Update AsyncDnsServer-related test cookbook parts

Add practical tips about specific handler classes.  Mention some good
practices and point developers at existing code written in the desired
manner.  Document common pitfalls.  Suggest preferred approaches for
splitting up complex response handling code.

bin/tests/system/COOKBOOK.md

@@ -174,12 +174,46 @@ glue, truncation, dropped queries, malformed records), add an `ansN`
 subdirectory containing an `ans.py` script based on `isctest.asyncserver`.
 The runner starts it automatically on 10.53.0.N, logging to `ans.run`.
 
-The general pattern: write a response handler class, scope it to the
-domains it owns, and install it into an `AsyncDnsServer`.  A handler
-typically subclasses `DomainHandler` — its `domains` list routes every
-query for those names and their subdomains to the handler — and implements
-`get_responses()`, an async generator that inspects the query context and
-yields response actions:
+Implementing a custom `ansN` server happens in two phases:
+
+  - define all static DNS data that the server needs to serve (if any) in `*.db`
+    files, like you would for a regular `named` instance,
+
+  - implement any non-standard behavior (modifying zone-based responses or
+    generating responses from scratch) by defining a response handler class,
+    scoping it to the QNAMEs/QTYPEs/domains it owns, and installing it into an
+    `AsyncDnsServer`.
+
+Most importantly, avoid the temptation to define all DNS responses that a given
+`ansN` server needs to serve using just dnspython APIs; zone files are much
+easier to follow for static DNS data.  Splitting up static DNS data and custom
+behavior also makes it easier to follow the idea behind each test.
+
+The most commonly subclassed handler classes are (ordered by descending
+specificity):
+
+  - `QnameQtypeHandler`
+  - `QnameHandler`
+  - `DomainHandler`
+
+These handler classes require certain properties (e.g. `qnames`, `qtypes`,
+`domains`) to be defined by their subclasses.  These properties define the set
+of queries that a given handler should be used for.  Please see
+`isctest/asyncserver.py` for up-to-date information on available handler classes
+and existing `ans.py` files for how they can be used in practice.  Consult the
+log files (`ans.run`) in case a query is not matched by its intended handler.
+
+**NOTE:** For readability (of both code and logs), defining separate handler
+classes for distinct queries is strongly preferred over using a single handler
+containing an `if`/`elif`/`else` chain.
+
+**NOTE:** If you find yourself implementing an `__init__()` method in your
+handler subclass, it often indicates that you're approaching the problem at hand
+from the wrong side; contact QA for guidance in such a case.
+
+When a query is matched to a handler, the latter is expected to yield a response
+action through its `get_responses()` method, an async generator that inspects
+the query context and decides how the server should react:
 
 ```python
 from collections.abc import AsyncGenerator
@@ -224,19 +258,53 @@ handler matches are answered from zone data — `AsyncDnsServer` loads every
 `*.db` zone file found in the `ansN` directory at startup — or with the
 server's default rcode (REFUSED unless configured otherwise).
 
-When one mock server accrues several unrelated domains — typical for a
-shared `ansN` in a multi-module family directory — keep each domain's
-handler logic in its own module next to `ans.py`, with `ans.py` reduced
-to a loader that imports and installs each handler:
+**NOTE:** For returning static responses, subclassing `StaticResponseHandler` is
+strongly recommended instead of implementing the `get_responses()` generator
+manually; see `resolver/ans3/ans.py` for practical examples.
+
+**NOTE:** Calling `yield` does **NOT** make `get_responses()` return!  This is
+by design: `get_responses()` can yield multiple DNS messages in response to a
+single query, so that it can also handle AXFR/IXFR queries, among others.  Be
+careful not to unintentionally cause multiple DNS messages to be returned for a
+single query.  If your handler's `get_responses()` method contains multiple
+`yield` statements, it might be a sign that it needs to be refactored into
+multiple separate handlers.
+
+If multiple `ansN` instances used in a given system test need to share common
+logic, extract that logic into a `<test-name>_ans.py` module in the system test
+directory.  See the `qmin` system test for a practical example.
+
+If multiple system tests would benefit from sharing some common logic, consider
+submitting a merge request adding that logic to `isctest/asyncserver.py` itself.
+
+To the extent possible, try to keep each `ans.py` file limited in length and
+scope.  Look at existing `ans.py` files to see what is meant by that.  If the
+response generation logic required for reproducing a given bug is particularly
+complex, consider dedicating the entire `ans.py` file just to that logic instead
+of appending it to an existing one; `ansN` instances are cheap to spawn and run
+compared to regular `named` instances.  If the number of `ansN` instances used
+in a given system test is becoming unwieldy, it usually indicates the need to
+start adding/moving code to a new system test directory.
+
+In some rare cases, it may be useful to reuse a common set of `nsN` server
+instances to reproduce a whole class of related issues, triggering which relies
+on some non-standard behavior and therefore needs a custom `ansN` server to be
+implemented.  If the logic necessary for reproducing each of these issues is
+complex and the amount of those issues makes it impractical to add a separate
+`ansN` server for each issue (as recommended in the previous paragraph), it is
+acceptable to split up the test logic for each issue into separate `ans_*.py`
+modules inside a single `ansN` directory and reduce `ans.py` itself to a loader
+that imports and installs handlers defined in those separate modules:
 
 ```python
-from mytest.ans4 import broken_example
+from mytest.ans1 import ans_some_bug, ans_some_other_bug
 from isctest.asyncserver import AsyncDnsServer
 
 
 def main() -> None:
     server = AsyncDnsServer()
-    server.install_response_handler(broken_example.BrokenExampleHandler())
+    server.install_response_handler(ans_some_bug.SomeBugHandler())
+    server.install_response_handler(ans_some_other_bug.SomeOtherBugHandler())
     server.run()
 
 
@@ -244,6 +312,23 @@ if __name__ == "__main__":
     main()
 ```
 
+However, in such a case it is particularly important to ensure consistency
+between the names of all the Python files related to a given issue - otherwise,
+chaos ensues.  Furthermore, avoid using cryptic file names (e.g. numeric bug
+identifiers).  The recommended naming scheme is:
+
+```
+mytest/
+├── ans1
+│   ├── ans.py
+│   ├── ans_some_bug.py
+│   └── ans_some_other_bug.py
+├── ns2
+│   └── ...
+├── tests_some_bug.py
+└── tests_some_other_bug.py
+```
+
 To point a resolver at the mock, delegate to it from the test's root zone
 (served by ns1) or list it as a forwarder; `cyclic_glue` shows the
 delegation pattern end to end.