + line for line in notice_lines))[4:]
      notice_element.appendChild(doc.createTextNode(indented_notice))

    d = self.default

    for r in sorted(self.remotes):
      self._RemoteToXml(self.remotes[r], doc, root)
    if self.remotes:
      root.appendChild(doc.createTextNode(''))

    have_default = False
    e = doc.createElement('default')
    if d.remote:
      have_default = True
      e.setAttribute('remote', d.remote.name)
    if d.revisionExpr:
      have_default = True
      e.setAttribute('revision', d.revisionExpr)
    if d.sync_j > 1:
      have_default = True
      e.setAttribute('sync-j', '%d' % d.sync_j)
    if d.sync_c:
      have_default = True
      e.setAttribute('sync-c', 'true')
    if d.sync_s:
      have_default = True
      e.setAttribute('sync-s', 'true')
    if have_default:
      root.appendChild(e)
      root.appendChild(doc.createTextNode(''))

    if self._manifest_server:
      e = doc.createElement('manifest-server')
      e.setAttribute('url', self._manifest_server)
      root.appendChild(e)
      root.appendChild(doc.createTextNode(''))

    def output_projects(parent, parent_node, projects):
      for p in projects:
        output_project(parent, parent_node, self.projects[p])

    def output_project(parent, parent_node, p):
      if not p.MatchesGroups(groups):
        return

      name = p.name
      relpath = p.relpath
      if parent:
        name = self._UnjoinName(parent.name, name)
        relpath = self._UnjoinRelpath(parent.relpath, relpath)

      e = doc.createElement('project')
      parent_node.appendChild(e)
      e.setAttribute('name', name)
      if relpath != name:
        e.setAttribute('path', relpath)
      remoteName = d.remote.remoteAlias or d.remote.name
      if not d.remote or p.remote.name != remoteName:
        e.setAttribute('remote', p.remote.name)
      if peg_rev:
        if self.IsMirror:
          value = p.bare_git.rev_parse(p.revisionExpr + '^0')
        else:
          value = p.work_git.rev_parse(HEAD + '^0')
        e.setAttribute('revision', value)
        if peg_rev_upstream and value != p.revisionExpr:
          # Only save the origin if the origin is not a sha1, and the default
          # isn't our value, and the if the default doesn't already have that
          # covered.
          e.setAttribute('upstream', p.revisionExpr)
      elif not d.revisionExpr or p.revisionExpr != d.revisionExpr:
        e.setAttribute('revision', p.revisionExpr)

      for c in p.copyfiles:
        ce = doc.createElement('copyfile')
        ce.setAttribute('src', c.src)
        ce.setAttribute('dest', c.dest)
        e.appendChild(ce)

      default_groups = ['all', 'name:%s' % p.name, 'path:%s' % p.relpath]
      egroups = [g for g in p.groups if g not in default_groups]
      if egroups:
        e.setAttribute('groups', ','.join(egroups))

      for a in p.annotations:
        if a.keep == "true":
          ae = doc.createElement('annotation')
          ae.setAttribute('name', a.name)
          ae.setAttribute('value', a.value)
          e.appendChild(ae)

      if p.sync_c:
        e.setAttribute('sync-c', 'true')

      if p.sync_s:
        e.setAttribute('sync-s', 'true')

      if p.subprojects:
        sort_projects = list(sorted([subp.name for subp in p.subprojects]))
        output_projects(p, e, sort_projects)

    sort_projects = list(sorted([key for key, value in self.projects.items()
                     if not value.parent]))
    sort_projects.sort()
    output_projects(None, root, sort_projects)

    if self._repo_hooks_project:
      root.appendChild(doc.createTextNode(''))
      e = doc.createElement('repo-hooks')
      e.setAttribute('in-project', self._repo_hooks_project.name)
      e.setAttribute('enabled-list',
                     ' '.join(self._repo_hooks_project.enabled_repo_hooks))
      root.appendChild(e)

    doc.writexml(fd, '', '  ', '\n', 'UTF-8')

  @property
  def projects(self):
    self._Load()
    return self._projects

  @property
  def remotes(self):
    self._Load()
    return self._remotes

  @property
  def default(self):
    self._Load()
    return self._default

  @property
  def repo_hooks_project(self):
    self._Load()
    return self._repo_hooks_project

  @property
  def notice(self):
    self._Load()
    return self._notice

  @property
  def manifest_server(self):
    self._Load()
    return self._manifest_server

  @property
  def IsMirror(self):
    return self.manifestProject.config.GetBoolean('repo.mirror')

  def _Unload(self):
    self._loaded = False
    self._projects = {}
    self._remotes = {}
    self._default = None
    self._repo_hooks_project = None
    self._notice = None
    self.branch = None
    self._manifest_server = None

  def _Load(self):
    if not self._loaded:
      m = self.manifestProject
      b = m.GetBranch(m.CurrentBranch).merge
      if b is not None and b.startswith(R_HEADS):
        b = b[len(R_HEADS):]
      self.branch = b

      nodes = []
      nodes.append(self._ParseManifestXml(self.manifestFile,
                                          self.manifestProject.worktree))

      local = os.path.join(self.repodir, LOCAL_MANIFEST_NAME)
      if os.path.exists(local):
        if not self.localManifestWarning:
          self.localManifestWarning = True
          print('warning: %s is deprecated; put local manifests in `%s` instead'
                % (LOCAL_MANIFEST_NAME, os.path.join(self.repodir, LOCAL_MANIFESTS_DIR_NAME)),
                file=sys.stderr)
        nodes.append(self._ParseManifestXml(local, self.repodir))

      local_dir = os.path.abspath(os.path.join(self.repodir, LOCAL_MANIFESTS_DIR_NAME))
      try:
        for local_file in sorted(os.listdir(local_dir)):
          if local_file.endswith('.xml'):
            local = os.path.join(local_dir, local_file)
            nodes.append(self._ParseManifestXml(local, self.repodir))
      except OSError:
        pass

      try:
        self._ParseManifest(nodes)
      except ManifestParseError as e:
        # There was a problem parsing, unload ourselves in case they catch
        # this error and try again later, we will show the correct error
        self._Unload()
        raise e

      if self.IsMirror:
        self._AddMetaProjectMirror(self.repoProject)
        self._AddMetaProjectMirror(self.manifestProject)

      self._loaded = True

  def _ParseManifestXml(self, path, include_root):
    try:
      root = xml.dom.minidom.parse(path)
    except (OSError, xml.parsers.expat.ExpatError) as e:
      raise ManifestParseError("error parsing manifest %s: %s" % (path, e))

    if not root or not root.childNodes:
      raise ManifestParseError("no root node in %s" % (path,))

    for manifest in root.childNodes:
      if manifest.nodeName == 'manifest':
        break
    else:
      raise ManifestParseError("no <manifest> in %s" % (path,))

    nodes = []
    for node in manifest.childNodes:  # pylint:disable=W0631
                                      # We only get here if manifest is initialised
      if node.nodeName == 'include':
        name = self._reqatt(node, 'name')
        fp = os.path.join(include_root, name)
        if not os.path.isfile(fp):
          raise ManifestParseError("include %s doesn't exist or isn't a file"
              % (name,))
        try:
          nodes.extend(self._ParseManifestXml(fp, include_root))
        # should isolate this to the exact exception, but that's
        # tricky.  actual parsing implementation may vary.
        except (KeyboardInterrupt, RuntimeError, SystemExit):
          raise
        except Exception as e:
          raise ManifestParseError(
              "failed parsing included manifest %s: %s", (name, e))
      else:
        nodes.append(node)
    return nodes

  def _ParseManifest(self, node_list):
    for node in itertools.chain(*node_list):
      if node.nodeName == 'remote':
        remote = self._ParseRemote(node)
        if remote:
          if remote.name in self._remotes:
            if remote != self._remotes[remote.name]:
              raise ManifestParseError(
                  'remote %s already exists with different attributes' %
                  (remote.name))
          else:
            self._remotes[remote.name] = remote

