plugin: resolve plugin import path issue

Resolve import paths to get plugin symbol prefixes.

Fixes #19534

Change-Id: Ic25d83e72465ba8f6be0337218a1627b5dc702dc
Reviewed-on: https://go-review.googlesource.com/40994
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

misc/cgo/testplugin/src/issue19534/main.go

+// Copyright 2017 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 main
+
+import "plugin"
+
+func main() {
+	p, err := plugin.Open("plugin.so")
+	if err != nil {
+		panic(err)
+	}
+
+	sym, err := p.Lookup("Foo")
+	if err != nil {
+		panic(err)
+	}
+	f := sym.(func() int)
+	if f() != 42 {
+		panic("expected f() == 42")
+	}
+}

misc/cgo/testplugin/src/issue19534/plugin.go

+// Copyright 2017 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 main
+
+func Foo() int {
+	return 42
+}

misc/cgo/testplugin/test.bash

@@ -16,7 +16,7 @@ goarch=$(go env GOARCH)
 
 function cleanup() {
 	rm -f plugin*.so unnamed*.so iface*.so
-	rm -rf host pkg sub iface issue18676
+	rm -rf host pkg sub iface issue18676 issue19534
 }
 trap cleanup EXIT
 
@@ -44,3 +44,9 @@ LD_LIBRARY_PATH=$(pwd) ./iface
 GOPATH=$(pwd) go build -buildmode=plugin -o plugin.so src/issue18676/plugin.go
 GOPATH=$(pwd) go build -o issue18676 src/issue18676/main.go
 timeout 10s ./issue18676
+
+# Test for issue 19534 - that we can load a plugin built in a path with non-alpha
+# characters
+GOPATH=$(pwd) go build -buildmode=plugin -ldflags='-pluginpath=issue.19534' -o plugin.so src/issue19534/plugin.go
+GOPATH=$(pwd) go build -o issue19534 src/issue19534/main.go
+./issue19534

src/plugin/plugin_dlopen.go

@@ -39,6 +39,47 @@ import (
 	"unsafe"
 )
 
+// avoid a dependency on strings
+func lastIndexByte(s string, c byte) int {
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}
+
+// pathToPrefix converts raw string to the prefix that will be used in the symbol
+// table. If modifying, modify the version in internal/obj/sym.go as well.
+func pathToPrefix(s string) string {
+	slash := lastIndexByte(s, '/')
+	// check for chars that need escaping
+	n := 0
+	for r := 0; r < len(s); r++ {
+		if c := s[r]; c <= ' ' || (c == '.' && r > slash) || c == '%' || c == '"' || c >= 0x7F {
+			n++
+		}
+	}
+
+	// quick exit
+	if n == 0 {
+		return s
+	}
+
+	// escape
+	const hex = "0123456789abcdef"
+	p := make([]byte, 0, len(s)+2*n)
+	for r := 0; r < len(s); r++ {
+		if c := s[r]; c <= ' ' || (c == '.' && r > slash) || c == '%' || c == '"' || c >= 0x7F {
+			p = append(p, '%', hex[c>>4], hex[c&0xF])
+		} else {
+			p = append(p, c)
+		}
+	}
+
+	return string(p)
+}
+
 func open(name string) (*Plugin, error) {
 	cPath := (*C.char)(C.malloc(C.PATH_MAX + 1))
 	defer C.free(unsafe.Pointer(cPath))
@@ -103,7 +144,7 @@ func open(name string) (*Plugin, error) {
 			delete(syms, symName)
 			symName = symName[1:]
 		}
-		cname := C.CString(pluginpath + "." + symName)
+		cname := C.CString(pathToPrefix(pluginpath) + "." + symName)
 		p := C.pluginLookup(h, cname, &cErr)
 		C.free(unsafe.Pointer(cname))
 		if p == nil {