Fix bridge issue with Linux >= 2.6.39 and bridge TAP-only interfaces.

In order to  be able to check  the uniqueness of IPv6 address  assigned to the
bridge,  the  network  interface  must  be up  from  an  administrative  *and*
operational point of view.

However, from  Linux 2.6.39, the bridge  reflects the state  of the underlying
device (e.g.   the bridge  asserts carrier if  at least  one of its  ports has
carrier) whereas it always asserted  carrier before. This should work fine for
"real" network interface, but will not  work properly if the bridge only binds
TAP  interfaces, which,  from  2.6.36, reports  carrier  only and  only if  an
userspace program is attached.

slapos/format/__init__.py

@@ -314,53 +314,66 @@ class Computer:
       os.chown(self.software_root, slapsoft_pw.pw_uid, slapsoft_pw.pw_gid)
     os.chmod(self.software_root, 0755)
 
-    for partition in self.partition_list:
-      # Reconstructing User's
-      partition.path = os.path.join(self.instance_root, partition.reference)
-      partition.user.setPath(partition.path)
-      partition.user.additional_group_list = [slapsoft.name]
-      if alter_user:
-        partition.user.create()
-
-      # Reconstructing Tap
-      if partition.user and partition.user.isAvailable():
-        owner = partition.user
-      else:
-        owner = User('root')
-
-      if alter_network:
-        partition.tap.createWithOwner(owner)
-        self.bridge.addTap(partition.tap)
-
-      # Reconstructing partition's directory
-      partition.createPath(alter_user)
-
-      # Reconstructing partition's address
-      # There should be two addresses on each Computer Partition:
-      #  * global IPv6
-      #  * local IPv4, took from slapformat:ipv4_local_network
-      if len(partition.address_list) == 0:
-        # regenerate
-        partition.address_list.append(self.bridge.addIPv4LocalAddress())
-        partition.address_list.append(self.bridge.addAddr())
-      elif alter_network:
-        # regenerate list of addresses
-        old_partition_address_list = partition.address_list
-        partition.address_list = []
-        if len(old_partition_address_list) != 2:
-          raise ValueError('There should be exactly 2 stored addresses')
-        if not any([netaddr.valid_ipv6(q['addr']) for q in old_partition_address_list]):
-          raise ValueError('Not valid ipv6 addresses loaded')
-        if not any([netaddr.valid_ipv4(q['addr']) for q in old_partition_address_list]):
-          raise ValueError('Not valid ipv6 addresses loaded')
-        for address in old_partition_address_list:
-          if netaddr.valid_ipv6(address['addr']):
-            partition.address_list.append(self.bridge.addAddr(address['addr'],
-              address['netmask']))
-          elif netaddr.valid_ipv4(address['addr']):
-            partition.address_list.append(self.bridge.addIPv4LocalAddress(address['addr']))
+    try:
+      for partition_index, partition in enumerate(self.partition_list):
+        # Reconstructing User's
+        partition.path = os.path.join(self.instance_root, partition.reference)
+        partition.user.setPath(partition.path)
+        partition.user.additional_group_list = [slapsoft.name]
+        if alter_user:
+          partition.user.create()
+
+        # Reconstructing Tap
+        if partition.user and partition.user.isAvailable():
+          owner = partition.user
+        else:
+          owner = User('root')
+
+        if alter_network:
+          # In case it has to be  attached to the TAP network device, only one
+          # is necessary for the bridge to assert carrier
+          if self.bridge.attach_to_tap and partition_index == 0:
+            partition.tap.createWithOwner(owner, attach_to_tap=True)
           else:
-            raise ValueError('Address %r is incorrect' % address['addr'])
+            partition.tap.createWithOwner(owner)
+
+          self.bridge.addTap(partition.tap)
+
+        # Reconstructing partition's directory
+        partition.createPath(alter_user)
+
+        # Reconstructing partition's address
+        # There should be two addresses on each Computer Partition:
+        #  * global IPv6
+        #  * local IPv4, took from slapformat:ipv4_local_network
+        if len(partition.address_list) == 0:
+          # regenerate
+          partition.address_list.append(self.bridge.addIPv4LocalAddress())
+          partition.address_list.append(self.bridge.addAddr())
+        elif alter_network:
+          # regenerate list of addresses
+          old_partition_address_list = partition.address_list
+          partition.address_list = []
+          if len(old_partition_address_list) != 2:
+            raise ValueError('There should be exactly 2 stored addresses')
+          if not any([netaddr.valid_ipv6(q['addr']) for q in old_partition_address_list]):
+            raise ValueError('Not valid ipv6 addresses loaded')
+          if not any([netaddr.valid_ipv4(q['addr']) for q in old_partition_address_list]):
+            raise ValueError('Not valid ipv6 addresses loaded')
+          for address in old_partition_address_list:
+            if netaddr.valid_ipv6(address['addr']):
+              partition.address_list.append(self.bridge.addAddr(address['addr'],
+                                                                address['netmask']))
+            elif netaddr.valid_ipv4(address['addr']):
+              partition.address_list.append(self.bridge.addIPv4LocalAddress(address['addr']))
+            else:
+              raise ValueError('Address %r is incorrect' % address['addr'])
+    finally:
+      if self.bridge.attach_to_tap:
+        try:
+          self.partition_list[0].tap.detach()
+        except IndexError:
+          pass
 
