cdylib-plugin.rs

Introduction

Plugin-style shared libraries are shared libraries designed for a specific host program. The host program calls into the plugin, and the plugin calls back into APIs in the host program. Importantly, this means that the plugin references symbols defined only in the host program; and these symbols are not defined at the time the plugin is built.

For example, a PostgreSQL extension (which uses a plugin-style shared library) may use SPI to execute queries in the current transaction in a running server. Symbols such as SPI_connect are defined in PostgreSQL and referenced by the plugin. Such symbols are resolved when the plugin is loaded, but cannot be resolved at the time the plugin is built.

Problem

In rust, there are two problems with plugin-style cdylib crates:

1. #62874 On some platforms, linking fails at build time due to the undefined symbols, unless special arguments are passed to the linker.
2. #8193 There is no good way to find the path of the library created.
   1. Makes integration testing difficult.
   2. Makes installation difficult.

Workarounds

Hopefully these problems are solved properly in the future. Until then, cdylib-plugin.rs offers workarounds to these problems.

Add a normal dependency and a build dependency to your crate:

[dependencies]
# ...
cdylib-plugin = "0.1"

[build-dependencies]
# ...
cdylib-plugin = "0.1"

Add a build.rs in your crate, such as:

extern crate cdylib_plugin;

fn main() {
	// ...
	cdylib_plugin::buildflags();
}

In your integration tests or installation code, find the library path with cdylib_plugin::cdylib_path().