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Refactor command-running to run oneshots after startup
This refactors the command-running utilities to take the fx.Options as variadic arguments, so that fxapps.OneShot can add its own fx.Option. It uses a bit of reflection to convince Fx to gather the arguments to a function without actually calling that function immediately, allowing the oneShot function to run after `app.Start` has finished, when we're sure that all components have started. This works around the startup ordering bug in uber-go/fx#918.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,73 @@ | ||
| // Unless explicitly stated otherwise all files in this repository are licensed | ||
| // under the Apache License Version 2.0. | ||
| // This product includes software developed at Datadog (https://www.datadoghq.com/). | ||
| // Copyright 2016-present Datadog, Inc. | ||
|
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| package fxapps | ||
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| import ( | ||
| "context" | ||
| "reflect" | ||
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| "go.uber.org/fx" | ||
| ) | ||
|
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| var errorInterface = reflect.TypeOf((*error)(nil)).Elem() | ||
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| // OneShot runs the given function in an fx.App using the supplied options. | ||
| // The function's arguments are supplied by Fx and can be any provided type. | ||
| // The function must return `error. | ||
| // | ||
| // The resulting app starts all components, then invokes the function, then | ||
| // immediately shuts down. This is typically used for command-line tools like | ||
| // `agent status`. | ||
| func OneShot(oneShotFunc interface{}, opts ...fx.Option) error { | ||
| ftype := reflect.TypeOf(oneShotFunc) | ||
| if ftype == nil || ftype.Kind() != reflect.Func { | ||
| panic("OneShot requires a function as its first argument") | ||
| } | ||
|
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| // verify it returns error | ||
| if ftype.NumOut() != 1 || !ftype.Out(0).Implements(errorInterface) { | ||
| panic("OneShot function must return error or nothing") | ||
| } | ||
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| // build an function with the same signature as oneShotFunc that will | ||
| // capture the args and do nothing. | ||
| var oneShotArgs []reflect.Value | ||
| captureArgs := reflect.MakeFunc( | ||
| ftype, | ||
| func(args []reflect.Value) []reflect.Value { | ||
| oneShotArgs = args | ||
| // return a single nil value of type error | ||
| return []reflect.Value{reflect.Zero(errorInterface)} | ||
| }) | ||
| // fx.Invoke that function to capture the args at startup | ||
| opt := fx.Invoke(captureArgs.Interface()) | ||
| opts = append(opts, opt) | ||
| app := fx.New(opts...) | ||
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| startCtx, cancel := context.WithTimeout(context.Background(), app.StartTimeout()) | ||
| defer cancel() | ||
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| if err := app.Start(startCtx); err != nil { | ||
| return err | ||
| } | ||
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| // call the original oneShotFunc with the args captured during | ||
| // app startup | ||
| res := reflect.ValueOf(oneShotFunc).Call(oneShotArgs) | ||
| if !res[0].IsNil() { | ||
| err := res[0].Interface().(error) | ||
| return err | ||
| } | ||
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| stopCtx, cancel := context.WithTimeout(context.Background(), app.StopTimeout()) | ||
| defer cancel() | ||
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| if err := app.Stop(stopCtx); err != nil { | ||
| return err | ||
| } | ||
|
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| return nil | ||
| } |
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