gopreload: LD_PRELOAD for the Gopher crowd

Published on 03/25/2017, 570 words, 3 minutes to read

A common pattern in Go libraries is to take advantage of init functions to do things like settings up defaults in loggers, automatic metrics instrumentation, flag values, debugging tools or database drivers. With monorepo culture prevalent in larger microservices based projects, this can lead to a few easily preventable problems:

Forgetting to set up a logger default or metrics submission, making operations teams blind to the performance of the app and developer teams blind to errors that come up during execution.
The requirement to make code changes to add things like metrics or HTTP routing extensions.

There is an environment variable in Linux libc's called LD_PRELOAD that will load arbitrary shared objects into ram before anything else is started. This has been used for good and evil, but the behavior is the same basic idea as underscore imports in Go.

My solution for this is gopreload. It emulates the behavior of LD_PRELOAD but with Go plugins. This allows users to explicitly automatically load arbitrary Go code into ram while the process starts.

Usage

To use this, add gopreload to your application's imports:

// gopreload.go
package main

/*
    This file is separate to make it very easy to both add into an application, but
    also very easy to remove.
*/

import _ "github.com/Xe/gopreload"

and then compile manhole:

$ go get -d github.com/Xe/gopreload/manhole
$ go build -buildmode plugin -o $GOPATH/manhole.so github.com/Xe/gopreload/manhole

then run your program with GO_PRELOAD set to the path of manhole.so:

$ export GO_PRELOAD=$GOPATH/manhole.so
$ go run *.go
2017/03/25 10:56:22 gopreload: trying to open: /home/xena/go/manhole.so
2017/03/25 10:56:22 manhole: Now listening on http://127.0.0.2:37588

That endpoint has pprof and a few other fun tools set up, making it a good stopgap "manhole" into the performance of a service.

Security Implications

This package assumes that programs run using it are never started with environment variables that are set by unauthenticated users. Any errors in loading the plugins will be logged using the standard library logger log and ignored.

This has about the same security implications as LD_PRELOAD does in most Linux distributions, but the risk is minimal compared to the massive benefit for being able to have arbitrary background services all be able to be dug into using the same tooling or being able to have metric submission be completely separated from the backend metric creation. Common logging setup processes can be always loaded, making the default logger settings into the correct settings.

Feedback

To give feedback about gopreload, please contact me on twitter or on the Gophers slack (I'm @xena there). For issues with gopreload please file an issue on Github.

Facts and circumstances may have changed since publication. Please contact me before jumping to conclusions if something seems wrong or unclear.

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