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+---
+title: gopreload: LD_PRELOAD for the Gopher crowd
+date: 207-03-25
+---
+
+gopreload: LD_PRELOAD for the Gopher crowd
+==========================================
+
+A common pattern in Go libraries is to take advantage of [init functions][initf]
+to do things like settings up defaults in loggers, automatic metrics instrumentation,
+flag values, [debugging tools][manhole] 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, causing errors reported to go nowhere
+  or metrics submission, making operations teams blind to the performance of the
+  app.
+- 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][good-ld-preload] and [evil][evil-ld-preload], but the
+behavior is the same basic idea as [underscore imports in Go][underscore-import].
+
+My solution for this is [gopreload][gopreload]. It emulates the behavior of
+`LD_PRELOAD` but with [Go plugins][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:
+
+```go
+// 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`:
+
+```console
+$ go get -d github.com/Xe/gopreload/manhole
+$ go build -buildmode plugin -o $GOPATH/manhole.so github.com/Xe/gopreload/manhole
+```
+
+then run your program with `GO_PRELOAD` set to the path of `manhole.so`:
+
+```console
+$ 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][manhole-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`][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][twitter-addr] or
+on the Gophers slack (I'm `@xena` there). For issues with gopreload please file
+[an issue on Github][gopreload-issues].
+
+[initf]: https://golang.org/doc/effective_go.html#init
+[manhole]: https://github.com/Xe/gopreload/tree/master/manhole
+[good-ld-preload]: http://www.logix.cz/michal/devel/faketime/
+[evil-ld-preload]: https://rafalcieslak.wordpress.com/2013/04/02/dynamic-linker-tricks-using-ld_preload-to-cheat-inject-features-and-investigate-programs/
+[underscore-import]: https://golang.org/doc/effective_go.html#blank
+[gopreload]: https://github.com/Xe/gopreload
+[go-plugins]: https://golang.org/pkg/plugin/
+[manhole-tools]: https://github.com/Xe/gopreload/blob/master/manhole/server.go
+[ld-preload]: https://rafalcieslak.wordpress.com/2013/04/02/dynamic-linker-tricks-using-ld_preload-to-cheat-inject-features-and-investigate-programs/
+[gopreload-issues]: https://github.com/Xe/gopreload/issues/new