feat: Single process optimization (canonical#5489)

Python interpreter initialization and module import time 
contributes a significant amount of wall clock time to
cloud-init's runtime (and therefore to total boot time).

Cloud-init has four stages. Each stage starts its own Python
interpreter and loads the same libraries. To eliminate the
redundant work of starting an interpreter and loading libraries,
this changes cloud-init to run as a single process. Systemd
service ordering is retained by using the existing cloud-init
services as shims which use a synchronization protocol to start
each cloud-init stage and to communicate that each stage is
complete to the init system. Since multiple cloud-init processes
sit in the critical chain of starting the system, this reduces
boot time (including time to ssh login and time to cloud-init
completion).

Currently only systemd is supported, but the synchronization
protocol should be capable of supporting other init systems
as well with minor changes.

Note: This enables many additional follow-on improvements that
eliminate redundant work. However, these potential improvements
are temporarily ignored. This commit has been structured to
minimize the changes required to capture the majority of primary
performance savings while preserving correctness and the ability
to preserve backwards compatibility.

Since this changes the semantics of the existing cloud-init unit
files, this change takes the opportunity to rename one of its
systemd units which causes frequent user confusion. The unit named
cloud-init.service is often mistaken by users for being the only
cloud-init service, when it is simply one of four stages. This
stage is documented as the "network" stage, so this service will
be renamed to "cloud-init-network.service". A new notify service
is added as part of this implementation which contains the
cloud-init process. This unit is named "cloud-init-main.service".
 
Synchronization protocol
========================

- create one Unix socket for each systemd service stage
- send sd_notify()
- For each of the four stages (local, network, config, final):
   - when init system sends "start" to the Unix socket, start the
     stage
   - when running stage is complete, send "done" to Unix socket

File changes
============

socket.py (new)
---------------

- define a systemd-notify helper function
- define a context manager which implements a multi-socket
  synchronization protocol

cloud-init.service -> cloud-init-network.service (renamed)
----------------------------------------------------------

- renamed to cloud-network.service

cloud-{init-local,init-network,config,final}.services
-------------------------------------------

- change ExecStart to use netcat to connect to Unix socket and:
  - send a start message
  - wait for completion response
- note: a pure Python equivalent is possible for any downstreams
  which do not package openbsd's netcat

cloud-init-main.service (new)
-----------------------------

 - use service type to 'notify'
 - invoke cloud-init in single process mode
 - adopt systemd ordering requirements from cloud-init-local.service
 - adopt KillMode from cloud-final.service

main.py
-------

 - Add command line flag to indicate "all stages" mode
 - In this mode run each stage followed by an IPC
   synchronization protocol step

cloud-final.services
--------------------

- drop KillMode

cloud-init-local.services
-------------------------

- drop dependencies made redundant by ordering after
  cloud-init-main.service

Performance Impact
==================

On Ubuntu 24.04, Python's wall clock start up time as measured with
`time python3 -c 'import cloudinit.cmd.main' on a few cloud types:

lxc container: 0.256s
QEMU machine:  0.300s
gce instance:  0.367s
ec2 instance:  0.491s

This change eliminates x1 this start up time from time to ssh.
This change eliminates x3 this start up time from cloud-init's total
completion. Total benefit varies based on the platform that the
instance is hosted by, but all platforms will measurably benefit from
this change.

BREAKING_CHANGE: Run all four cloud-init services as a single systemd service.

cloudinit/cmd/main.py

@@ -24,6 +24,7 @@
 from cloudinit import netinfo
 from cloudinit import signal_handler
 from cloudinit import sources
+from cloudinit import socket
 from cloudinit import stages
 from cloudinit import url_helper
 from cloudinit import util
@@ -38,7 +39,12 @@
 from cloudinit.config.schema import validate_cloudconfig_schema
 from cloudinit import log
 from cloudinit.reporting import events
-from cloudinit.settings import PER_INSTANCE, PER_ALWAYS, PER_ONCE, CLOUD_CONFIG
+from cloudinit.settings import (
+    PER_INSTANCE,
+    PER_ALWAYS,
+    PER_ONCE,
+    CLOUD_CONFIG,
+)
 
