improve strict undefined

[beckjake]

Fixes #1389

• During parsing, leave undefined values as non-strict
• Create a warning cache to avoid duplicate warnings when macros contain an undefined value
• When compiling hooks, add empty results and schemas to their context.
• Added some tests to expose this behavior

[cmcarthur]

@beckjake how does that global flag interact across threads? if we are running an RPC server where one thread is in "parse mode" and the other is in "compile mode", what happens?

(same question for WARNING_CACHE)

[beckjake]

The RPC server runs dbt in processes, not threads, so globals aren't shared! (technically it's a thread that starts a process, but that's not important)

[cmcarthur]

The RPC server runs dbt in processes, not threads, so globals aren't shared! (technically it's a thread that starts a process, but that's not important)

Yep, ok, makes sense. But it's also true to say that this adds to the list of things that we would have to change if we wanted to switch from processes to threads in the future, right?

(I don't know that we'd want to do that, just curious)

[beckjake]

Yeah. I think at this point threads are pretty infeasible, honestly - just from a control standpoint, you can't kill a thread (yes, there is pthread_kill, never ever use it).

I also realized that when parsing this branch does not set PARSE_MODE=True when it's parsing RPC-provided macros - I'll add that.

[drewbanin]

I think something funky happens when recording the name of the macro where the warning is generated:

-- models/my_model.sql

select 1 as id

{{ macro_1() }}
{{ macro_2() }}

-- macros/my_macro.sql

{% macro macro_1() %}

    {{ kwargs['abc'] }}

{% endmacro %}

{% macro macro_2() %}

    {{ kwargs['def'] }}

{% endmacro %}

This generates the following warnings on dbt run

Compilation warning in macro macro_1 (macros/my_macro.sql):
        'abc' is undefined
Compilation warning in macro macro_1 (macros/my_macro.sql):
        'def' is undefined

I'd expect the second warning to say macro_2, not macro_1

[beckjake]

Let's just revert this feature