pytest: support simple subdaemon debugging.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/HACKING.md

@@ -174,7 +174,7 @@ A modern desktop can build and run through all the tests in a couple of minutes
 
     make -j12 check PYTEST_PAR=24 DEVELOPER=1 VALGRIND=0
 
-Adust `-j` and `PYTEST_PAR` accordingly for your hardware.
+Adjust `-j` and `PYTEST_PAR` accordingly for your hardware.
 
 There are three kinds of tests:
 
@@ -200,7 +200,9 @@ There are three kinds of tests:
 
   `PYTHONPATH=contrib/pylightning py.test -v tests/`.
 
-  You can also append `-k TESTNAME` to run a single test.
+  You can also append `-k TESTNAME` to run a single test.  Environment variables
+  `DEBUG_SUBD=<subdaemon>` and `TIMEOUT=<seconds>` can be useful for debugging
+  subdaemons on individual tests.
 
 Our Travis CI instance (see `.travis.yml`) runs all these for each
 pull request.

tests/utils.py

@@ -703,6 +703,8 @@ class NodeFactory(object):
         if DEVELOPER:
             daemon.opts["dev-fail-on-subdaemon-fail"] = None
             daemon.env["LIGHTNINGD_DEV_MEMLEAK"] = "1"
+            if os.getenv("DEBUG_SUBD"):
+                daemon.opts["dev-debugger"] = os.getenv("DEBUG_SUBD")
             if VALGRIND:
                 daemon.env["LIGHTNINGD_DEV_NO_BACKTRACE"] = "1"
             if not may_reconnect: