Add barebones example of a custom protoc plugin that writes a bash script

Author: ryapric

protobuf-grpc/protoc-gen-list-messages/.gitignore

@@ -0,0 +1 @@
+build/

protobuf-grpc/protoc-gen-list-messages/Makefile

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+.PHONY: build gen
+
+build:
+	@go build -o ./build/ .
+
+gen: build
+	@mkdir -p ./pb
+	@protoc \
+		--plugin=./build/protoc-gen-list-messages \
+		-I../proto \
+		--list-messages_out=./pb \
+		--list-messages_opt=paths=source_relative \
+		../proto/echo/v1/*.proto

protobuf-grpc/protoc-gen-list-messages/main.go

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+package main
+
+import "google.golang.org/protobuf/compiler/protogen"
+
+func main() {
+	opts := protogen.Options{}
+	opts.Run(func(gen *protogen.Plugin) error {
+		for _, f := range gen.Files {
+			if f.Generate {
+				generateFile(gen, f)
+			}
+		}
+		return nil
+	})
+}
+
+func generateFile(gen *protogen.Plugin, file *protogen.File) {
+	filename := file.GeneratedFilenamePrefix + "_list-messages.pb.sh"
+	g := gen.NewGeneratedFile(filename, file.GoImportPath)
+	g.P("#!/usr/bin/env bash")
+	g.P("set -euo pipefail")
+	g.P()
+	g.P("##########################################################")
+	g.P("# Code generated by protoc-gen-list-messages. DO NOT EDIT.")
+	g.P("##########################################################")
+	g.P()
+	g.P(`printf 'The messages defined in the provided proto file are:\n'`)
+
+	for _, msg := range file.Messages {
+		g.P(`printf '`, msg.Desc.FullName(), `\n'`)
+	}
+}

protobuf-grpc/protoc-gen-list-messages/pb/echo/v1/echo_list-messages.pb.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+##########################################################
+# Code generated by protoc-gen-list-messages. DO NOT EDIT.
+##########################################################
+
+printf 'The messages defined in the provided proto file are:\n'
+printf 'echo.v1.EchoRequest\n'
+printf 'echo.v1.EchoResponse\n'