integrations/perforce: Skip messages for missing streams.

For a large perforce server with many users, this allows projects
to opt-in to zulip notifications by creating the stream for the
branch to be monitored.

Commits to paths for which a stream mapping yields a result,
but no stream exists in Zulip, are simply dropped silently.

This behaviour is opt-in, by setting the configuration key
ZULIP_IGNORE_MISSING_STREAM = True
in the zulip_perforce_config.py file.

api/integrations/perforce/zulip_change-commit.py

@@ -72,10 +72,22 @@ except ValueError:
 metadata = git_p4.p4_describe(changelist)  # type: Dict[str, str]
 
 destination = config.commit_notice_destination(changeroot, changelist)  # type: Optional[Dict[str, str]]
+
 if destination is None:
     # Don't forward the notice anywhere
     sys.exit(0)
 
+ignore_missing_stream = None
+if hasattr(config, "ZULIP_IGNORE_MISSING_STREAM"):
+    ignore_missing_stream = config.ZULIP_IGNORE_MISSING_STREAM
+
+if ignore_missing_stream:
+    # Check if the destination stream exists yet
+    stream_state = client.get_stream_id(destination["stream"])
+    if stream_state["result"] == "error":
+        # Silently discard the message
+        sys.exit(0)
+
 change = metadata["change"]
 p4web = None
 if hasattr(config, "P4_WEB"):

api/integrations/perforce/zulip_perforce_config.py

@@ -26,6 +26,11 @@ ZULIP_USER = "p4-bot@example.com"
 ZULIP_API_KEY = "0123456789abcdef0123456789abcdef"
 ZULIP_SITE = "https://zulip.example.com"
 
+# Set this to True to silently drop messages if the destination stream
+# does not exist. This prevents the warnings from Zulip's Notification Bot
+# when commits are made on a branch for which no stream has been created.
+ZULIP_IGNORE_MISSING_STREAM = False
+
 # Set this to point at a p4web installation to get changelist IDs as links
 # P4_WEB = "https://p4web.example.com"
 P4_WEB = None