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Provide internal types first
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Consider the following example:
```go
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, se 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)
```
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@JacobOaks
JacobOaks committed Apr 19, 2024
1 parent 9814dd3 commit 81d3ff8
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Showing 3 changed files with 24 additions and 14 deletions.
11 changes: 7 additions & 4 deletions app.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -480,10 +480,9 @@ func New(opts ...Option) *App {
m.build(app, app.container)
}

for _, m := range app.modules {
m.provideAll()
}

// Provide Fx types first to increase the change a custom logger
// can be successfully built in the face of unrelated DI failure.
// E.g., for a custom logger that relies on the Lifecycle type.
frames := fxreflect.CallerStack(0, 0) // include New in the stack for default Provides
app.root.provide(provide{
Target: func() Lifecycle { return app.lifecycle },
Expand All @@ -492,6 +491,10 @@ func New(opts ...Option) *App {
app.root.provide(provide{Target: app.shutdowner, Stack: frames})
app.root.provide(provide{Target: app.dotGraph, Stack: frames})

for _, m := range app.modules {
m.provideAll()
}

// Run decorators before executing any Invokes -- including the one
// inside constructCustomLogger.
app.err = multierr.Append(app.err, app.root.decorateAll())
Expand Down
17 changes: 12 additions & 5 deletions app_test.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -113,7 +113,13 @@ func TestNewApp(t *testing.T) {
[]string{"Provided", "Provided", "Provided", "Provided", "LoggerInitialized", "Started"},
spy.EventTypes())

assert.Contains(t, spy.Events()[0].(*fxevent.Provided).OutputTypeNames, "struct {}")
// Fx types get provided first to increase change of
// successful custom logger build.
assert.Contains(t, spy.Events()[0].(*fxevent.Provided).OutputTypeNames, "fx.Lifecycle")
assert.Contains(t, spy.Events()[1].(*fxevent.Provided).OutputTypeNames, "fx.Shutdowner")
assert.Contains(t, spy.Events()[2].(*fxevent.Provided).OutputTypeNames, "fx.DotGraph")
// Our type should be index 3.
assert.Contains(t, spy.Events()[3].(*fxevent.Provided).OutputTypeNames, "struct {}")
})

t.Run("CircularGraphReturnsError", func(t *testing.T) {
Expand Down Expand Up @@ -575,7 +581,7 @@ func TestWithLogger(t *testing.T) {
)

assert.Equal(t, []string{
"Supplied", "Provided", "Provided", "Provided", "Run", "LoggerInitialized",
"Provided", "Provided", "Provided", "Supplied", "Run", "LoggerInitialized",
}, spy.EventTypes())

spy.Reset()
Expand Down Expand Up @@ -605,7 +611,7 @@ func TestWithLogger(t *testing.T) {
"must provide constructor function, got (type *bytes.Buffer)",
)

assert.Equal(t, []string{"Supplied", "Provided", "Run", "LoggerInitialized"}, spy.EventTypes())
assert.Equal(t, []string{"Provided", "Provided", "Provided", "Supplied", "Provided", "Run", "LoggerInitialized"}, spy.EventTypes())
})

t.Run("logger failed to build", func(t *testing.T) {
Expand Down Expand Up @@ -1166,8 +1172,9 @@ func TestOptions(t *testing.T) {
Provide(&bytes.Buffer{}), // error, not a constructor
WithLogger(func() fxevent.Logger { return spy }),
)
require.Equal(t, []string{"Provided", "LoggerInitialized"}, spy.EventTypes())
assert.Contains(t, spy.Events()[0].(*fxevent.Provided).Err.Error(), "must provide constructor function")
require.Equal(t, []string{"Provided", "Provided", "Provided", "Provided", "LoggerInitialized"}, spy.EventTypes())
// First 3 provides are Fx types (Lifecycle, Shutdowner, DotGraph).
assert.Contains(t, spy.Events()[3].(*fxevent.Provided).Err.Error(), "must provide constructor function")
})
}

Expand Down
10 changes: 5 additions & 5 deletions module_test.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -261,15 +261,15 @@ func TestModuleSuccess(t *testing.T) {
desc: "custom logger for module",
giveWithLogger: fx.NopLogger,
wantEvents: []string{
"Supplied", "Provided", "Provided", "Provided",
"Provided", "Provided", "Provided", "Supplied",
"Run", "LoggerInitialized", "Invoking", "Invoked",
},
},
{
desc: "Not using a custom logger for module defaults to app logger",
giveWithLogger: fx.Options(),
wantEvents: []string{
"Supplied", "Provided", "Provided", "Provided", "Provided", "Run",
"Provided", "Provided", "Provided", "Supplied", "Provided", "Run",
"LoggerInitialized", "Invoking", "Run", "Invoked", "Invoking", "Invoked",
},
},
Expand Down Expand Up @@ -660,7 +660,7 @@ func TestModuleFailures(t *testing.T) {
giveAppOpts: spyAsLogger,
wantErrContains: []string{"error building logger"},
wantEvents: []string{
"Supplied", "Provided", "Provided", "Provided", "Run",
"Provided", "Provided", "Provided", "Supplied", "Run",
"LoggerInitialized", "Provided", "LoggerInitialized",
},
},
Expand All @@ -678,7 +678,7 @@ func TestModuleFailures(t *testing.T) {
giveAppOpts: spyAsLogger,
wantErrContains: []string{"error building logger dependency"},
wantEvents: []string{
"Supplied", "Provided", "Provided", "Provided", "Run",
"Provided", "Provided", "Provided", "Supplied", "Run",
"LoggerInitialized", "Provided", "Provided", "Run", "LoggerInitialized",
},
},
Expand All @@ -690,7 +690,7 @@ func TestModuleFailures(t *testing.T) {
"fx.WithLogger", "from:", "Failed",
},
wantEvents: []string{
"Supplied", "Provided", "Provided", "Provided", "Run",
"Provided", "Provided", "Provided", "Supplied", "Run",
"LoggerInitialized", "Provided", "LoggerInitialized",
},
},
Expand Down

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