Provide Fx types before user types

[JacobOaks]

Consider the following example:

func opts() fx.Option {
        return fx.Options(
                fx.WithLogger(func(fx.Lifecycle) fxevent.Logger {
                        return &fxevent.ConsoleLogger{ W: os.Stdout }
                }),
                fx.Provide(func() string { return "" }),
                fx.Provide(func() string { return "" }),
        )
}

func main() {
        fx.New(opts()).Run()
}

The relevant issue to surface to the user is that they are double providing the same type. However, the actual error message is:

[Fx] ERROR              Failed to start: the following errors occurred:
 -  fx.Provide(main.opts.func3()) from:
    main.opts
        /home/user/go/src/scratch/fx_provide_order/main.go:17
    main.main
        /home/user/go/src/scratch/fx_provide_order/main.go:22
    runtime.main
        /opt/go/root/src/runtime/proc.go:271
    Failed: cannot provide function "main".opts.func3
(/home/user/go/src/scratch/fx_provide_order/main.go:17): cannot provide
string from [0]: already provided by "main".opts.func2
(/home/user/go/src/scratch/fx_provide_order/main.go:16)
 -  could not build arguments for function
"go.uber.org/fx".(*module).constructCustomLogger.func2
        /home/user/go-repos/pkg/mod/go.uber.org/[email protected]/module.go:292:
    failed to build fxevent.Logger:
    missing dependencies for function "main".opts.func1
        /home/user/go/src/scratch/fx_provide_order/main.go:11:
    missing type:
        - fx.Lifecycle (did you mean to Provide it?)

Which contains an additional error related to how the custom logger could not be built.

This is because Fx will try to continue to build the custom logger in the face of DI failure, theoretically to report issues through the right channels. But after an error occurs when providing anything, Fx refuses to provide any more types - leading to a subsequent error when trying to build this custom logger that depends on the fx.Lifecycle type.

This is a common issue that can be misleading for new engineers debugging their fx apps.

I couldn't find any particular reason why user-provided provides are registered before these Fx types, so this PR switches this ordering so that custom loggers can still be built if they rely on the Fx types, which cleans up the error message:

[Fx] ERROR              Failed to start: fx.Provide(main.opts.func3())
from:
main.opts
        /home/user/go/src/scratch/fx_provide_order/main.go:17
main.main
        /home/user/go/src/scratch/fx_provide_order/main.go:22
runtime.main
        /opt/go/root/src/runtime/proc.go:271
Failed: cannot provide function "main".opts.func3
(/home/user/go/src/scratch/fx_provide_order/main.go:17): cannot provide
string from [0]: already provided by "main".opts.func2
(/home/user/go/src/scratch/fx_provide_order/main.go:16)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.73%. Comparing base (9814dd3) to head (4626474).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1191   +/-   ##
=======================================
  Coverage   98.73%   98.73%           
=======================================
  Files          31       31           
  Lines        2851     2851           
=======================================
  Hits         2815     2815           
  Misses         29       29           
  Partials        7        7           

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[sywhang]

Did we validate this in internal CI? I don't see a reason it should affect anything but in case.

[JacobOaks]

Did we validate this in internal CI? I don't see a reason it should affect anything but in case.

Yes, this has been validated in internal CI @sywhang

[sywhang]

@JacobOaks do you want to cut a release?