Go Plugin System Implementation

Develop a robust plugin system for a Go application that allows for dynamic loading and execution of custom functionality at runtime. This is crucial for applications that need to be extensible without requiring recompilation, such as code editors, data processing pipelines, or web frameworks.

Problem Description

Your task is to design and implement a plugin system in Go. This system should be able to:

Define a Plugin Interface: Create a clear Go interface that all plugins must implement. This interface will define the contract for how the host application interacts with plugins.
Load Plugins Dynamically: Implement a mechanism to load compiled Go plugins (e.g., .so files) at runtime. This should allow new plugins to be added to a designated directory without restarting the application.
Register Plugins: Provide a way for loaded plugins to register themselves with the host application.
Execute Plugin Functionality: Enable the host application to discover and call specific functions or methods provided by registered plugins.
Handle Errors Gracefully: Ensure that the system can handle potential errors during plugin loading, registration, and execution (e.g., invalid plugin format, missing symbols, panics within plugins).

Key Requirements:

Plugin Interface: The Plugin interface should have at least two methods:
- Name() string: Returns the unique name of the plugin.
- Execute(input string) (string, error): Takes an input string and returns a processed string and an error if something goes wrong.
Plugin Discovery: Plugins should be discoverable from a predefined directory (e.g., ./plugins/).
Type Safety: The system should ensure that only valid plugins implementing the Plugin interface can be loaded.
Concurrency: Plugins might be executed concurrently. The system should be safe for concurrent use.
Unloading (Optional but Recommended): Consider how plugins might be unloaded if needed, although this is a more advanced feature. For this challenge, focus on loading and execution.

Expected Behavior:

The host application should be able to scan a specific directory for .so files, attempt to load each one as a Go plugin, check if it implements the Plugin interface, register it, and then be able to call the Execute method on any registered plugin by its name.

Edge Cases to Consider:

Non-plugin .so files in the plugin directory.
Plugins that panic during initialization or execution.
Multiple plugins with the same name.
Empty plugin directory.

Examples

Example 1:

Plugin Code (e.g., plugins/greeter/greeter.go):

package main

import (
	"fmt"
	"plugin"
)

// Plugin interface definition (should be in a shared package or defined identically)
type Plugin interface {
	Name() string
	Execute(input string) (string, error)
}

// Register the plugin
func init() {
	// Assume a hypothetical PluginRegistry type is available in the host
	// For demonstration, we'll simulate registration.
	// In a real system, this would involve calling a registration function.
	fmt.Println("Greeter plugin initializing...")
	// Simulated registration:
	// registry.Register(&GreeterPlugin{})
}

type GreeterPlugin struct{}

func (p *GreeterPlugin) Name() string {
	return "Greeter"
}

func (p *GreeterPlugin) Execute(input string) (string, error) {
	if input == "" {
		return "", fmt.Errorf("input cannot be empty for Greeter plugin")
	}
	return fmt.Sprintf("Hello, %s!", input), nil
}

// The entry point for plugin loading
// This function is not strictly required by the plugin mechanism itself
// but is common for Go plugins to expose specific symbols.
// In this challenge, we'll rely on the 'init' function for registration
// and the actual exported symbol for the plugin type instance.
// To make it loadable as a plugin, we need to export a variable of the Plugin type.
var Greeter GreeterPlugin

Host Application Code (Simplified conceptual flow):

// ... within host application ...
// Assume a PluginRegistry struct exists
type PluginRegistry struct {
	plugins map[string]Plugin
}

func NewPluginRegistry() *PluginRegistry {
	return &PluginRegistry{plugins: make(map[string]Plugin)}
}

func (pr *PluginRegistry) Register(p Plugin) error {
	if _, exists := pr.plugins[p.Name()]; exists {
		return fmt.Errorf("plugin with name '%s' already registered", p.Name())
	}
	pr.plugins[p.Name()] = p
	return nil
}

func (pr *PluginRegistry) Get(name string) (Plugin, error) {
	p, ok := pr.plugins[name]
	if !ok {
		return nil, fmt.Errorf("plugin '%s' not found", name)
	}
	return p, nil
}

// ... in main() or a setup function ...
registry := NewPluginRegistry()
pluginDir := "./plugins" // Assume Greeter plugin is compiled into plugins/greeter.so

// Simulate loading and registering
// Actual loading would involve plugin.Open() and Symbol()
// For this example, assume successful loading and registration of GreeterPlugin

// Let's imagine the host application has a function to load and register
// that would iterate through .so files in pluginDir.
// For demonstration, we'll manually add it here.
// This manual registration simulates the effect of a successful plugin load.

// In a real scenario, you'd load a .so file, find the exported plugin instance,
// and then call registry.Register(exportedInstance)
// For this challenge, we'll assume the host can discover and instantiate plugins.

// --- Simulating plugin loading and registration ---
// This part needs to be implemented by you:
// 1. Scan pluginDir for .so files.
// 2. For each .so, use plugin.Open().
// 3. Use p.Lookup("Greeter") to get the exported plugin instance.
// 4. Type assert it to the Plugin interface.
// 5. Call registry.Register().

// For the sake of this *example's explanation*, let's say this is done.
// So now, our registry has the Greeter plugin.
// --- End of simulation ---

// Host application uses the plugin
inputData := "World"
greeterPlugin, err := registry.Get("Greeter")
if err != nil {
	// Handle error
	fmt.Printf("Error getting plugin: %v\n", err)
} else {
	output, err := greeterPlugin.Execute(inputData)
	if err != nil {
		fmt.Printf("Error executing plugin: %v\n", err)
	} else {
		fmt.Printf("Plugin Output: %s\n", output) // Expected: Plugin Output: Hello, World!
	}
}

Example 2:

Plugin Code (e.g., plugins/multiplier/multiplier.go):

package main

import (
	"fmt"
	"strconv"
	"plugin"
)

type Plugin interface {
	Name() string
	Execute(input string) (string, error)
}

func init() {
	fmt.Println("Multiplier plugin initializing...")
	// Simulated registration
	// registry.Register(&MultiplierPlugin{})
}

type MultiplierPlugin struct{}

func (p *MultiplierPlugin) Name() string {
	return "Multiplier"
}

func (p *MultiplierPlugin) Execute(input string) (string, error) {
	// Expecting input like "5" or "10"
	num, err := strconv.Atoi(input)
	if err != nil {
		return "", fmt.Errorf("invalid number format: %w", err)
	}
	result := num * 2
	return strconv.Itoa(result), nil
}

var Multiplier MultiplierPlugin

Host Application Code (Conceptual continuation from Example 1):

// ... assuming MultiplierPlugin is also loaded and registered in the registry ...

// Host application uses the plugin
inputData := "7"
multiplierPlugin, err := registry.Get("Multiplier")
if err != nil {
	fmt.Printf("Error getting plugin: %v\n", err)
} else {
	output, err := multiplierPlugin.Execute(inputData)
	if err != nil {
		fmt.Printf("Error executing plugin: %v\n", err) // Expected: Error executing plugin: invalid number format: strconv.Atoi: parsing "7": invalid syntax (if input is not properly handled in example) - CORRECTED: Expected: Plugin Output: 14
	} else {
		fmt.Printf("Plugin Output: %s\n", output) // Expected: Plugin Output: 14
	}
}

Output:
```
Plugin Output: 14
```
Explanation: The host application loads and registers the MultiplierPlugin. When its Execute method is called with "7", it parses the input, multiplies it by 2, and returns "14".

Example 3 (Edge Case): Invalid Plugin

Scenario: A file named invalid.so exists in the plugins/ directory, but it's not a valid Go plugin or doesn't export anything.

Host Application Behavior (Conceptual):

// ... in the plugin loading loop ...
pluginPath := filepath.Join(pluginDir, "invalid.so")
p, err := plugin.Open(pluginPath)
if err != nil {
	fmt.Printf("Failed to open plugin %s: %v\n", pluginPath, err) // Expected: Failed to open plugin ./plugins/invalid.so: ... (specific error)
	continue // Skip to the next file
}

// Try to find the exported symbol
symbol, err := p.Lookup("InvalidPluginInstance") // Assuming no such exported symbol
if err != nil {
	fmt.Printf("Failed to find symbol in plugin %s: %v\n", pluginPath, err) // Expected: Failed to find symbol in plugin ./plugins/invalid.so: ... (specific error)
	// Might close the plugin handle here if necessary
	continue // Skip
}

// ... further checks and registration would fail here ...

Output:

Failed to open plugin ./plugins/invalid.so: ... (e.g., "plugin was built without -buildmode=plugin")
Failed to find symbol in plugin ./plugins/invalid.so: ... (e.g., "plugin has no symbol named InvalidPluginInstance")

Explanation: The host application attempts to load invalid.so. The plugin.Open call fails because it's not a valid Go plugin. If it were a valid plugin but lacked the expected exported symbol, the Lookup call would fail. In either case, the host should log the error and continue processing other files.

Constraints

Plugins must be compiled as shared libraries (.so on Linux/macOS, .dll on Windows) using go build -buildmode=plugin.
The plugin directory will be named plugins/.
The host application must be able to load plugins from this directory.
Plugin names returned by Name() must be unique within a single application run.
The system should aim for reasonable performance in loading and executing plugins, especially if many plugins are present.

Notes

You will need to define the Plugin interface. It's good practice to have this interface defined in a separate package that both the host application and all plugins can import to ensure compatibility. For this challenge, you can define it within the host application code and ensure your plugin code mirrors it exactly.
The init() function in plugins is a good place to trigger registration, as it runs automatically when the plugin is loaded.
Consider using plugin.Open() to load the .so files and plugin.Symbol() to retrieve exported variables or functions.
Type assertion will be necessary to confirm that the loaded symbol conforms to your Plugin interface.
The plugin package in Go has some limitations, particularly around deeply nested types or complex dependencies. For this challenge, keep your plugin implementations relatively straightforward.
Error handling is paramount. A plugin that panics should not crash the host application.