Segfault when having a cycle in imports

[dlespiau]

With:

$ cat foo.js 
import * as bar from 'bar.js';
$ cat bar.js 
import * as foo from 'foo.js';

jk segfaults trying to resolve an infinite import loop. --debug-imports can be used to trace what happens, continuously resolving the two imports:

$ jk run foo.js --debug-imports
debug:   File: ✔ import foo.js from /home/damien/go/src/github.com/jkcfg/jk/bar.js (base=/home/damien/go/src/github.com/jkcfg/jk) -> /home/damien/go/src/github.com/jkcfg/jk/foo.js
debug:   File: ✔ import bar.js from /home/damien/go/src/github.com/jkcfg/jk/foo.js (base=/home/damien/go/src/github.com/jkcfg/jk) -> /home/damien/go/src/github.com/jkcfg/jk/bar.js
debug:   File: ✔ import foo.js from /home/damien/go/src/github.com/jkcfg/jk/bar.js (base=/home/damien/go/src/github.com/jkcfg/jk) -> /home/damien/go/src/github.com/jkcfg/jk/foo.js
debug:   File: ✔ import bar.js from /home/damien/go/src/github.com/jkcfg/jk/foo.js (base=/home/damien/go/src/github.com/jkcfg/jk) -> /home/damien/go/src/github.com/jkcfg/jk/bar.js
...

We should detect import cycles and stop them before they explode, displaying the cycle for users to debug.

[dlespiau]

es6 is supposed to be able to load modules with cyclic dependencies. The cycle is entirely in our import code, not the v8 one, so maybe, once we break the cycle instead of segfaulting, v8 will be able to load the script.

https://2ality.com/2014/09/es6-modules-final.html#support-for-cyclic-dependencies-between-modules

[dlespiau]

• The import code is in the pkg/resolve package
• The main import resolving function is https://github.com/jkcfg/jk/blob/master/pkg/resolve/resolver.go#L89.
• ResolveModule has a recursive behaviour: when importing a module, we need to resolve the modules it itself imports.
• The recursion when having cyclic imports doesn't currently terminate
• We could have a cycle detection in ResolveModule. path is the canonical path of an imported module and can be used to track if a module has already been loaded
• --debug-imports can be used to trace what's happening in the import code.

The thing we want to do here is not to segfault. Supporting cyclic import may start to "just work" once we break cycles.