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Create a Buf plugin – Tutorial

Buf's lint and breaking change checks come with pre-defined rules and categories that cover the vast majority of customer needs. However, organizations sometimes need to enforce different or additional rules. For these cases, you can create Buf plugins that work with the Buf checkers so you can integrate your own rules and categories into your workflows.

This tutorial demonstrates how to define your own rules in a Buf plugin and how to install and use it locally.

Prerequisites

  • Install the Buf CLI or update your existing version to >=1.40
  • Clone the buf-tour repo: 
    $ git clone git@github.com:bufbuild/buf-tour.git

This tutorial shows how to write a Buf plugin in Go, taking advantage of the bufplugin-go library, but you can write them in any language as long as the plugin conforms to the Bufplugin framework.

Inspect the workspace

In this tutorial, we provide you with a sample Protobuf module, so start in its directory:

$ cd buf-tour/start/tutorial-create-buf-plugin

It contains a buf.yaml and a pet.proto with definitions related to a pet store:

.
├── buf.yaml
└── proto
    └── pet
        └── v1
            └── pet.proto

For the sake of this tutorial, pet.proto includes an undesirable naming style:

~/.../buf-tour/start/tutorial-create-buf-plugin/proto/pet/v1/pet.proto
service PetStoreService {
  rpc GetPetMethod(GetPetRequest) returns (GetPetResponse) {}
}

Notice that GetPetMethod is an RPC method but shouldn't end with the word Method, so you'll write a lint plugin that checks for this style error.

Write a simple plugin

Run the following to scaffold the plugin:

$ go mod init plugin/tutorial
$ mkdir -p cmd/rpc-suffix
$ touch cmd/rpc-suffix/main.go

Copy and paste the following content into cmd/rpc-suffix/main.go:

~/.../buf-tour/start/tutorial-create-buf-plugin/cmd/rpc-suffix/main.go
package main

import (
    "context"

    "buf.build/go/bufplugin/check"
    "buf.build/go/bufplugin/check/checkutil"
    "google.golang.org/protobuf/reflect/protoreflect"
)

var (
    rpcSuffixRuleSpec = &check.RuleSpec{
        ID:      "RPC_SUFFIX",
        Default: true,
        Purpose: "Checks that RPC names don't end with an illegal suffix.",
        Type:    check.RuleTypeLint,
        Handler: checkutil.NewMethodRuleHandler(checkRPCSuffix, checkutil.WithoutImports()),
    }
)

func main() {
    check.Main(&check.Spec{
        Rules: []*check.RuleSpec{
            rpcSuffixRuleSpec,
        },
    })
}

func checkRPCSuffix(
    _ context.Context,
    responseWriter check.ResponseWriter,
    _ check.Request,
    methodDescriptor protoreflect.MethodDescriptor,
) error {
    responseWriter.AddAnnotation(
        check.WithMessage("hello world"),
    )
    return nil
}

The plugin's main.go file imports the SDK and has three components:

  • An rpcSuffixRuleSpec definition, which defines the lint rule with fields like its ID and purpose.
  • A main function that calls check.Main, which creates a fully functional plugin function using the lint rule you just defined.
  • A checkRPCSuffix handler function that will contain our linting logic but for now just returns "hello world" regardless of the Protobuf file's content.

To see the plugin in action, first install its binary:

$ go mod tidy
$ go install ./cmd/rpc-suffix

then add the plugin and its rule ID to the buf.yaml config file:

 version: v2
 modules:
   - path: proto
     name: buf.build/tutorials/breaking
 lint:
   use:
     - STANDARD
+    - RPC_SUFFIX
+ plugins:
+   - plugin: rpc-suffix

You can now verify that the new rule is being checked when you lint:

$ buf lint
<input>:1:1:hello world (rpc-suffix)

With the dummy plugin working, you now need to add the rule logic:

package main

import (
    "context"
+   "strings"
+
    "buf.build/go/bufplugin/check"
    "buf.build/go/bufplugin/check/checkutil"
    "google.golang.org/protobuf/reflect/protoreflect"
)

+const forbiddenRPCSuffix = "Method"
+
var (
    rpcSuffixRuleSpec = &check.RuleSpec{
        ID:      "RPC_SUFFIX",
        Default: true,
        Purpose: "Checks that RPC names don't end with an illegal suffix.",
        Type:    check.RuleTypeLint,
        Handler: checkutil.NewMethodRuleHandler(checkRPCSuffix, checkutil.WithoutImports()),
    }
)

func main() {
    check.Main(&check.Spec{
        Rules: []*check.RuleSpec{
                rpcSuffixRuleSpec,
        },
    })
}

func checkRPCSuffix(
    _ context.Context,
    responseWriter check.ResponseWriter,
    _ check.Request,
    methodDescriptor protoreflect.MethodDescriptor,
) error {
-   responseWriter.AddAnnotation(
-       check.WithMessage("hello world"),
-   )
+   methodName := string(methodDescriptor.Name())
+   if strings.HasSuffix(methodName, forbiddenRPCSuffix) {
+       responseWriter.AddAnnotation(
+           check.WithDescriptor(methodDescriptor),
+           check.WithMessagef("method name should not end with %q", forbiddenRPCSuffix),
+       )
+   }
    return nil
}

Now the Buf linter only prints error when a method ends with the keyword Method. In addition, the plugin passes the check.WithDescriptor(methodDescriptor) option for printing, which provides location information about where the error happened. To verify this, re-install and run the plugin:

$ go install ./cmd/rpc-suffix
$ buf lint

proto/pet/v1/pet.proto:6:3:method name should not end with "Method" (rpc-suffix)

Now that you've implemented the rule logic, let's look at how to make a rule configurable.

