Verbose errors on lookup error

Author: Mateusz Gajewski

README.md

@@ -9,7 +9,7 @@
 
 **go-bind-plugin** generates neat API around symbols exported by a plugin built with `go build -buildmode=plugin` in upcoming go 1.8. [plugin.Plugin](https://tip.golang.org/pkg/plugin/#Plugin) holds information about exported symbols as map[string]interface{}. go-bind-plugins uses reflection to find out actual types of symbols and generates typed API for a provided plugin with additional functionalities (like dereferencing exported variables and checking sha256 sum). Basic usage does not require plugin sources as wrapper can be generated using only `.so` file.
 
-For example if plugin exports `AddTwoInts(a, b int) int` function instead of using [Plugin.Lookup](https://tip.golang.org/pkg/plugin/#Plugin.Lookup) directly:
+For example if plugin exports `AddTwoInts(a, b int) int` function and `BuildVersion string` variable instead of using [Plugin.Lookup](https://tip.golang.org/pkg/plugin/#Plugin.Lookup) directly:
 
 ```
 plug, err := plugin.Open("plugin.so")
@@ -25,6 +25,19 @@ if err != nil {
 
 if typed, ok := symbol.(func(int, int) int); ok {
   result := typed(10, 20)
+} else {
+  panic("AddTwoInts has different type than exported by plugin")
+}
+
+symbol, err := plug.Lookup("BuildVersion")
+if err != nil {
+  panic(err)
+}
+
+if typed, ok := symbol.(*string); ok {
+  fmt.Println(*typed)
+} else {
+  panic("BuildVersion is not a string reference")
 }
 ```
 
@@ -38,6 +51,7 @@ if err != nil {
 }
 
 result := plug.AddTwoInts(10, 20)
+fmt.Println(plug.BuildVersion) // or fmt.Println(*plug.BuildVersion) if -dereference-vars is not used
 ```
 
 `BingPluginAPI()` will ensure that plugin exports `AddTwoInts` functions and its type is `func(int, int) int`.
@@ -47,11 +61,8 @@ result := plug.AddTwoInts(10, 20)
 ```
 go get -u github.com/wendigo/go-bind-plugin
 go-bind-plugin -help
-```
 
-Available flags:
 
-```
 Usage of go-bind-plugin:
   -dereference-vars
     	Dereference plugin variables

cli/template.go

@@ -96,24 +96,24 @@ func Bind{{.Config.OutputName}}(path string) (*{{.Config.OutputName}}, error) {
     {{range .Plugin.Functions}}
     func{{ .Name }}, err := p.Lookup("{{ .Name }}")
     if err != nil {
-      return nil, err
+      return nil, fmt.Errorf("Could not import function '{{ .Name }}', symbol not found: %s", err)
     }
 
     if typed, ok := func{{ .Name }}.({{ .Signature }}); ok {
       ret._{{ .Name }} = typed
     } else {
-      return nil, fmt.Errorf("Could not export function '{{ .Name }}', incompatible types '{{ .Signature }}' and '%s'", reflect.TypeOf(func{{ .Name }}))
+      return nil, fmt.Errorf("Could not import function '{{ .Name }}', incompatible types '{{ .Signature }}' and '%s'", reflect.TypeOf(func{{ .Name }}))
     }
     {{end}}{{range .Plugin.Variables}}
     var{{ .Name }}, err := p.Lookup("{{ .Name }}")
     if err != nil {
-      return nil, err
+      return nil, fmt.Errorf("Could not import variable '{{ .Name }}', symbol not found: %s", err)
     }
 
     if typed, ok := var{{ .Name }}.(*{{.Signature}}); ok {
       ret.{{ .Name }} = {{if $useVarReference|not}}*{{end}}typed
     } else {
-      return nil, fmt.Errorf("Could not export variable '{{ .Name }}', incompatible types '{{ .Signature }}' and '%s'", reflect.TypeOf(var{{ .Name }}))
+      return nil, fmt.Errorf("Could not import variable '{{ .Name }}', incompatible types '{{ .Signature }}' and '%s'", reflect.TypeOf(var{{ .Name }}))
     }
     {{end}}
     return ret, nil