Merge #12718: [Tests] Require exact match in assert_start_raises_init_eror (jnewbery)

fae1374 qa: Allow for partial_match when checking init error (MarcoFalke)
5812273 [Tests] Require exact match in assert_start_raises_init_eror() (John Newbery)
0ec08a6 [Tests] Move assert_start_raises_init_error method to TestNode (John Newbery)

Pull request description:

  Extracted from #12379, because the changes are important on their own.

  This allows for exact testing, since the match can be specified with a strict regex. Internal details (such as exact formatting of the error message) can still be fuzzed away by regex wildcards.

Tree-SHA512: 605d2c9c42362a32d42321b066637577a026d0bb8cfc1c9f5737a4ca6503ffe85457a5122cea6e1101053ccc6c8aa1bbae3602e1fa7d2988bf7d5c275f412f66

test/functional/test_framework/test_node.py

@@ -12,6 +12,7 @@ import logging
 import os
 import re
 import subprocess
+import tempfile
 import time
 
 from .authproxy import JSONRPCException
@@ -165,6 +166,41 @@ class TestNode():
     def wait_until_stopped(self, timeout=BITCOIND_PROC_WAIT_TIMEOUT):
         wait_until(self.is_node_stopped, timeout=timeout)
 
+    def assert_start_raises_init_error(self, extra_args=None, expected_msg=None, partial_match=False, *args, **kwargs):
+        """Attempt to start the node and expect it to raise an error.
+
+        extra_args: extra arguments to pass through to bitcoind
+        expected_msg: regex that stderr should match when bitcoind fails
+
+        Will throw if bitcoind starts without an error.
+        Will throw if an expected_msg is provided and it does not match bitcoind's stdout."""
+        with tempfile.SpooledTemporaryFile(max_size=2**16) as log_stderr:
+            try:
+                self.start(extra_args, stderr=log_stderr, *args, **kwargs)
+                self.wait_for_rpc_connection()
+                self.stop_node()
+                self.wait_util_stopped()
+            except Exception as e:
+                assert 'bitcoind exited' in str(e)  # node must have shutdown
+                self.running = False
+                self.process = None
+                # Check stderr for expected message
+                if expected_msg is not None:
+                    log_stderr.seek(0)
+                    stderr = log_stderr.read().decode('utf-8').strip()
+                    if partial_match:
+                        if re.search(expected_msg, stderr, flags=re.MULTILINE) is None:
+                            raise AssertionError('Expected message "{}" does not partially match stderr:\n"{}"'.format(expected_msg, stderr))
+                    else:
+                        if re.fullmatch(expected_msg, stderr) is None:
+                            raise AssertionError('Expected message "{}" does not fully match stderr:\n"{}"'.format(expected_msg, stderr))
+            else:
+                if expected_msg is None:
+                    assert_msg = "bitcoind should have exited with an error"
+                else:
+                    assert_msg = "bitcoind should have exited with expected error " + expected_msg
+                raise AssertionError(assert_msg)
+
     def node_encrypt_wallet(self, passphrase):
         """"Encrypts the wallet.