recurse_link infinite recursion for external library circular dependency crashes forge

[daltyboy11]

Component

Forge

Have you ensured that all of these are up to date?

• Foundry
• Foundryup

What version of Foundry are you on?

forge 0.2.0 (59f026f 2023-05-23T17:21:42.593814000Z)

What command(s) is the bug in?

forge test

Operating System

macOS (Apple Silicon)

Describe the bug

If two libs have external functions that call each other then recurse_link enters an infinite loop. It attempts to link Lib A to Lib B, then B to A, then A to B..., until a stack overflow. Below is a minimal repro demonstrating the issue

1. Create a starter project via forge init
2. Append the following code to Counter.sol

library Foo {
  function foo() external {
    Bar.bar();
  }

  function flum() external {}
}

library Bar {
  function bar() external {
    Foo.flum();
  }
}

3. Run RUST_LOG=forge forge test
4. Observe crash output (duplicate lines omitted for brevity):

2023-05-23T20:21:53.130321Z TRACE forge::link: linking contract target="Foo.json:Foo"
2023-05-23T20:21:53.130325Z TRACE forge::link: get dependency dependency="Bar.json:Bar" file="src/Counter.sol" key="Bar" version=Version { major: 0, minor: 8, patch: 20, build: BuildMetadata("commit.a1b79de6.Darwin.appleclang") }
2023-05-23T20:21:53.130333Z TRACE forge::link: linking contract target="Bar.json:Bar"
2023-05-23T20:21:53.130362Z TRACE forge::link: get dependency dependency="Foo.json:Foo" file="src/Counter.sol" key="Foo" version=Version { major: 0, minor: 8, patch: 20, build: BuildMetadata("commit.a1b79de6.Darwin.appleclang") }
2023-05-23T20:21:53.130376Z TRACE forge::link: linking contract target="Foo.json:Foo"
2023-05-23T20:21:53.130380Z TRACE forge::link: get dependency dependency="Bar.json:Bar" file="src/Counter.sol" key="Bar" version=Version { major: 0, minor: 8, patch: 20, build: BuildMetadata("commit.a1b79de6.Darwin.appleclang") }
2023-05-23T20:21:53.130388Z TRACE forge::link: linking contract target="Bar.json:Bar"
2023-05-23T20:21:53.130392Z TRACE forge::link: get dependency dependency="Foo.json:Foo" file="src/Counter.sol" key="Foo" version=Version { major: 0, minor: 8, patch: 20, build: BuildMetadata("commit.a1b79de6.Darwin.appleclang") }

thread 'main' has overflowed its stack
fatal runtime error: stack overflow

The trace logs narrowed the issue down to recurse_link. Note this only happens when the lib's functions are external.