scheduler.py: Use threads instead of processes for jobs

This changes how the scheduler works and adapts all the code that needs
adapting in order to be able to run in threads instead of in
subprocesses, which helps with Windows support, and will allow some
simplifications in the main pipeline.

This addresses the following issues:

* Fix #810: All CAS calls are now made in the master process, and thus
  share the same connection to the cas server

* Fix #93: We don't start as many child processes anymore, so the risk
  of starving the machine are way less

* Fix #911: We now use `forkserver` for starting processes. We also
  don't use subprocesses for jobs so we should be starting less
  subprocesses

And the following highlevel changes where made:

* cascache.py: Run the CasCacheUsageMonitor in a thread instead of a
  subprocess.

* casdprocessmanager.py: Ensure start and stop of the process are thread
  safe.

* job.py: Run the child in a thread instead of a process, adapt how we
  stop a thread, since we ca't use signals anymore.

* _multiprocessing.py: Not needed anymore, we are not using `fork()`.

* scheduler.py: Run the scheduler with a threadpool, to run the child
  jobs in. Also adapt how our signal handling is done, since we are not
  receiving signals from our children anymore, and can't kill them the
  same way.

* sandbox: Stop using blocking signals to wait on the process, and use
  timeouts all the time.

* messenger.py: Use a thread-local context for the handler, to allow for
  multiple parameters in the same process.

* _remote.py: Ensure the start of the connection is thread safe

* _signal.py: Allow blocking entering in the signal's context managers
  by setting an event. This is to ensure no thread runs long-running
  code while we asked the scheduler to pause. This also ensures all the
  signal handlers is thread safe.

* source.py: Change check around saving the source's ref. We are now
  running in the same process, and thus the ref will already have been
  changed.

2 files changed, 12 insertions, 6 deletions

diff --git a/src/buildstream/sandbox/_sandboxbuildboxrun.py b/src/buildstream/sandbox/_sandboxbuildboxrun.py
index 3d71b7440..1c187d7fd 100644
--- a/src/buildstream/sandbox/_sandboxbuildboxrun.py
+++ b/src/buildstream/sandbox/_sandboxbuildboxrun.py
@@ -184,17 +184,27 @@ class SandboxBuildBoxRun(SandboxREAPI):
             try:
                 while True:
                     try:
-                        returncode = process.wait()
+                        # Here, we don't use `process.wait()` directly without a timeout
+                        # This is because, if we were to do that, and the process would never
+                        # output anything, the control would never be given back to the python
+                        # process, which might thus not be able to check for request to
+                        # shutdown, or kill the process.
+                        # We therefore loop with a timeout, to ensure the python process
+                        # can act if it needs.
+                        returncode = process.wait(timeout=1)
                         # If the process exits due to a signal, we
                         # brutally murder it to avoid zombies
                         if returncode < 0:
                             utils._kill_process_tree(process.pid)
 
+                    except subprocess.TimeoutExpired:
+                        continue
+
                     # Unlike in the bwrap case, here only the main
                     # process seems to receive the SIGINT. We pass
                     # on the signal to the child and then continue
                     # to wait.
-                    except KeyboardInterrupt:
+                    except _signals.TerminateException:
                         process.send_signal(signal.SIGINT)
                         continue
 
diff --git a/src/buildstream/sandbox/_sandboxremote.py b/src/buildstream/sandbox/_sandboxremote.py
index 6cba7d611..2ac159337 100644
--- a/src/buildstream/sandbox/_sandboxremote.py
+++ b/src/buildstream/sandbox/_sandboxremote.py
@@ -26,7 +26,6 @@ from functools import partial
 
 import grpc
 
-from .. import utils
 from ..node import Node
 from .._message import Message, MessageType
 from ._sandboxreapi import SandboxREAPI
@@ -59,9 +58,6 @@ class SandboxRemote(SandboxREAPI):
         if config is None:
             return
 
-        # gRPC doesn't support fork without exec, which is used in the main process.
-        assert not utils._is_main_process()
-
         self.storage_url = config.storage_service["url"]
         self.exec_url = config.exec_service["url"]