This is just a scratchpad of notes for development.
After developer documentation is authored, this file should be deleted.

Terminology can be a bit weird regarding client vs server.
Instead, I prefer to use "front end" and "back end".
Ghidra is always the front end, as it provides the UI.
The actual debugger is always the "back end" is it provides the actual instrumentation and access to the target.
wrt/ TCP, the connection can go either way, but once established, Ghidra still plays the front end role.

Client/Server otherwise depends on context.
For the trace-recording channel, the back-end is the client, and the front-end (Ghidra) is the server.
The back-end invokes remote methods on the DBTrace, and those cause DomainObjectChange events, updating the UI.
The front-end replies with minimal information.
	(More on this and sync/async/batching later)

For the command channel, the front-end (Ghidra) is the client, and the back-end is the server.
The user presses a button, which invokes a remote method on the back-end.
Often, that method and/or its effects on the target and back-end result in it updating the trace, and the loop is complete.
Again, the back-end replies with minimal information.
	One notable exception is the `execute` method, which can optionally return captured console output.
	In general, methods should only respond with actual information that doesn't belong in the trace.
	While I've not yet needed this, I suppose another exception could be for methods that want to return the path to an object, to clarify association of cause and effect.

Regarding sync/async and batching:
One of the goals of TraceRmi was to simplify the trace-recording process.
It does this in three ways:

1. Providing direct control to write the Trace database.
   The ObjectModel approach was more descriptive.
   It would announce the existence of things, and a recorder at the front end would decide (applying some arcane rules) what to record and display.
   Almost every new model required some adjustment to the recorder.
   
2. Changing to a synchronous RMI scheme.
   The decision to use an asynchronous scheme was to avoid accidental lock-ups of the Swing thread.
   In practice, it just poisoned every API that depended on it, and we still got Swing lock-ups.
   And worse, they were harder to diagnose, because the stack traces were obscured.
   And still worse, execution order and threading was difficult to predict.

   We've only been somewhat successful in changing to a fully synchronous scheme, but even then, we've (attempted to) mitigate each of the above complaints.
   On the front-end, the internals still use CompletableFuture, but we're more apt to use .get(), which keeps the stack together on the thread waiting for the result.
   In essence, there's little difference in blocking on .get() vs blocking on .recv().
   The reason we need a dedicated background thread to receive is to sort out the two channels.
   The recommended public API method is RemoteMethod.invoke(), which uses .get() internally, so this is mostly transparent, except when debugging the front end.
   There is still an .invokeAsync(), if desired, giving better control of timeouts, which is actually a feature we would not have using a purely synchronous .recv() (at least not without implementing non-blocking IO)
   To mitigate Swing lock-ups the .get() methods are overridden to explicitly check for the Swing thread.

   On the back end, the internals work similarly to the front end.
   We use a Future to handle waiting for the result, and the implementation of each trace modification method will immediately invoke .result().
   Unfortunately, this does slow things down far too much, since every miniscule operation requires a round trip.
   We mitigate this by implementing a `batch` context manager.
   Inside this context, most of the trace modification methods will now return the Future.
   However, a reference to each such future is stored off in the context.
   When the context is exited, all the Futures' results are waited on.
   This maintains a mostly synchronous behavior, while alleviating the repeated round-trip costs.

3. Simplifying the back end implementation, and providing it in Python.
   It turns out no debugger we've encountered up to this point provides Java language bindings out of the box.
   The closest we've seen is LLDB, which has specified their interfaces using SWIG, which lent itself to exporting Java bindings.
   And that was lucky, too, because accessing C++ virtual functions from JNA is fraught with peril.
   For gdb, we've been using a pseudo-terminal or ssh connection to its Machine Interface, which aside from the piping delays, has been pretty nice.
   It's not been great on Windows, though -- their ConPTY stuff has some ANSI oddities, the handling of which has slowed our performance.
   For dbgeng/dbgmodel, we've been fortunate that they follow COM+, which is fairly well understood by JNA.
   Nevertheless, all of these have required us to hack some kind of native bindings in Java.
   This introduces risks of crashing the JVM, and in some cases can cause interesting conflicts, e.g., the JVM and dbgeng may try to handle the same signals differently.
   dbgeng also only allows a single session.
   If the user connects twice to it using IN-VM (this is easy to do by accident), then the two connections are aliases of the same dbgeng session.

   Both gdb and lldb offer Python bindings, so it is an obvious choice for back end implementations.
   We are already using protobuf, so we keep it, but developed a new protocol specification.
   The trace modification methods are prescribed by Ghidra, so each is implemented specifically in the trace client.
   The back end remote methods are described entirely by the back end.
   They are enumerated during connection negotiation; otherwise, there is only one generic "Invoke" message.


Because we're more tightly integrated with the debugger, there may be some interesting caveats.

Pay careful attention to synchronization and session tear down.
At one point, I was using gdb's post_event as a Python Executor.
A separate thread handled the method invocation requests, scheduled it on the executor, waited for the result, and then responded.
This worked until the front end invoked `execute("quit")`.
I was expecting gdb to just quit, and the front end would expect the connection to die.
However, this never happened.
Instead, during execution of the `quit`, gdb wanted to clean up the Python interpreter.
Part of that was gracefully cleaning up all the Python threads, one of which was blocking indefinitely on execution of the `quit`.
Thus, the two threads were waiting on each other, and gdb locked up.

Depending on the debugger, the Python API may be more or less mature, and there could be much variation among versions we'd like to support.
For retrieving information, we at least have console capture as a fallback; however, there's not always a reliable way to detect certain events without a direct callback.
At worst, we can always hook something like `prompt`, but if we do, we must be quick in our checks.
Dealing with multiple versions, there's at least two ways:
1. Probe for the feature.
   This is one case where Python's dynamic nature helps out.
   Use `hasattr` to check for the existence of various features and choose accordingly.
2. Check the version string.
   Assuming version information can be consistently and reliably retrieved across all the supported versions, parse it first thing.
   If the implementation of a feature various across versions, the appropriate one can be selected.
   This may not work well for users of development branches, or are otherwise off the standard releases of their debuggers.

This is probably well understood by the Python community, but I'll overstate it here:
If you've written something, but you haven't unit tested it yet, then you haven't really written it.
This may be mitigated by some static analysis tools and type annotations, but I didn't use them.
In fact, you might even say I abused type annotations for remote method specifications.

For gdb, I did all of my unit testing using JUnit as the front end in Java.
This is perhaps not ideal, since this is inherently an integration test; nevertheless, it does allow me to test each intended feature of the back end separately.


# Package installation

I don't know what the community preference will be here, but now that we're playing in the Python ecosystem, we have to figure out how to play nicely.
Granted, some of this depends on how nicely the debugger plays in the Python ecosystem.
My current thought is distribute our stuff as Python packages, and let the user figure it out.
We'll still want to figure out the best way, if possible, to make things work out of the box.
Nevertheless, a `pip install` command may not be *that* offensive for a set-up step.

That said, for unit testing, I've had to incorporate package installation as a @BeforeClass method.
There's probably a better way, and that way may also help with out-of-the-box support.
Something like setting PYTHON_PATH before invoking the debugger?
There's still the issue of installing protobuf, though.
And the version we use is not the latest, which may put users who already have protobuf in dependency hell.
We use version 3.20, while the latest is 4.something.
According to protobuf docs, major versions are not guaranteed backward compatible.
To upgrade, we'd also have to upgrade the Java side.

# Protobuf headaches

Protobufs in Java have these nifty `writeDelimitedTo` and `parseDelimitedFrom` methods.
There's no equivalent for Python :(
That said, according to a stackoverflow post (which I've lost track of, but it's easily confirmed by examining protobufs Java source), you can hand-spin this by prepending a varint giving each message's length.
If only the varint codec were part of protobuf's public Python API....
They're pretty easily accessed in Python by importing the `internal` package, but that's probably not a good idea.
Also, (as I had been doing that), it's easy to goof up receiving just variable-length int and keeping the encoded message in tact for parsing.
I instead just use a fixed 32-bit int now.

