add BORG_HOST_ID, fixes #3985

(cherry picked from commit 90348c1de9)

docs/usage_general.rst.inc

@@ -185,6 +185,13 @@ General:
         Borg assumes that it can derive a unique hostname / identity (see ``borg debug info``).
         If this is not the case or you do not want Borg to automatically remove stale locks,
         set this to *no*.
+    BORG_HOST_ID
+        Borg usually computes a host id from the FQDN plus the results of ``uuid.getnode()`` (which usually returns
+        a unique id based on the MAC address of the network interface. Except if that MAC happens to be all-zero - in
+        that case it returns a random value, which is not what we want (because it kills automatic stale lock removal).
+        So, if you have a all-zero MAC address or other reasons to better externally control the host id, just set this
+        environment variable to a unique value. If all your FQDNs are unique, you can just use the FQDN. If not,
+        use fqdn@uniqueid.
     BORG_LOGGING_CONF
         When set, use the given filename as INI_-style logging configuration.
     BORG_RSH

src/borg/platform/base.py

@@ -210,7 +210,13 @@ def getfqdn(name=''):
 # XXX this sometimes requires live internet access for issuing a DNS query in the background.
 hostname = socket.gethostname()
 fqdn = getfqdn(hostname)
-hostid = '%s@%s' % (fqdn, uuid.getnode())
+
+# uuid.getnode() is problematic in some environments (e.g. OpenVZ, see #3968) where the virtual MAC address
+# is all-zero. uuid.getnode falls back to returning a random value in that case, which is not what we want.
+# thus, we offer BORG_HOST_ID where a user can set an own, unique id for each of his hosts.
+hostid = os.environ.get('BORG_HOST_ID')
+if not hostid:
+    hostid = '%s@%s' % (fqdn, uuid.getnode())
 
 
 def get_process_id():