Some kind of cycle is causing a build failure

[alecthomas]

It's strange, because there are no cycles in the FTL data type system itself, only in "calls" which should not impact the Go compiler, but still somehow the Go build will get into a loop like this:

backend | info: Building go module 'foo'
backend | error: Error deploying module /Users/alec/dev/project/backend/modules/foo. Will retry: failed to build module: failed to extract module schema: ftl/foo: /Users/alec/dev/project/backend/modules/foo/foo.go:14:2: could not import ftl/bar (invalid package name: "")
backend | info: Building go module 'bar'
backend | error: Error deploying module /Users/alec/dev/project/backend/modules/bar. Will retry: failed to build module: failed to extract module schema: ftl/bar: /Users/alec/dev/project/backend/modules/bar/bar.go:19:2: could not import ftl/foo (invalid package name: "")

Even stranger is that if we comment out the verbs that trigger it, let everything rebuild, then uncomment them, things seem to work in a flaky kind of way.

[alecthomas]

I don't think there's a solution that FTL can provide in the near term. What's happening is actually quite obvious in hindsight, but the errors were really obtuse:

1. FTL tries to build foo. It fails because it needs to import bar .
2. FTL tries to build bar. It fails because it needs to import foo.
3. Repeat.

This cycle is triggered by the ftl.Call()s. In theory we could make the schema extractor fault tolerant to any unresolved imports that aren't part of the module's verb or data type signatures. The Go type analyser is actually fault tolerant and replaces unresolved types with ast.SelectorExpr which has sufficient information for us to resolve to a reference, such that FTL could still extract the types.

That said, this would really only be a bandaid in certain circumstances I think. If you had data types or verbs in different modules that referenced each other in a cycle, this would still break.

I think the real issue here is that the reported errors are useless. We should fix those.