Live reload breaks debugging session in elixirLS debugger

[lukaszsamson]

I know that this most likely not be planned but there are repeated bugs and support requests related to that issue.

elixir-lsp/elixir-ls#797
elixir-lsp/elixir-ls#798

There are 2 problems when phoenix is run under elixirLS debugger

1. The code reloader (https://github.com/phoenixframework/phoenix/blob/master/lib/phoenix/code_reloader/server.ex) is purging and reloading modules on the fly with no regard for the ongoing debug session. The reloaded modules are not properly interpreted via erlang debugger.
2. It is messing with IO interception. Debug Adapter Protocol has strict requirements on stdout IO. All standard IO needs to be sent as JSONRPC protocol messages.

[josevalim]

This is tricky because I can’t come with good suggestions to address this. At best we could block the code reloader but I am not sure that would help either.

[CharlesIrvineKC]

Thought I would add a note specifying the impact of and work around for the problem. Basically, any time code is changed while in a debugging session, the Elixir compiler will likely throw spurious (false positive) errors. The work around is to bounce the debugger. This clears the problem and allows debugging to continue.

[josevalim]

@lukaszsamson if you can reproduce it we should do a pairing session :)

[CharlesIrvineKC]

There may be multiple error paths, but here is one that comes up often. To reproduce in vscode:

1. Create a new Phoenix 1.6 app: "mix phx.new hello".
2. Create a launch config. Add: "task": "phx.server".
3. In "page_controller.ex" add a break point to line 5.
4. Execute: Run/Start Debugging.
5. Go to "localhost:4000" to trigger the breakpoint. Continue past the breakpoint.
6. Insert "IO.inspect(conn)" above line 5.
7. Reset breakpoint to new line 5 and refresh "localhost:4000".
8. The following error occurs:

Compiling 1 file (.ex)

== Compilation error in file lib/hello_web/controllers/page_controller.ex ==
** (UndefinedFunctionError) function HelloWeb.__using__/1 is undefined or private. However there is a macro with the same name and arity. Be sure to require HelloWeb if you intend to invoke this macro
    (hello 0.1.0) HelloWeb.__using__(:controller)
    lib/hello_web/controllers/page_controller.ex:2: (module)
    (elixir 1.14.2) lib/kernel/parallel_compiler.ex:346: anonymous fn/5 in Kernel.ParallelCompiler.spawn_workers/7