Add options to a rule

Instead of hard-coding the check against the Method suffix, suppose you want the user to be able to set which suffixes to check for. You can enable this by making the plugin configurable from the buf.yaml config file. From the user's perspective, it looks like this:

 version: v2
 modules:
   - path: proto
     name: buf.build/tutorials/lint-plugin
 lint:
   use:
     - STANDARD
     - RPC_SUFFIX
 plugins:
   - plugin: rpc-suffix
+    options:
+      forbidden_rpc_suffixes:
+        - Method
+        - RPC

Plugin options are key-value pairs, so this configuration passes the forbidden_rpc_suffixes key and its values ["Method", "RPC"] to the plugin. To enable the plugin to interpret the option, you remove the hard-coded value and add the key in its place, then check to see if the user has configured it:

package main

import (
    "context"
    "strings"

    "buf.build/go/bufplugin/check"
    "buf.build/go/bufplugin/check/checkutil"
+   "buf.build/go/bufplugin/option"
    "google.golang.org/protobuf/reflect/protoreflect"
)

-const forbiddenRPCSuffix = "Method"
+const (
+   defaultForbiddenRPCSuffix     = "Method"
+   forbiddenRPCSuffixesOptionKey = "forbidden_rpc_suffixes"
+)

var (
    rpcSuffixRuleSpec = &check.RuleSpec{
        ID:      "RPC_SUFFIX",
        Default: true,
        Purpose: "Checks that RPC names don't end with an illegal suffix.",
        Type:    check.RuleTypeLint,
        Handler: checkutil.NewMethodRuleHandler(checkRPCSuffix, checkutil.WithoutImports()),
    }
)

func main() {
    check.Main(&check.Spec{
        Rules: []*check.RuleSpec{
            rpcSuffixRuleSpec,
        },
    })
}

func checkRPCSuffix(
    _ context.Context,
    responseWriter check.ResponseWriter,
-   _ check.Request,
+   request check.Request,
    methodDescriptor protoreflect.MethodDescriptor,
) error {
    methodName := string(methodDescriptor.Name())
-   if strings.HasSuffix(methodName, forbiddenRPCSuffix) {
-       responseWriter.AddAnnotation(
-           check.WithDescriptor(methodDescriptor),
-           check.WithMessagef("method name should not end with %q", forbiddenRPCSuffix),
-       )
+   forbiddenRPCSuffixes, err := option.GetStringSliceValue(request.Options(), forbiddenRPCSuffixesOptionKey)
+   if err != nil {
+       return err
+   }
+   if len(forbiddenRPCSuffixes) == 0 {
+       forbiddenRPCSuffixes = append(forbiddenRPCSuffixes, defaultForbiddenRPCSuffix)
+   }
+   for _, forbiddenRPCSuffix := range forbiddenRPCSuffixes {
+       if strings.HasSuffix(methodName, forbiddenRPCSuffix) {
+           responseWriter.AddAnnotation(
+               check.WithDescriptor(methodDescriptor),
+               check.WithMessagef("method name should not end with %q", forbiddenRPCSuffix),
+           )
+       }
    }
    return nil
}

Now the plugin uses option.GetStringSliceValue(request.Options(), forbiddenRPCSuffixesOptionKey) to check whether the option is specified by the users and use its list of values if so. To verify that it uses the option values, re-install and run the plugin:

$ go install ./cmd/rpc-suffix
$ buf lint

proto/pet/v1/pet.proto:6:3:method name should not end with "Method" (rpc-suffix)
proto/pet/v1/pet.proto:7:3:method name should not end with "RPC" (rpc-suffix)

Add a breaking change rule to the same plugin

Buf plugins can support breaking change rules in addition to lint rules. Adding a breaking change rule to your plugin is almost identical to adding a lint rule, but there are three key differences:

  • A breaking change checking function requires an additional descriptor prefixed with against because it compares the input against another set of files.
  • Checking functions for breaking change rules should be passed to checkutil.NewFieldPairRuleHandler handler helpers instead of checkutil.NewMethodRuleHandler.
  • The checking function's Type should be set to type should check.RuleTypeBreaking.

For example, this rule checks that a field option stays set to True in both versions of a file:

fieldOptionSafeForMLStaysTrueRuleSpec = &check.RuleSpec{
    ...
    Type:    check.RuleTypeBreaking,
    Handler: checkutil.NewFieldPairRuleHandler(checkFieldOptionSafeForMLStaysTrue, checkutil.WithoutImports()),
}

func checkFieldOptionSafeForMLStaysTrue(
    _ context.Context,
    responseWriter check.ResponseWriter,
    _ check.Request,
    fieldDescriptor protoreflect.FieldDescriptor,
    againstFieldDescriptor protoreflect.FieldDescriptor,
) error {

Default property

Each rule can set a Default boolean property that controls whether the rule is called by default if no rules are specified in buf.yaml. For example, adding it to the preceding example ensures that the field option is always checked when the plugin is present:

fieldOptionSafeForMLStaysTrueRuleSpec = &check.RuleSpec{
    Default: True,
    Type:    check.RuleTypeBreaking,
    Handler: checkutil.NewFieldPairRuleHandler(checkFieldOptionSafeForMLStaysTrue, checkutil.WithoutImports()),
}

Now that you've built your first Buf plugin, check out these docs for more detailed information on the Buf plugin framework and example code: