tests: add utility to assert node memory usage hasn't increased

Adds a utility to get resident set size memory usage for a test
node and a context manager that allows assertions based upon
maximum memory use increase.

test/functional/test_framework/test_node.py

@@ -81,6 +81,28 @@ def __init__(self, i, dirname, extra_args, rpchost, timewait, binary, stderr, mo
 
         self.p2ps = []
 
+    def get_mem_rss(self):
+        """Get the memory usage (RSS) per `ps`.
+
+        If process is stopped or `ps` is unavailable, return None.
+        """
+        if not (self.running and self.process):
+            self.log.warning("Couldn't get memory usage; process isn't running.")
+            return None
+
+        try:
+            return int(subprocess.check_output(
+                "ps h -o rss {}".format(self.process.pid),
+                shell=True, stderr=subprocess.DEVNULL).strip())
+
+        # Catching `Exception` broadly to avoid failing on platforms where ps
+        # isn't installed or doesn't work as expected, e.g. OpenBSD.
+        #
+        # We could later use something like `psutils` to work across platforms.
+        except Exception:
+            self.log.exception("Unable to get memory usage")
+            return None
+
     def __del__(self):
         # Ensure that we don't leave any bitcoind processes lying around after
         # the test ends
@@ -225,6 +247,29 @@ def assert_debug_log(self, expected_msgs):
                 if re.search(re.escape(expected_msg), log, flags=re.MULTILINE) is None:
                     raise AssertionError('Expected message "{}" does not partially match log:\n\n{}\n\n'.format(expected_msg, print_log))
 
+    @contextlib.contextmanager
+    def assert_memory_usage_stable(self, perc_increase_allowed=0.03):
+        """Context manager that allows the user to assert that a node's memory usage (RSS)
+        hasn't increased beyond some threshold percentage.
+        """
+        before_memory_usage = self.get_mem_rss()
+
+        yield
+
+        after_memory_usage = self.get_mem_rss()
+
+        if not (before_memory_usage and after_memory_usage):
+            self.log.warning("Unable to detect memory usage (RSS) - skipping memory check.")
+            return
+
+        perc_increase_memory_usage = 1 - (float(before_memory_usage) / after_memory_usage)
+
+        if perc_increase_memory_usage > perc_increase_allowed:
+            raise AssertionError(
+                "Memory usage increased over threshold of {:.3f}% from {} to {} ({:.3f}%)".format(
+                    perc_increase_allowed * 100, before_memory_usage, after_memory_usage,
+                    perc_increase_memory_usage * 100))
+
     def node_encrypt_wallet(self, passphrase):
         """"Encrypts the wallet.