# How-To?

For now, I'd say just take the gdb implementation as a template / guide.
Just beware, the whole thing is a bit unstable, so the code may change, but still, I don't expect it to change so drastically that integration work would be scrapped.

If you're writing Python, create a Python package following the template for gdb's.
I'd like the version numbers to match Ghidra's, though this may need discussion.
Currently, only Python 3 is supported.
I expect older versions of gdb may not support Py3, so we may need some backporting.
That said, if your distro's package for whatever debugger is compiled for Py2, you may need to build from source, assuming it supports Py3 at all.
I recommend mirroring the file layout:

__init__.py:
    Python package marker, but also initialization.
    For gdb, this file gets executed when the user types `python import ghidragdb`.
    Thus, that's how they load the extension.
arch.py:
    Utilities for mapping architecture-specific things between back and front ends.
    Technically, you should just be able to use the "DATA" processor for your trace, things will generally work better if you can map.
commands.py:
    These are commands we add to the debugger's CLI.
    For gdb, we use classes that extend `gdb.Command`, which allows the user to access them whether or not connected to Ghidra.
    For now, this is the recommendation, as I expect it'll allow users to "hack" on it more easily, either to customize or to retrieve diagnostics, etc.
    Notice that I use gdb's expression evaluator wherever that can enhance the command's usability, e.g., `ghidra trace putval`
hooks.py:
    These are event callbacks from the debugger as well as whatever plumbing in necessary to actually install them.
    That "plumbing" may vary, since the debugger may not directly support the callback you're hoping for.
    In gdb, there are at least 3 flavors:
    1. A directly-supported callback, i.e., in `gdb.events`
    2. A breakpoint callback, which also breaks down into two sub-flavors:
       * Internal breakpoint called back via `gdb.Breakpoint.stop`
       * Normal breakpoint whose commands invoke a CLI command
    3. A hooked command to invoke a CLI command, e.g., `define hook-inferior`
method.py:
    These are remote methods available to the front end.
    See the `MethodRegistry` object in the Python implementation, or the `RemoteMethod` interface in the Java implementation.
parameters.py:
    These are for gdb parameters, which may not map to anything in your debugger, so adjust as necessary.
    They're preferred to custom commands whose only purpose is to access a variable.
schema.xml:
    This is exactly what you think it is.
    It is recommended you copy this directly from the ObjectModel-based implementation and make adjustments as needed.
    See `commands.start_trace` to see how to load this file from your Python package.
util.py:
    Just utilities and such.
    For the gdb connector, this is where I put my version-specific implementations, e.g., to retrieve the memory map and module list.

For testing, similarly copy the JUnit tests (they're in the IntegrationTests project) into a separate properly named package.
I don't intend to factor out test cases, except for a few utilities.
The only real service that did in the past was to remind you what cases you ought to test.
Prescribing exactly *how* to test those and the scenarios, I think, was a mistake.
If I provide a base test class, it might just be to name some methods that all fail by default.
Then, as a tester, the failures would remind you to override each method with the actual test code.


For manual testing, I've used two methods
1. See `GdbCommandsTest#testManual`.
   Uncomment it to have JUnit start a trace-rmi front-end listener.
   You can then manually connect from inside your debugger and send/diagnose commands one at a time.
   Typically, I'd use the script from another test that was giving me trouble.
2. Start the full Ghidra Debugger and use a script to connect.
   At the moment, there's little UI integration beyond what is already offered by viewing a populated trace.
   Use either ConnectTraceRmiScript or ListenTraceRmiScript and follow the prompts / console.
   The handler will activate the trace when commanded, and it will follow the latest snapshot.