 # Welcome message template
 WELCOME_MSG_TPL = (
@@ -362,8 +368,11 @@ def main_init(name, args):
     outfmt = None
     errfmt = None
     try:
-        close_stdin(lambda msg: early_logs.append((logging.DEBUG, msg)))
-        outfmt, errfmt = util.fixup_output(init.cfg, name)
+        if not args.skip_log_setup:
+            close_stdin(lambda msg: early_logs.append((logging.DEBUG, msg)))
+            outfmt, errfmt = util.fixup_output(init.cfg, name)
+        else:
+            outfmt, errfmt = util.get_output_cfg(init.cfg, name)
     except Exception:
         msg = "Failed to setup output redirection!"
         util.logexc(LOG, msg)
@@ -375,8 +384,9 @@ def main_init(name, args):
             "Logging being reset, this logger may no longer be active shortly"
         )
         log.reset_logging()
-    log.setup_logging(init.cfg)
-    apply_reporting_cfg(init.cfg)
+    if not args.skip_log_setup:
+        log.setup_logging(init.cfg)
+        apply_reporting_cfg(init.cfg)
 
     # Any log usage prior to setup_logging above did not have local user log
     # config applied.  We send the welcome message now, as stderr/out have
@@ -633,8 +643,9 @@ def main_modules(action_name, args):
     mods = Modules(init, extract_fns(args), reporter=args.reporter)
     # Stage 4
     try:
-        close_stdin()
-        util.fixup_output(mods.cfg, name)
+        if not args.skip_log_setup:
+            close_stdin()
+            util.fixup_output(mods.cfg, name)
     except Exception:
         util.logexc(LOG, "Failed to setup output redirection!")
     if args.debug:
@@ -643,8 +654,9 @@ def main_modules(action_name, args):
             "Logging being reset, this logger may no longer be active shortly"
         )
         log.reset_logging()
-    log.setup_logging(mods.cfg)
-    apply_reporting_cfg(init.cfg)
+    if not args.skip_log_setup:
+        log.setup_logging(mods.cfg)
+        apply_reporting_cfg(init.cfg)
 
     # now that logging is setup and stdout redirected, send welcome
     welcome(name, msg=w_msg)
@@ -804,9 +816,10 @@ def status_wrapper(name, args):
         )
 
     v1[mode]["start"] = float(util.uptime())
-    preexisting_recoverable_errors = next(
+    handler = next(
         filter(lambda h: isinstance(h, log.LogExporter), root_logger.handlers)
-    ).export_logs()
+    )
+    preexisting_recoverable_errors = handler.export_logs()
 
     # Write status.json prior to running init / module code
     atomic_helper.write_json(status_path, status)
@@ -847,11 +860,8 @@ def status_wrapper(name, args):
         v1["stage"] = None
 
         # merge new recoverable errors into existing recoverable error list
-        new_recoverable_errors = next(
-            filter(
-                lambda h: isinstance(h, log.LogExporter), root_logger.handlers
-            )
-        ).export_logs()
+        new_recoverable_errors = handler.export_logs()
+        handler.clean_logs()
         for key in new_recoverable_errors.keys():
             if key in preexisting_recoverable_errors:
                 v1[mode]["recoverable_errors"][key] = list(
@@ -953,9 +963,19 @@ def main(sysv_args=None):
         default=False,
     )
 
+    parser.add_argument(
+        "--all-stages",
+        dest="all_stages",
+        action="store_true",
+        help=(
+            "Run cloud-init's stages under a single process using a "
+            "syncronization protocol. This is not intended for CLI usage."
+        ),
+        default=False,
+    )
+
     parser.set_defaults(reporter=None)
     subparsers = parser.add_subparsers(title="Subcommands", dest="subcommand")
-    subparsers.required = True
 
