plugin.go

// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package plugin implements loading and symbol resolution of Go plugins.
//
// A plugin is a Go main package with exported functions and variables that
// has been built with:
//
//	go build -buildmode=plugin
//
// When a plugin is first opened, the init functions of all packages not
// already part of the program are called. The main function is not run.
// A plugin is only initialized once, and cannot be closed.
//
// # Warnings
//
// The ability to dynamically load parts of an application during
// execution, perhaps based on user-defined configuration, may be a
// useful building block in some designs. In particular, because
// applications and dynamically loaded functions can share data
// structures directly, plugins may enable very high-performance
// integration of separate parts.
//
// However, the plugin mechanism has many significant drawbacks that
// should be considered carefully during the design. For example:
//
//   - Plugins are currently supported only on Linux, FreeBSD, and
//     macOS, making them unsuitable for applications intended to be
//     portable.
//
//   - Applications that use plugins may require careful configuration
//     to ensure that the various parts of the program be made available
//     in the correct location in the file system (or container image).
//     By contrast, deploying an application consisting of a single static
//     executable is straightforward.
//
//   - Reasoning about program initialization is more difficult when
//     some packages may not be initialized until long after the
//     application has started running.
//
//   - Bugs in applications that load plugins could be exploited by
//     an attacker to load dangerous or untrusted libraries.
//
//   - Runtime crashes are likely to occur unless all parts of the
//     program (the application and all its plugins) are compiled
//     using exactly the same version of the toolchain, the same build
//     tags, and the same values of certain flags and environment
//     variables.
//
//   - Similar crashing problems are likely to arise unless all common
//     dependencies of the application and its plugins are built from
//     exactly the same source code.
//
//   - Together, these restrictions mean that, in practice, the
//     application and its plugins must all be built together by a
//     single person or component of a system. In that case, it may
//     be simpler for that person or component to generate Go source
//     files that blank-import the desired set of plugins and then
//     compile a static executable in the usual way.
//
// For these reasons, many users decide that traditional interprocess
// communication (IPC) mechanisms such as sockets, pipes, remote
// procedure call (RPC), shared memory mappings, or file system
// operations may be more suitable despite the performance overheads.
package plugin

// Plugin is a loaded Go plugin.
type Plugin struct {
	pluginpath string
	err        string        // set if plugin failed to load
	loaded     chan struct{} // closed when loaded
	syms       map[string]any
}

// Open opens a Go plugin.
// If a path has already been opened, then the existing *[Plugin] is returned.
// It is safe for concurrent use by multiple goroutines.
func Open(path string) (*Plugin, error) {
	return open(path)
}

// Lookup searches for a symbol named symName in plugin p.
// A symbol is any exported variable or function.
// It reports an error if the symbol is not found.
// It is safe for concurrent use by multiple goroutines.
func (p *Plugin) Lookup(symName string) (Symbol, error) {
	return lookup(p, symName)
}

// A Symbol is a pointer to a variable or function.
//
// For example, a plugin defined as
//
//	package main
//
//	import "fmt"
//
//	var V int
//
//	func F() { fmt.Printf("Hello, number %d\n", V) }
//
// may be loaded with the [Open] function and then the exported package
// symbols V and F can be accessed
//
//	p, err := plugin.Open("plugin_name.so")
//	if err != nil {
//		panic(err)
//	}
//	v, err := p.Lookup("V")
//	if err != nil {
//		panic(err)
//	}
//	f, err := p.Lookup("F")
//	if err != nil {
//		panic(err)
//	}
//	*v.(*int) = 7
//	f.(func())() // prints "Hello, number 7"
type Symbol any