    for node in itertools.chain(*node_list):
      if node.nodeName == 'default':
        if self._default is not None:
          raise ManifestParseError(
              'duplicate default in %s' %
              (self.manifestFile))
        self._default = self._ParseDefault(node)
    if self._default is None:
      self._default = _Default()

    for node in itertools.chain(*node_list):
      if node.nodeName == 'notice':
        if self._notice is not None:
          raise ManifestParseError(
              'duplicate notice in %s' %
              (self.manifestFile))
        self._notice = self._ParseNotice(node)

    for node in itertools.chain(*node_list):
      if node.nodeName == 'manifest-server':
        url = self._reqatt(node, 'url')
        if self._manifest_server is not None:
          raise ManifestParseError(
              'duplicate manifest-server in %s' %
              (self.manifestFile))
        self._manifest_server = url

    def recursively_add_projects(project):
      if self._projects.get(project.name):
        raise ManifestParseError(
            'duplicate project %s in %s' %
            (project.name, self.manifestFile))
      self._projects[project.name] = project
      for subproject in project.subprojects:
        recursively_add_projects(subproject)

    for node in itertools.chain(*node_list):
      if node.nodeName == 'project':
        project = self._ParseProject(node)
        recursively_add_projects(project)
      if node.nodeName == 'repo-hooks':
        # Get the name of the project and the (space-separated) list of enabled.
        repo_hooks_project = self._reqatt(node, 'in-project')
        enabled_repo_hooks = self._reqatt(node, 'enabled-list').split()

        # Only one project can be the hooks project
        if self._repo_hooks_project is not None:
          raise ManifestParseError(
              'duplicate repo-hooks in %s' %
              (self.manifestFile))

        # Store a reference to the Project.
        try:
          self._repo_hooks_project = self._projects[repo_hooks_project]
        except KeyError:
          raise ManifestParseError(
              'project %s not found for repo-hooks' %
              (repo_hooks_project))

        # Store the enabled hooks in the Project object.
        self._repo_hooks_project.enabled_repo_hooks = enabled_repo_hooks
      if node.nodeName == 'remove-project':
        name = self._reqatt(node, 'name')
        try:
          del self._projects[name]
        except KeyError:
          raise ManifestParseError('remove-project element specifies non-existent '
                                   'project: %s' % name)

        # If the manifest removes the hooks project, treat it as if it deleted
        # the repo-hooks element too.
        if self._repo_hooks_project and (self._repo_hooks_project.name == name):
          self._repo_hooks_project = None


  def _AddMetaProjectMirror(self, m):
    name = None
    m_url = m.GetRemote(m.remote.name).url
    if m_url.endswith('/.git'):
      raise ManifestParseError('refusing to mirror %s' % m_url)

    if self._default and self._default.remote:
      url = self._default.remote.resolvedFetchUrl
      if not url.endswith('/'):
        url += '/'
      if m_url.startswith(url):
        remote = self._default.remote
        name = m_url[len(url):]

    if name is None:
      s = m_url.rindex('/') + 1
      manifestUrl = self.manifestProject.config.GetString('remote.origin.url')
      remote = _XmlRemote('origin', fetch=m_url[:s], manifestUrl=manifestUrl)
      name = m_url[s:]

    if name.endswith('.git'):
      name = name[:-4]

    if name not in self._projects:
      m.PreSync()
      gitdir = os.path.join(self.topdir, '%s.git' % name)
      project = Project(manifest = self,
                        name = name,
                        remote = remote.ToRemoteSpec(name),
                        gitdir = gitdir,
                        worktree = None,
                        relpath = None,
                        revisionExpr = m.revisionExpr,
                        revisionId = None)
      self._projects[project.name] = project

  def _ParseRemote(self, node):
    """
    reads a <remote> element from the manifest file
    """
    name = self._reqatt(node, 'name')
    alias = node.getAttribute('alias')
    if alias == '':
      alias = None
    fetch = self._reqatt(node, 'fetch')
    review = node.getAttribute('review')
    if review == '':
      review = None
    manifestUrl = self.manifestProject.config.GetString('remote.origin.url')
    return _XmlRemote(name, alias, fetch, manifestUrl, review)

