Circular dependencies in SSR not detected and causes indefinite hang

[antimatter15]

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Describe the bug

Let's say we have three files: A, B, and C, where A is the entry point. B and C cyclically depend on each other. A depends on both B and C. In this situation, Vite will not detect the cyclic dependency and simply hang indefinitely.

Reproduction

Create the following 3 files inside of the src/pages directory of the ssr-react playground. Attempt to load the /A page, and notice that it hangs forever and the logs are empty.

// A.jsx
import { addFour } from './B'
import { addTwo } from './C'

export default function Main(){
    return addFour(addTwo(1))
}

// B.js
import { addTwo } from './C'
export function addOne(x){
    return x + 1
}
export function addFour(x){
    return addTwo(addTwo(x))
}

// C.js
import { addOne } from './A'
export function addTwo(x){
    return addOne(addOne(x))
}

Explanation

This bug happens because ssrLoadModule first gets called with A, the entry point. Within instantiateModule, it then tries to load the dependencies B and C in parallel, and caches their promises in the pendingModules map.

When instantiateModule finally runs on B, it blocks until ssrLoadModule completes for its sole dependency: C. When ssrLoadModule is called, the urlStack only has [A, B] so it bypasses the circular dependency test. Since C is already in the pendingModules map, it returns the existing promise waiting for C to load.

Essentially this results in a state where A depends on B and C, while B and C both depend on each other. At this point ssrLoadModule will never complete.

System Info

• vite version: 2.0.5
• Operating System: macOS 11.2
• Node version: 15.11
• Package manager (npm/yarn/pnpm) and version: Yarn 1.22.10

[raythurnvoid]

@fairbanksg I've improved your fix with another case:

A -> B means: B is imported by A and B has A in its stack
A ... B means: A is waiting for B to ssrLoadModule()

H -> [X] ... (Y)
H -> [X] -> (Y) ... {B}
H -> A -> {B} ... |M|
H -> A -> {B} -> |M| ... [X]

Deadlock description:
[X] is waiting for (Y) to resolve 
	which is waiting for {B} 
	which is waiting for |M|
	which is waiting for [X]

H is the first module in common by all modules

https://github.com/raythurnevoid/vite/commit/4bffced904bb04c32ae48084cbda1fb156e26e9e