pyln: Use a fair FS lock to throttle node startups

We were getting a couple of starvations, so we need a fair filelock. I
also wasn't too happy with the lock as is, so I hand-coded it quickly.

Should be correct, but the overall timeout will tell us how well we
are doing on CI.

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -198,8 +198,8 @@ def teardown_checks(request):
 
 
 @pytest.fixture
-def throttler():
-    yield Throttler()
+def throttler(test_base_dir):
+    yield Throttler(test_base_dir)
 
 
 @pytest.fixture

contrib/pyln-testing/pyln/testing/utils.py

@@ -1,12 +1,13 @@
 from bitcoin.core import COIN  # type: ignore
 from bitcoin.rpc import RawProxy as BitcoinProxy  # type: ignore
 from bitcoin.rpc import JSONRPCError
+from contextlib import contextmanager
+from pathlib import Path
 from pyln.client import RpcError
 from pyln.testing.btcproxy import BitcoinRpcProxy
 from collections import OrderedDict
 from decimal import Decimal
 from ephemeral_port_reserve import reserve  # type: ignore
-from filelock import FileLock  # type: ignore
 from pyln.client import LightningRpc
 from pyln.client import Millisatoshi
 
@@ -1064,6 +1065,43 @@ class LightningNode(object):
             return None
 
 
+@contextmanager
+def flock(directory: Path):
+    """A fair filelock, based on atomic fs operations.
+    """
+    if not isinstance(directory, Path):
+        directory = Path(directory)
+    d = directory / Path(".locks")
+    os.makedirs(str(d), exist_ok=True)
+    fname = None
+
+    while True:
+        # Try until we find a filename that doesn't exist yet.
+        try:
+            fname = d / Path("lock-{}".format(time.time()))
+            fd = os.open(str(fname), flags=os.O_CREAT | os.O_EXCL)
+            os.close(fd)
+            break
+        except FileExistsError:
+            time.sleep(0.1)
+
+    # So now we have a position in the lock, let's check if we are the
+    # next one to go:
+    while True:
+        files = sorted([f.resolve() for f in d.iterdir() if f.is_file()])
+        # We're queued, so it should at least have us.
+        assert len(files) >= 1
+        if files[0] == fname:
+            break
+        time.sleep(0.1)
+
+    # We can continue
+    yield fname
+
+    # Remove our file, so the next one can go ahead.
+    fname.unlink()
+
+
 class Throttler(object):
     """Throttles the creation of system-processes to avoid overload.
 
@@ -1082,26 +1120,24 @@ class Throttler(object):
     tests, which could cause more timeouts.
 
     """
-    def __init__(self, target: float = 75):
+    def __init__(self, directory: str, target: float = 75):
         """If specified we try to stick to a load of target (in percent).
         """
         self.target = target
-        self.lock = FileLock("/tmp/ltest.lock")
         self.current_load = self.target  # Start slow
         psutil.cpu_percent()  # Prime the internal load metric
+        self.directory = directory
 
     def wait(self):
         start_time = time.time()
-        with self.lock.acquire(poll_intervall=0.250):
+        with flock(self.directory):
             # We just got the lock, assume someone else just released it
             self.current_load = 100
             while self.load() >= self.target:
                 time.sleep(1)
 
-            delay = time.time() - start_time
-            with open("/tmp/ltest-throttler.csv", "a") as f:
-                f.write("{}, {}, {}, {}\n".format(time.time(), self.load(), self.target, delay))
             self.current_load = 100  # Back off slightly to avoid triggering right away
+        print("Throttler delayed startup for {} seconds".format(time.time() - start_time))
 
     def load(self):
         """An exponential moving average of the load

contrib/pyln-testing/requirements.txt

@@ -1,7 +1,6 @@
 Flask==1.1.*
 cheroot==8.5.*
 ephemeral-port-reserve==1.1.1
-filelock==3.0.*
 flaky ~= 3.7.0
 psutil==5.7.*
 psycopg2-binary==2.8.*