call socket.gethostname only once

(cherry picked from commit d2e2f1b89d)

src/borg/archive.py

@@ -40,7 +40,7 @@
 from .helpers import ellipsis_truncate, ProgressIndicatorPercent, log_multi
 from .patterns import PathPrefixPattern, FnmatchPattern, IECommand
 from .item import Item, ArchiveItem
-from .platform import acl_get, acl_set, set_flags, get_flags, swidth
+from .platform import acl_get, acl_set, set_flags, get_flags, swidth, hostname
 from .remote import cache_if_remote
 from .repository import Repository, LIST_SCAN_LIMIT
 
@@ -468,7 +468,7 @@ def save(self, name=None, comment=None, timestamp=None, additional_metadata=None
             'comment': comment or '',
             'items': self.items_buffer.chunks,
             'cmdline': sys.argv,
-            'hostname': socket.gethostname(),
+            'hostname': hostname,
             'username': getuser(),
             'time': start.strftime(ISO_FORMAT),
             'time_end': end.strftime(ISO_FORMAT),

src/borg/helpers.py

@@ -664,13 +664,13 @@ def format_line(format, data):
 
 def replace_placeholders(text):
     """Replace placeholders in text with their values."""
-    from .platform import fqdn
+    from .platform import fqdn, hostname
     current_time = datetime.now(timezone.utc)
     data = {
         'pid': os.getpid(),
         'fqdn': fqdn,
         'reverse-fqdn': '.'.join(reversed(fqdn.split('.'))),
-        'hostname': socket.gethostname(),
+        'hostname': hostname,
         'now': DatetimeWrapper(current_time.astimezone(None)),
         'utcnow': DatetimeWrapper(current_time),
         'user': uid2user(os.getuid(), os.getuid()),