bitcoind backend: assign port at start time, not init.

I found a similar race in our core-lightning CI: when a node restarts,
there's a race where another node could start on the same port.

In this case, we catch the exception, which was covering up the failure.
The test then talks to the wrong bitcoind, which has unpredictable results.

Signed-off-by: Rusty Russell <[email protected]>

lnprototest/backend/bitcoind.py

@@ -71,9 +71,8 @@ def __init__(self, basedir: str):
             "-logtimestamps",
             "-nolisten",
         ]
-        self.port = reserve()
         self.btc_version = None
-        logging.debug("Port is {}, dir is {}".format(self.port, self.bitcoin_dir))
+        logging.debug("dir is {}".format(self.bitcoin_dir))
 
     def __init_bitcoin_conf(self):
         """Init the bitcoin core directory with all the necessary information
@@ -124,6 +123,10 @@ def __is__bitcoind_ready(self) -> bool:
         return True
 
     def start(self) -> None:
+        # Now we're about to start, we can allocate the port.
+        # We used to do this at init, but in parallel cases another
+        # bitcoin could use the port when we restart!
+        self.port = reserve()
         if self.rpc is None:
             self.__init_bitcoin_conf()
         # TODO: We can move this to a single call and not use Popen
@@ -148,7 +151,7 @@ def stop(self) -> None:
         shutil.rmtree(os.path.join(self.bitcoin_dir, "regtest"))
 
     def restart(self) -> None:
-        # Only restart if we have to.
+        # Only restart if we have to: may change port!
         if self.rpc.getblockcount() != 101 or self.rpc.getrawmempool() != []:
             self.stop()
             self.start()

lnprototest/clightning/clightning.py

@@ -118,11 +118,7 @@ def start(self) -> None:
         self.__init_sandbox_dir()
         if self.bitcoind is None:
             self.bitcoind = Bitcoind(self.directory)
-            try:
-                self.bitcoind.start()
-            except Exception as ex:
-                self.logger.debug(f"Exception with message {ex}")
-                self.logger.debug("RUN Bitcoind")
+            self.bitcoind.start()
         self.proc = subprocess.Popen(
             [
                 "{}/lightningd/lightningd".format(LIGHTNING_SRC),