 class Partition:
   "Represent a computer partition"
@@ -461,8 +474,16 @@ class User:
     except KeyError:
       return False
 
+import struct
+import fcntl
+import errno
+import threading
+
 class Tap:
   "Tap represent a tap interface on the system"
+  IFF_TAP = 0x0002
+  TUNSETIFF = 0x400454ca
+  KEEP_TAP_ATTACHED_EVENT = threading.Event()
 
   def __init__(self, tap_name):
     """
@@ -476,7 +497,58 @@ class Tap:
   def __getinitargs__(self):
     return (self.name,)
 
-  def createWithOwner(self,owner):
+  def attach(self):
+    """
+    Attach to the TAP interface, meaning  that it just opens the TAP interface
+    and wait for the caller to notify that it can be safely detached.
+
+    Linux  distinguishes administrative  and operational  state of  an network
+    interface.  The  former can be set  manually by running ``ip  link set dev
+    <dev> up|down'', whereas the latter states that the interface can actually
+    transmit  data (for  a wired  network interface,  it basically  means that
+    there is  carrier, e.g.  the network  cable is plugged  into a  switch for
+    example).
+
+    In order  to be able to check  the uniqueness of IPv6  address assigned to
+    the bridge, the network interface  must be up from an administrative *and*
+    operational point of view.
+
+    However,  from  Linux  2.6.39,  the  bridge  reflects  the  state  of  the
+    underlying device (e.g.  the bridge asserts carrier if at least one of its
+    ports has carrier) whereas it  always asserted carrier before. This should
+    work fine for "real" network interface,  but will not work properly if the
+    bridge only binds TAP interfaces, which, from 2.6.36, reports carrier only
+    and only if an userspace program is attached.
+    """
+    tap_fd = os.open("/dev/net/tun", os.O_RDWR)
+
+    try:
+      # Attach to the TAP interface which has previously been created
+      fcntl.ioctl(tap_fd, self.TUNSETIFF,
+                  struct.pack("16sI", self.name, self.IFF_TAP))
+
+    except IOError, error:
+      # If  EBUSY, it  means another  program is  already attached,  thus just
+      # ignore it...
+      if error.errno != errno.EBUSY:
+        os.close(tap_fd)
+        raise
+    else:
+      # Block until the  caller send an event stating that  the program can be
+      # now detached safely,  thus bringing down the TAP  device (from 2.6.36)
+      # and the bridge at the same time (from 2.6.39)
+      self.KEEP_TAP_ATTACHED_EVENT.wait()
+    finally:
+      os.close(tap_fd)
+
+  def detach(self):
+    """
+    Detach to the  TAP network interface by notifying  the thread which attach
+    to the TAP and closing the TAP file descriptor
+    """
+    self.KEEP_TAP_ATTACHED_EVENT.set()
+
+  def createWithOwner(self, owner, attach_to_tap=False):
     """
     Create a tap interface on the system.
 
@@ -497,6 +569,9 @@ class Tap:
       callAndRead(['tunctl', '-t', self.name, '-u', owner.name])
     callAndRead(['ip', 'link', 'set', self.name, 'up'])
 
+    if attach_to_tap:
+      threading.Thread(target=self.attach).start()
+
     return True
 
 class Bridge:
@@ -511,6 +586,11 @@ class Bridge:
     self.name = str(name)
     self.ipv4_local_network = ipv4_local_network
 
+    # Attach to TAP  network interface, only if the  bridge interface does not
+    # report carrier
+    returncode, result = callAndRead(['ip', 'addr', 'list', self.name])
+    self.attach_to_tap = 'DOWN' in result.split('\n', 1)[0]
+
   def __getinitargs__(self):
     return (self.name,)