Set User-Agent header on both REST and graphql requests

In #256 we starting setting a custom User-Agent header on REST API
calls, to help us understand the ways our API is used. It looks like this:

    Terraform/ (+https://www.terraform.io) Terraform-Plugin-SDK/2.0.3 buildkite/0.15.0

This builds on that work and sets the same User-Agent on graphql API
requests as well.

It drops the unnescary use of golang.org/x/oauth2 to set a static bearer
token header, and uses the tripperware pattern [1] to set both headers we
care about:

1. the static bearer token for auth
2. the user agent

The same http.Client is used for both REST and graphql, so we'll get
identical behaviour on both APIs.

[1] https://dev.to/stevenacoffman/tripperwares-http-client-middleware-chaining-roundtrippers-3o00

buildkite/client.go

@@ -2,7 +2,6 @@ package buildkite
 
 import (
 	"bytes"
-	"context"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -11,7 +10,6 @@ import (
 
 	genqlient "github.com/Khan/genqlient/graphql"
 	"github.com/shurcooL/graphql"
-	"golang.org/x/oauth2"
 )
 
 // Client can be used to interact with the Buildkite API
@@ -21,7 +19,6 @@ type Client struct {
 	http         *http.Client
 	organization string
 	restUrl      string
-	userAgent    string
 }
 
 type clientConfig struct {
@@ -32,21 +29,55 @@ type clientConfig struct {
 	userAgent  string
 }
 
+type headerRoundTripper struct {
+	next   http.RoundTripper
+	Header http.Header
+}
+
 // NewClient creates a client to use for interacting with the Buildkite API
 func NewClient(config *clientConfig) *Client {
-	token := oauth2.StaticTokenSource(&oauth2.Token{AccessToken: config.apiToken})
-	httpClient := oauth2.NewClient(context.Background(), token)
+
+	// Setup a HTTP Client that can be used by all REST and graphql API calls,
+	// with suitable headers for authentication and user agent identification
+	var rt http.RoundTripper
+	rt = http.DefaultTransport
+	header := make(http.Header)
+	header.Set("Authorization", "Bearer "+config.apiToken)
+	header.Set("User-Agent", config.userAgent)
+	rt = newHeaderRoundTripper(rt, header)
+
+	httpClient := &http.Client{
+		Transport: rt,
+	}
 
 	return &Client{
 		graphql:      graphql.NewClient(config.graphqlURL, httpClient),
 		genqlient:    genqlient.NewClient(config.graphqlURL, httpClient),
 		http:         httpClient,
 		organization: config.org,
 		restUrl:      config.restURL,
-		userAgent:    config.userAgent,
 	}
 }
 
+func newHeaderRoundTripper(next http.RoundTripper, Header http.Header) *headerRoundTripper {
+	if next == nil {
+		next = http.DefaultTransport
+	}
+	return &headerRoundTripper{
+		next:   next,
+		Header: Header,
+	}
+}
+
+func (rt *headerRoundTripper) RoundTrip(req *http.Request) (resp *http.Response, err error) {
+	if rt.Header != nil {
+		for k, v := range rt.Header {
+			req.Header[k] = v
+		}
+	}
+	return rt.next.RoundTrip(req)
+}
+
 func (client *Client) makeRequest(method string, path string, postData interface{}, responseObject interface{}) error {
 	var bodyBytes io.Reader
 	if postData != nil {
@@ -64,8 +95,6 @@ func (client *Client) makeRequest(method string, path string, postData interface
 		return fmt.Errorf("failed to create request: %w", err)
 	}
 
-	req.Header.Add("User-Agent", client.userAgent)
-
 	resp, err := client.http.Do(req)
 	if err != nil {
 		return fmt.Errorf("failed to send request: %w", err)

go.mod

@@ -6,7 +6,6 @@ require (
 	github.com/Khan/genqlient v0.4.0
 	github.com/hashicorp/terraform-plugin-sdk/v2 v2.0.3
 	github.com/shurcooL/graphql v0.0.0-20181231061246-d48a9a75455f
-	golang.org/x/oauth2 v0.7.0
 )
 
 require (
@@ -17,6 +16,7 @@ require (
 	github.com/google/uuid v1.3.0 // indirect
 	github.com/googleapis/enterprise-certificate-proxy v0.2.3 // indirect
 	github.com/klauspost/compress v1.15.11 // indirect
+	golang.org/x/oauth2 v0.7.0 // indirect
 	golang.org/x/xerrors v0.0.0-20220907171357-04be3eba64a2 // indirect
 )