1564163 - (dbg-impl-cleanup) [jsdbg2] Debugger implementation cleanup [meta]

Status: NEW

(Core :: JavaScript Engine, task, P3)

Description

[Jim Blandy :jimb]

SpiderMonkey's Debugger API, first added in 2011, has cheerfully supported the Firefox devtools across several generations of JavaScript JITs, but its implementation is showing its age. Its interactions with the garbage collector and just-in-time compilers are obscure. Despite having extensive unit tests, Debugger is frequently implicated in fuzzer-blocking bugs. Its support for Web Assembly evolves slowly, if at all. SpiderMonkey engineers cringe when tracking down a bug leads them into the Debugger's territory. Even Debugger's original designers encounter a steady stream of unexpected obstacles when adding support for new ECMAScript features like generators and asynchronous functions.

In any software project, for engineers to be able to accurately estimate the time required to implement a change, certain preconditions must hold:

1. The features that Debugger provides must have clear, accurate documentation.

2. The demands that Debugger places on the rest of SpiderMonkey in order to implement those features must be documented similarly.

3. Within the implementation, each component's requirements and promises must be evident to any reasonably skilled developer.

4. The code must be thoroughly tested.

At the moment, only condition 4) is well in hand. As a result, estimates for Debugger work have a lot of uncertainty, and it is difficult for new engineers to contribute to the code. To have accurate estimates and a comfortable “bus factor”, we need to invest in refactoring and simplifying the Debugger implementation.