     # Each action and its sub-options (if any)
     parser_init = subparsers.add_parser(
@@ -1143,8 +1163,76 @@ def main(sysv_args=None):
 
             status_parser(parser_status)
             parser_status.set_defaults(action=("status", handle_status_args))
+    else:
+        parser.error("a subcommand is required")
 
     args = parser.parse_args(args=sysv_args)
+    setattr(args, "skip_log_setup", False)
+    if not args.all_stages:
+        return sub_main(args)
+    return all_stages(parser)
+
+
+def all_stages(parser):
+    """Run all stages in a single process using an ordering protocol."""
+    LOG.info("Running cloud-init in single process mode.")
+
+    # this _must_ be called before sd_notify is called otherwise netcat may
+    # attempt to send "start" before a socket exists
+    sync = socket.SocketSync("local", "network", "config", "final")
+
+    # notify systemd that this stage has completed
+    socket.sd_notify("READY=1")
+    # wait for cloud-init-local.service to start
+    with sync("local"):
+        # set up logger
+        args = parser.parse_args(args=["init", "--local"])
+        args.skip_log_setup = False
+        # run local stage
+        sync.systemd_exit_code = sub_main(args)
+
+    # wait for cloud-init-network.service to start
+    with sync("network"):
+        # skip re-setting up logger
+        args = parser.parse_args(args=["init"])
+        args.skip_log_setup = True
+        # run init stage
+        sync.systemd_exit_code = sub_main(args)
+
+    # wait for cloud-config.service to start
+    with sync("config"):
+        # skip re-setting up logger
+        args = parser.parse_args(args=["modules", "--mode=config"])
+        args.skip_log_setup = True
+        # run config stage
+        sync.systemd_exit_code = sub_main(args)
+
+    # wait for cloud-final.service to start
+    with sync("final"):
+        # skip re-setting up logger
+        args = parser.parse_args(args=["modules", "--mode=final"])
+        args.skip_log_setup = True
+        # run final stage
+        sync.systemd_exit_code = sub_main(args)
+
+    # signal completion to cloud-init-main.service
+    if sync.experienced_any_error:
+        message = "a stage of cloud-init exited non-zero"
+        if sync.first_exception:
+            message = f"first exception received: {sync.first_exception}"
+        socket.sd_notify(
+            f"STATUS=Completed with failure, {message}. Run 'cloud-init status"
+            " --long' for more details."
+        )
+        socket.sd_notify("STOPPING=1")
+        # exit 1 for a fatal failure in any stage
+        return 1
+    else:
+        socket.sd_notify("STATUS=Completed")
+        socket.sd_notify("STOPPING=1")
+
+
+def sub_main(args):
 
     # Subparsers.required = True and each subparser sets action=(name, functor)
     (name, functor) = args.action

cloudinit/cmd/status.py

@@ -318,8 +318,9 @@ def systemd_failed(wait: bool) -> bool:
     for service in [
         "cloud-final.service",
         "cloud-config.service",
-        "cloud-init.service",
+        "cloud-init-network.service",
         "cloud-init-local.service",
+        "cloud-init-main.service",
     ]:
         try:
             stdout = query_systemctl(

cloudinit/config/cc_mounts.py

@@ -524,7 +524,7 @@ def handle(name: str, cfg: Config, cloud: Cloud, args: list) -> None:
     # fs_spec, fs_file, fs_vfstype, fs_mntops, fs-freq, fs_passno
     uses_systemd = cloud.distro.uses_systemd()
     default_mount_options = (
-        "defaults,nofail,x-systemd.after=cloud-init.service,_netdev"
+        "defaults,nofail,x-systemd.after=cloud-init-network.service,_netdev"
         if uses_systemd
         else "defaults,nobootwait"
     )

cloudinit/config/schemas/schema-cloud-config-v1.json

@@ -2022,12 +2022,12 @@
         },
         "mount_default_fields": {
           "type": "array",
-          "description": "Default mount configuration for any mount entry with less than 6 options provided. When specified, 6 items are required and represent ``/etc/fstab`` entries. Default: ``defaults,nofail,x-systemd.after=cloud-init.service,_netdev``",
+          "description": "Default mount configuration for any mount entry with less than 6 options provided. When specified, 6 items are required and represent ``/etc/fstab`` entries. Default: ``defaults,nofail,x-systemd.after=cloud-init-network.service,_netdev``",
           "default": [
             null,
             null,
             "auto",
-            "defaults,nofail,x-systemd.after=cloud-init.service",
+            "defaults,nofail,x-systemd.after=cloud-init-network.service",
             "0",
             "2"
           ],

cloudinit/log.py

@@ -152,6 +152,9 @@ def emit(self, record: logging.LogRecord):
     def export_logs(self):
         return copy.deepcopy(self.holder)
 
+    def clean_logs(self):
+        self.holder = defaultdict(list)
+
     def flush(self):
         pass