# User installation instructions:

The intent is to provide .whl or whatever Python packages as part of the Ghidra distribution.
A user should be able to install them using `pip3 install ...`, however:
We've recently encountered issues where the version of Python that gdb is linked to may not be the same version of Python the user gets when the type `python`, `python3` or `pip3`.
To manually check for this version, a user must type, starting in their shell:

```bash
gdb
python-interactive
import sys
print(sys.version)
```

Suppose they get `3.9.19`.
They'd then take the major and minor numbers to invoke `python3.9` directly:

```bash
python3.9 -m pip install ...
```

A fancy way to just have gdb print the python command for you is:

```bash
gdb --batch -ex 'python import sys' -ex 'python print(f"python{sys.version_info.major}.{sys.version_info.minor}")'
```

Regarding method registry, the executor has to be truly asynchronous.
You cannot just invoke the method synchronously and return a completed future.
If you do, you'll hang the message receiver thread, which may need to be free if the invoked method interacts with the trace.

We've currently adopted a method-naming convention that aims for a somewhat consistent API across back-end plugins.
In general, the method name should match the action name exactly, e.g., the method corresponding the Ghidra's `resume` action should be defined as:

   @REGISTRY.method
   def resume(...):
       ...

Not:

   @REGISTRY.method(name='continue', action='resume')
   def _continue(...):
      ...

Even though the back-end's command set and/or API may call it "continue."
If you would like to provide a hint to the user regarding the actual back-end command, do so in the method's docstring:

   @REGISTRY.method
   def resume(...):
       """Continue execution of the current target (continue)."""
       ...

There are exceptions:

1. When there is not a one-to-one mapping from the method to an action.
   This is usually the case for delete, toggle, refresh, etc.
   For these, use the action as the prefix, and then some suffix, usually describing the type of object affected, e.g., delete_breakpoint.
2. When using an "_ext" class of action, e.g., step_ext or break_ext.
   There is almost certainly not a one-to-one method for such an action.
   The naming convention is the same as 1, but omitting the "_ext", e.g., step_advance or break_event
   Even if you only have one method that maps to step_ext, the method should *never* be called step_ext.
3. There is no corresponding action at all.
   In this case, call it what you want, but strive for consistency among related methods in this category for your back-end.
   Act as though there could one day be a Ghidra action that you'd like to map them to.

There may be some naming you find annoying, e.g., "resume" (not "continue") or "launch" (not "start")
We also do not use the term "watchpoint." We instead say "write breakpoint."
Thus, the method for placing one is named `break_write_whatever`, not `watch_whatever`.


# Regarding transactions:

At the moment, I've defined two modes for transaction management on the client side.
The server side couldn't care less. A transactions is a transaction.
For hooks, i.e., things driven by events on the back end, use the client's transaction manager directly.
For commands, i.e., things driven by the user via the CLI, things are a little dicey.
I wouldn't expect the user to manage multiple transaction objects.
The recommendation is that the CLI can have at most one active transaction.
For the user to open a second transaction may be considered an error.
Take care as you're coding (and likely re-using command logic) that you don't accidentally take or otherwise conflict with the CLI's transaction manager when processing an event.


# Regarding launcher shell scripts:

Need to document all the @metadata stuff
In particular, "Image" is a special parameter that will get the program executable by default.


# Regarding the schema and method signatures

An interface like Togglable requires that a TOGGLE action takes the given schema as a parameter
(e.g., a BreakpointLocation)


# Regarding registers

The register container has to exist, even if its left empty in favor of the register space.

1. The space is named after the container
2. The UI uses the container as an anchor in its searches

To allow register writes, each writable register must exist as an object it the register container.

1. I might like to relax this....
2. The UI it to validate the register is editable, even though the RemoteMethod may accept
   frame/thread,name,val.


# Regarding reading and writing memory

The process parameter, if accepted, is technically redundant.
Because all address spaces among all targets must be unique, the address space encodes the process (or other target object).
If taken, the back end must validate that the address space belongs to the given process.
Otherwise, the back end must figure out the applicable target based on the space name.
