Allow User-Agent header to be overwritten

This commit allows users to overwrite the `User-Agent` header. It will
still default to:

```
"elastic/"+Version+" ("+runtime.GOOS+"-"+runtime.GOARCH+")"
```

Close #1438

Author: olivere

client.go

@@ -14,6 +14,7 @@ import (
 	"net/http/httputil"
 	"net/url"
 	"os"
+	"runtime"
 	"strings"
 	"sync"
 	"time"
@@ -983,6 +984,10 @@ func (c *Client) sniffNode(ctx context.Context, url string) []*conn {
 	}
 	c.mu.RUnlock()
 
+	if req.Header.Get("User-Agent") == "" {
+		req.Header.Add("User-Agent", "elastic/"+Version+" ("+runtime.GOOS+"-"+runtime.GOARCH+")")
+	}
+
 	res, err := c.c.Do((*http.Request)(req).WithContext(ctx))
 	if err != nil {
 		return nodes
@@ -1126,6 +1131,9 @@ func (c *Client) healthcheck(parentCtx context.Context, timeout time.Duration, f
 					}
 				}
 			}
+			if req.Header.Get("User-Agent") == "" {
+				req.Header.Add("User-Agent", "elastic/"+Version+" ("+runtime.GOOS+"-"+runtime.GOARCH+")")
+			}
 			res, err := c.c.Do((*http.Request)(req).WithContext(ctx))
 			if res != nil {
 				status = res.StatusCode
@@ -1387,11 +1395,9 @@ func (c *Client) PerformRequest(ctx context.Context, opt PerformRequestOptions)
 		if opt.ContentType != "" {
 			req.Header.Set("Content-Type", opt.ContentType)
 		}
-		if len(opt.Headers) > 0 {
-			for key, value := range opt.Headers {
-				for _, v := range value {
-					req.Header.Add(key, v)
-				}
+		for key, value := range opt.Headers {
+			for _, v := range value {
+				req.Header.Add(key, v)
 			}
 		}
 		if len(defaultHeaders) > 0 {
@@ -1401,6 +1407,9 @@ func (c *Client) PerformRequest(ctx context.Context, opt PerformRequestOptions)
 				}
 			}
 		}
+		if req.Header.Get("User-Agent") == "" {
+			req.Header.Set("User-Agent", "elastic/"+Version+" ("+runtime.GOOS+"-"+runtime.GOARCH+")")
+		}
 
 		// Set body
 		if opt.Body != nil {

client_test.go

@@ -17,6 +17,7 @@ import (
 	"net/url"
 	"reflect"
 	"regexp"
+	"runtime"
 	"strings"
 	"sync"
 	"testing"
@@ -1716,7 +1717,40 @@ func TestPerformRequestWithCustomHTTPHeadersOnClient(t *testing.T) {
 	}
 	// Request-level headers have preference
 	if want, have := "123456", res.Header.Get("X-Opaque-Id"); want != have {
-		t.Fatalf("want response header X-Opaque-Id=%q, have %q", want, have)
+		t.Fatalf("want X-Opaque-Id=%q, have %q", want, have)
+	}
+}
+
+func TestPerformRequestSetsDefaultUserAgent(t *testing.T) {
+	var req *http.Request
+	h := func(r *http.Request) (*http.Response, error) {
+		req = new(http.Request)
+		*req = *r
+		return &http.Response{Request: r, StatusCode: http.StatusOK, Body: http.NoBody}, nil
+	}
+	tr := &failingTransport{path: "/", fail: h}
+	httpClient := &http.Client{Transport: tr}
+
+	client, err := NewClient(SetHttpClient(httpClient), SetSniff(false), SetHealthcheck(false))
+	if err != nil {
+		t.Fatal(err)
+	}
+	res, err := client.PerformRequest(context.TODO(), PerformRequestOptions{
+		Method: "GET",
+		Path:   "/",
+		Params: url.Values{
+			"pretty": []string{"true"},
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res == nil {
+		t.Fatal("expected response to be != nil")
+	}
+	// Have a default for User-Agent
+	if want, have := "elastic/"+Version+" ("+runtime.GOOS+"-"+runtime.GOARCH+")", req.Header.Get("User-Agent"); want != have {
+		t.Fatalf("want User-Agent=%q, have %q", want, have)
 	}
 }
 
