Generate host functions for using as exportable library or SDK (#35)

This PR makes it possible to generate a plugin consisting only of host functions that can be built into other plugins. Allows to create and distribute resusable SDK/library solutions that are compatible with this plugin.

Signed-off-by: Dmitry Volodin <[email protected]>

examples/host-function-library/README.md

@@ -0,0 +1,106 @@
+# Register Host Function Library Example
+This example shows how to embed functions defined in a host into plugins to expand the plugin functionalities.
+Host functions can be defined either in the current plugin distribution in the proto file or imported from another module defined as a source for these host functions.
+This allows host functions to be distributed as a library or SDK instead of having to copy them every time you create plugins.
+
+## Generate Go code for distributed host functions
+A proto file is under `library/json-parser/export`.
+
+```protobuf
+// Distributing host functions without plugin code
+// go:plugin type=host module=json-parser
+service ParserLibrary {
+  rpc ParseJson(ParseJsonRequest) returns (ParseJsonResponse) {}
+}
+```
+
+> **_NOTE:_** You must specify `type=host` in the comment `module=json-parser` and module name to be unique for the host function redistribution and registration.
+It represents the service is for host functions.
+
+Then, generate source code for `json-parser` module hosts functions.
+
+```shell
+$ protoc --go-plugin_out=. --go-plugin_opt=paths=source_relative library/json-parser/export/library.proto 
+```
+
+## Implement `json-parser` module host functions
+The following interface is generated.
+
+```go
+// Distributing host functions without plugin code
+// go:plugin type=host module=json-parser
+type ParserLibrary interface {
+	ParseJson(context.Context, *ParseJsonRequest) (*ParseJsonResponse, error)
+}
+```
+
+Implement that interface in separate package which will be exported to the main plugin implementation.
+
+```go
+// ParserLibraryImpl implements export.ParserLibrary functions
+type ParserLibraryImpl struct{}
+
+// ParseJson is embedded into the plugin and can be called by the plugin.
+func (ParserLibraryImpl) ParseJson(_ context.Context, request *export.ParseJsonRequest) (*export.ParseJsonResponse, error) {
+    var person export.Person
+    if err := json.Unmarshal(request.GetContent(), &person); err != nil {
+        return nil, err
+    }
+
+    return &export.ParseJsonResponse{Response: &person}, nil
+}
+```
+
+Then, generate source code stubs for the base plugin.
+
+```shell
+$ protoc --go-plugin_out=. --go-plugin_opt=paths=source_relative proto/greer.proto 
+```
+
+Register exported hosts functions module int while providing new WazeroRuntime in the `proto.NewGreeterPlugin()`.
+
+```go
+ctx := context.Background()
+p, err := proto.NewGreeterPlugin(ctx, proto.WazeroRuntime(func(ctx context.Context) (wazero.Runtime, error) {
+    r, err := proto.DefaultWazeroRuntime()(ctx)
+    if err != nil {
+        return nil, err
+    }
+    return r, export.Instantiate(ctx, r, impl.ParserLibraryImpl{})
+}))
+```
+
+## Call host functions in a plugin
+The exported as a library or SDK host functions can be called in plugins.
+
+```go
+parserLibrary := export.NewParserLibrary()
+
+// Call the host function to parse JSON
+resp, err := parserLibrary.ParseJson(ctx, &export.ParseJsonRequest{
+    Content: []byte(fmt.Sprintf(`{"name": "%s", "age": 20}`, sanrequest.Message)),
+})
+if err != nil {
+    return nil, err
+}
+```
+
+## Compile a plugin
+Use TinyGo to compile the plugin to Wasm.
+
+```shell
+$ go generate main.go
+```
+
+## Run
+`main.go` loads the above plugin.
+
+```shell
+$ go run main.go
+```
+```shll
+2022/08/28 10:13:57 Sending a HTTP request...
+Hello, Sato. This is Yamada-san (age 20).
+```
+
+

examples/host-function-library/library/json-parser/export/library.proto

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+syntax = "proto3";
+package host;
+
+option go_package = "github.com/knqyf263/go-plugin/examples/host-functions-library/library/json-parser/export";
+
+// Distributing host functions without plugin code
+// go:plugin type=host module=json-parser
+service ParserLibrary {
+  rpc ParseJson(ParseJsonRequest) returns (ParseJsonResponse) {}
+}
+
+message ParseJsonRequest {
+  bytes content = 1;
+}
+
+message ParseJsonResponse {
+  Person response = 1;
+}
+
+message Person {
+  string name = 1;
+  int64  age  = 2;
+}