Add scripting (tengo) support for every incoming message (#731)

TengoModifyMessage allows you to specify the location of a tengo (https://github.com/d5/tengo/) script.
This script will receive every incoming message and can be used to modify the Username and the Text of that message.
The script will have the following global variables:
to modify: msgUsername and msgText
to read: msgChannel and msgAccount

The script is reloaded on every message, so you can modify the script on the fly.

Example script can be found in https://github.com/42wim/matterbridge/tree/master/gateway/bench.tengo
and https://github.com/42wim/matterbridge/tree/master/contrib/example.tengo

The example below will check if the text contains blah and if so, it'll replace the text and the username of that message.
text := import("text")
if text.re_match("blah",msgText) {
    msgText="replaced by this"
    msgUsername="fakeuser"
}

More information about tengo on: https://github.com/d5/tengo/blob/master/docs/tutorial.md and
https://github.com/d5/tengo/blob/master/docs/stdlib.md

vendor/github.com/d5/tengo/compiler/compiler_module.go

@@ -0,0 +1,123 @@
+package compiler
+
+import (
+	"io/ioutil"
+	"strings"
+
+	"github.com/d5/tengo/compiler/ast"
+	"github.com/d5/tengo/compiler/parser"
+	"github.com/d5/tengo/objects"
+)
+
+func (c *Compiler) compileModule(expr *ast.ImportExpr) (*objects.CompiledFunction, error) {
+	compiledModule, exists := c.loadCompiledModule(expr.ModuleName)
+	if exists {
+		return compiledModule, nil
+	}
+
+	moduleName := expr.ModuleName
+
+	// read module source from loader
+	var moduleSrc []byte
+	if c.moduleLoader == nil {
+		// default loader: read from local file
+		if !strings.HasSuffix(moduleName, ".tengo") {
+			moduleName += ".tengo"
+		}
+
+		if err := c.checkCyclicImports(expr, moduleName); err != nil {
+			return nil, err
+		}
+
+		var err error
+		moduleSrc, err = ioutil.ReadFile(moduleName)
+		if err != nil {
+			return nil, c.errorf(expr, "module file read error: %s", err.Error())
+		}
+	} else {
+		if err := c.checkCyclicImports(expr, moduleName); err != nil {
+			return nil, err
+		}
+
+		var err error
+		moduleSrc, err = c.moduleLoader(moduleName)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	compiledModule, err := c.doCompileModule(moduleName, moduleSrc)
+	if err != nil {
+		return nil, err
+	}
+
+	c.storeCompiledModule(moduleName, compiledModule)
+
+	return compiledModule, nil
+}
+
+func (c *Compiler) checkCyclicImports(node ast.Node, moduleName string) error {
+	if c.moduleName == moduleName {
+		return c.errorf(node, "cyclic module import: %s", moduleName)
+	} else if c.parent != nil {
+		return c.parent.checkCyclicImports(node, moduleName)
+	}
+
+	return nil
+}
+
+func (c *Compiler) doCompileModule(moduleName string, src []byte) (*objects.CompiledFunction, error) {
+	modFile := c.file.Set().AddFile(moduleName, -1, len(src))
+	p := parser.NewParser(modFile, src, nil)
+	file, err := p.ParseFile()
+	if err != nil {
+		return nil, err
+	}
+
+	symbolTable := NewSymbolTable()
+
+	// inherit builtin functions
+	for idx, fn := range objects.Builtins {
+		s, _, ok := c.symbolTable.Resolve(fn.Name)
+		if ok && s.Scope == ScopeBuiltin {
+			symbolTable.DefineBuiltin(idx, fn.Name)
+		}
+	}
+
+	// no global scope for the module
+	symbolTable = symbolTable.Fork(false)
+
+	// compile module
+	moduleCompiler := c.fork(modFile, moduleName, symbolTable)
+	if err := moduleCompiler.Compile(file); err != nil {
+		return nil, err
+	}
+
+	// add OpReturn (== export undefined) if export is missing
+	if !moduleCompiler.lastInstructionIs(OpReturnValue) {
+		moduleCompiler.emit(nil, OpReturn)
+	}
+
+	compiledFunc := moduleCompiler.Bytecode().MainFunction
+	compiledFunc.NumLocals = symbolTable.MaxSymbols()
+
+	return compiledFunc, nil
+}
+
+func (c *Compiler) loadCompiledModule(moduleName string) (mod *objects.CompiledFunction, ok bool) {
+	if c.parent != nil {
+		return c.parent.loadCompiledModule(moduleName)
+	}
+
+	mod, ok = c.compiledModules[moduleName]
+
+	return
+}
+
+func (c *Compiler) storeCompiledModule(moduleName string, module *objects.CompiledFunction) {
+	if c.parent != nil {
+		c.parent.storeCompiledModule(moduleName, module)
+	}
+
+	c.compiledModules[moduleName] = module
+}