plugin: new package for loading plugins

Includes a linux implementation.

Change-Id: Iacc2ed7da760ae9deebc928adf2b334b043b07ec
Reviewed-on: https://go-review.googlesource.com/27823
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/plugin/plugin_dlopen.go

@@ -0,0 +1,125 @@
+// 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.
+
+// +build linux,cgo
+
+package plugin
+
+/*
+#cgo linux LDFLAGS: -ldl
+#include <dlfcn.h>
+#include <limits.h>
+#include <stdlib.h>
+#include <stdint.h>
+
+static uintptr_t pluginOpen(const char* path, char** err) {
+	void* h = dlopen(path, RTLD_NOW|RTLD_GLOBAL);
+	if (h == NULL) {
+		*err = dlerror();
+	}
+	return (uintptr_t)h;
+}
+
+static void* pluginLookup(uintptr_t h, const char* name, char** err) {
+	void* r = dlsym((void*)h, name);
+	if (r == NULL) {
+		*err = dlerror();
+	}
+	return r;
+}
+*/
+import "C"
+
+import (
+	"errors"
+	"sync"
+	"unsafe"
+)
+
+func open(name string) (*Plugin, error) {
+	pluginsMu.Lock()
+	cRelName := C.CString(name)
+	cPath := C.realpath(cRelName, nil)
+	C.free(unsafe.Pointer(cRelName))
+	defer C.free(unsafe.Pointer(cPath))
+	path := C.GoString(cPath)
+	if p := plugins[path]; p != nil {
+		pluginsMu.Unlock()
+		<-p.loaded
+		return p, nil
+	}
+	var cErr *C.char
+	h := C.pluginOpen(cPath, &cErr)
+	if h == 0 {
+		pluginsMu.Unlock()
+		return nil, errors.New("plugin.Open: " + C.GoString(cErr))
+	}
+	// TODO(crawshaw): look for plugin note, confirm it is a Go plugin
+	// and it was built with the correct toolchain.
+	// TODO(crawshaw): get full plugin name from note.
+	if len(name) > 3 && name[len(name)-3:] == ".so" {
+		name = name[:len(name)-3]
+	}
+	syms := lastmoduleinit()
+	if plugins == nil {
+		plugins = make(map[string]*Plugin)
+	}
+	// This function can be called from the init function of a plugin.
+	// Drop a placeholder in the map so subsequent opens can wait on it.
+	p := &Plugin{
+		name:   name,
+		loaded: make(chan struct{}),
+		syms:   syms,
+	}
+	plugins[path] = p
+	pluginsMu.Unlock()
+
+	initStr := C.CString(name + ".init")
+	initFuncPC := C.pluginLookup(h, initStr, &cErr)
+	C.free(unsafe.Pointer(initStr))
+	if initFuncPC != nil {
+		initFuncP := &initFuncPC
+		initFunc := *(*func())(unsafe.Pointer(&initFuncP))
+		initFunc()
+	}
+
+	// Fill out the value of each plugin symbol.
+	for symName, sym := range syms {
+		isFunc := symName[0] == '.'
+		if isFunc {
+			delete(syms, symName)
+			symName = symName[1:]
+		}
+
+		cname := C.CString(name + "." + symName)
+		p := C.pluginLookup(h, cname, &cErr)
+		C.free(unsafe.Pointer(cname))
+		if p == nil {
+			return nil, errors.New("plugin.Open: could not find symbol " + symName + ": " + C.GoString(cErr))
+		}
+		valp := (*[2]unsafe.Pointer)(unsafe.Pointer(&sym))
+		if isFunc {
+			(*valp)[1] = unsafe.Pointer(&p)
+		} else {
+			(*valp)[1] = p
+		}
+		syms[symName] = sym
+	}
+	close(p.loaded)
+	return p, nil
+}
+
+func lookup(p *Plugin, symName string) (Symbol, error) {
+	if s := p.syms[symName]; s != nil {
+		return s, nil
+	}
+	return nil, errors.New("plugin: symbol " + symName + " not found in plugin " + p.name)
+}
+
+var (
+	pluginsMu sync.Mutex
+	plugins   map[string]*Plugin
+)
+
+func lastmoduleinit() map[string]interface{} // in package runtime