@@ -1732,6 +1766,7 @@ func TestPerformRequestWithCustomHTTPHeadersPriority(t *testing.T) {
 
 	client, err := NewClient(SetHttpClient(httpClient), SetHeaders(http.Header{
 		"Custom-Id":   []string{"olivere"},
+		"User-Agent":  []string{"My user agent"},
 		"X-Opaque-Id": []string{"sandra"}, // <- will be overridden by request-level header
 	}), SetSniff(false), SetHealthcheck(false))
 	if err != nil {
@@ -1758,13 +1793,16 @@ func TestPerformRequestWithCustomHTTPHeadersPriority(t *testing.T) {
 		t.Fatal("expected to record HTTP request")
 	}
 	if want, have := "123456", req.Header.Get("X-Opaque-Id"); want != have {
-		t.Fatalf("want HTTP header X-Opaque-Id=%q, have %q", want, have)
+		t.Fatalf("want X-Opaque-Id=%q, have %q", want, have)
 	}
 	if want, have := "olivere", req.Header.Get("Custom-Id"); want != have {
-		t.Fatalf("want HTTP header Custom-Id=%q, have %q", want, have)
+		t.Fatalf("want Custom-Id=%q, have %q", want, have)
+	}
+	if want, have := "My user agent", req.Header.Get("User-Agent"); want != have {
+		t.Fatalf("want User-Agent=%q, have %q", want, have)
 	}
 	if want, have := "somewhat", req.Header.Get("X-Somewhat"); want != have {
-		t.Fatalf("want HTTP header X-Somewhat=%q, have %q", want, have)
+		t.Fatalf("want X-Somewhat=%q, have %q", want, have)
 	}
 }
 

ping.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/url"
+	"runtime"
 	"strings"
 )
 
@@ -160,11 +161,9 @@ func (s *PingService) Do(ctx context.Context) (*PingResult, int, error) {
 	if err != nil {
 		return nil, 0, err
 	}
-	if len(s.headers) > 0 {
-		for key, values := range s.headers {
-			for _, v := range values {
-				req.Header.Add(key, v)
-			}
+	for key, values := range s.headers {
+		for _, v := range values {
+			req.Header.Add(key, v)
 		}
 	}
 	if len(defaultHeaders) > 0 {
@@ -179,6 +178,10 @@ func (s *PingService) Do(ctx context.Context) (*PingResult, int, error) {
 		req.SetBasicAuth(basicAuthUsername, basicAuthPassword)
 	}
 
+	if req.Header.Get("User-Agent") == "" {
+		req.Header.Add("User-Agent", "elastic/"+Version+" ("+runtime.GOOS+"-"+runtime.GOARCH+")")
+	}
+
 	res, err := s.client.c.Do((*http.Request)(req).WithContext(ctx))
 	if err != nil {
 		return nil, 0, err

request.go

@@ -11,7 +11,6 @@ import (
 	"io"
 	"io/ioutil"
 	"net/http"
-	"runtime"
 	"strings"
 )
 
@@ -24,7 +23,6 @@ func NewRequest(method, url string) (*Request, error) {
 	if err != nil {
 		return nil, err
 	}
-	req.Header.Add("User-Agent", "elastic/"+Version+" ("+runtime.GOOS+"-"+runtime.GOARCH+")")
 	req.Header.Add("Accept", "application/json")
 	req.Header.Set("Content-Type", "application/json")
 	return (*Request)(req), nil

request_test.go

@@ -14,11 +14,14 @@ func TestRequestSetContentType(t *testing.T) {
 		t.Fatal(err)
 	}
 	if want, have := "application/json", req.Header.Get("Content-Type"); want != have {
-		t.Fatalf("want %q, have %q", want, have)
+		t.Fatalf("want Content-Type=%q, have %q", want, have)
 	}
 	req.Header.Set("Content-Type", "application/x-ndjson")
 	if want, have := "application/x-ndjson", req.Header.Get("Content-Type"); want != have {
-		t.Fatalf("want %q, have %q", want, have)
+		t.Fatalf("want Content-Type=%q, have %q", want, have)
+	}
+	if want, have := "", req.Header.Get("User-Agent"); want != have {
+		t.Fatalf("want User-Agent=%q, have %q", want, have)
 	}
 }