  def _ParseDefault(self, node):
    """
    reads a <default> element from the manifest file
    """
    d = _Default()
    d.remote = self._get_remote(node)
    d.revisionExpr = node.getAttribute('revision')
    if d.revisionExpr == '':
      d.revisionExpr = None

    d.destBranchExpr = node.getAttribute('dest-branch') or None

    sync_j = node.getAttribute('sync-j')
    if sync_j == '' or sync_j is None:
      d.sync_j = 1
    else:
      d.sync_j = int(sync_j)

    sync_c = node.getAttribute('sync-c')
    if not sync_c:
      d.sync_c = False
    else:
      d.sync_c = sync_c.lower() in ("yes", "true", "1")

    sync_s = node.getAttribute('sync-s')
    if not sync_s:
      d.sync_s = False
    else:
      d.sync_s = sync_s.lower() in ("yes", "true", "1")
    return d

  def _ParseNotice(self, node):
    """
    reads a <notice> element from the manifest file

    The <notice> element is distinct from other tags in the XML in that the
    data is conveyed between the start and end tag (it's not an empty-element
    tag).

    The white space (carriage returns, indentation) for the notice element is
    relevant and is parsed in a way that is based on how python docstrings work.
    In fact, the code is remarkably similar to here:
      http://www.python.org/dev/peps/pep-0257/
    """
    # Get the data out of the node...
    notice = node.childNodes[0].data

    # Figure out minimum indentation, skipping the first line (the same line
    # as the <notice> tag)...
    minIndent = sys.maxsize
    lines = notice.splitlines()
    for line in lines[1:]:
      lstrippedLine = line.lstrip()
      if lstrippedLine:
        indent = len(line) - len(lstrippedLine)
        minIndent = min(indent, minIndent)

    # Strip leading / trailing blank lines and also indentation.
    cleanLines = [lines[0].strip()]
    for line in lines[1:]:
      cleanLines.append(line[minIndent:].rstrip())

    # Clear completely blank lines from front and back...
    while cleanLines and not cleanLines[0]:
      del cleanLines[0]
    while cleanLines and not cleanLines[-1]:
      del cleanLines[-1]

    return '\n'.join(cleanLines)

  def _JoinName(self, parent_name, name):
    return os.path.join(parent_name, name)

  def _UnjoinName(self, parent_name, name):
    return os.path.relpath(name, parent_name)

  def _ParseProject(self, node, parent = None):
    """
    reads a <project> element from the manifest file
    """
    name = self._reqatt(node, 'name')
    if parent:
      name = self._JoinName(parent.name, name)

    remote = self._get_remote(node)
    if remote is None:
      remote = self._default.remote
    if remote is None:
      raise ManifestParseError("no remote for project %s within %s" %
            (name, self.manifestFile))

    revisionExpr = node.getAttribute('revision')
    if not revisionExpr:
      revisionExpr = self._default.revisionExpr
    if not revisionExpr:
      raise ManifestParseError("no revision for project %s within %s" %
            (name, self.manifestFile))

    path = node.getAttribute('path')
    if not path:
      path = name
    if path.startswith('/'):
      raise ManifestParseError("project %s path cannot be absolute in %s" %
            (name, self.manifestFile))

    rebase = node.getAttribute('rebase')
    if not rebase:
      rebase = True
    else:
      rebase = rebase.lower() in ("yes", "true", "1")

    sync_c = node.getAttribute('sync-c')
    if not sync_c:
      sync_c = False
    else:
      sync_c = sync_c.lower() in ("yes", "true", "1")

    sync_s = node.getAttribute('sync-s')
    if not sync_s:
      sync_s = self._default.sync_s
    else:
      sync_s = sync_s.lower() in ("yes", "true", "1")

    clone_depth = node.getAttribute('clone-depth')
    if clone_depth:
      try:
        clone_depth = int(clone_depth)
        if  clone_depth <= 0:
          raise ValueError()
      except ValueError:
        raise ManifestParseError('invalid clone-depth %s in %s' %
                                 (clone_depth, self.manifestFile))

    dest_branch = node.getAttribute('dest-branch') or self._default.destBranchExpr

    upstream = node.getAttribute('upstream')

    groups = ''
    if node.hasAttribute('groups'):
      groups = node.getAttribute('groups')
    groups = [x for x in re.split(r'[,\s]+', groups) if x]

    if parent is None:
      relpath, worktree, gitdir = self.GetProjectPaths(name, path)
    else:
      relpath, worktree, gitdir = self.GetSubprojectPaths(parent, path)

    default_groups = ['all', 'name:%s' % name, 'path:%s' % relpath]
    groups.extend(set(default_groups).difference(groups))

    if self.IsMirror and node.hasAttribute('force-path'):
      if node.getAttribute('force-path').lower() in ("yes", "true", "1"):
        gitdir = os.path.join(self.topdir, '%s.git' % path)

    project = Project(manifest = self,
                      name = name,
                      remote = remote.ToRemoteSpec(name),
                      gitdir = gitdir,
                      worktree = worktree,
                      relpath = relpath,
                      revisionExpr = revisionExpr,
                      revisionId = None,
                      rebase = rebase,
                      groups = groups,
                      sync_c = sync_c,
                      sync_s = sync_s,
                      clone_depth = clone_depth,
                      upstream = upstream,
                      parent = parent,
                      dest_branch = dest_branch)

    for n in node.childNodes:
      if n.nodeName == 'copyfile':
        self._ParseCopyFile(project, n)
      if n.nodeName == 'annotation':
        self._ParseAnnotation(project, n)
      if n.nodeName == 'project':
        project.subprojects.append(self._ParseProject(n, parent = project))

    return project

  def GetProjectPaths(self, name, path):
    relpath = path
    if self.IsMirror:
      worktree = None
      gitdir = os.path.join(self.topdir, '%s.git' % name)
    else:
      worktree = os.path.join(self.topdir, path).replace('\\', '/')
      gitdir = os.path.join(self.repodir, 'projects', '%s.git' % path)
    return relpath, worktree, gitdir

  def GetSubprojectName(self, parent, submodule_path):
    return os.path.join(parent.name, submodule_path)

  def _JoinRelpath(self, parent_relpath, relpath):
    return os.path.join(parent_relpath, relpath)

  def _UnjoinRelpath(self, parent_relpath, relpath):
    return os.path.relpath(relpath, parent_relpath)

  def GetSubprojectPaths(self, parent, path):
    relpath = self._JoinRelpath(parent.relpath, path)
    gitdir = os.path.join(parent.gitdir, 'subprojects', '%s.git' % path)
    if self.IsMirror:
      worktree = None
    else:
      worktree = os.path.join(parent.worktree, path).replace('\\', '/')
    return relpath, worktree, gitdir

  def _ParseCopyFile(self, project, node):
    src = self._reqatt(node, 'src')
    dest = self._reqatt(node, 'dest')
    if not self.IsMirror:
      # src is project relative;
      # dest is relative to the top of the tree
      project.AddCopyFile(src, dest, os.path.join(self.topdir, dest))

  def _ParseAnnotation(self, project, node):
    name = self._reqatt(node, 'name')
    value = self._reqatt(node, 'value')
    try:
      keep = self._reqatt(node, 'keep').lower()
    except ManifestParseError:
      keep = "true"
    if keep != "true" and keep != "false":
      raise ManifestParseError('optional "keep" attribute must be '
            '"true" or "false"')
    project.AddAnnotation(name, value, keep)

  def _get_remote(self, node):
    name = node.getAttribute('remote')
    if not name:
      return None

    v = self._remotes.get(name)
    if not v:
      raise ManifestParseError("remote %s not defined in %s" %
            (name, self.manifestFile))
    return v

  def _reqatt(self, node, attname):
    """
    reads a required attribute from the node.
    """
    v = node.getAttribute(attname)
    if not v:
      raise ManifestParseError("no %s in <%s> within %s" %
            (attname, node.nodeName, self.manifestFile))
    return v