HttpGetter: Add ability to set headers

This adds the ability to set headers that will be sent out on every
request of a particular HttpGetter. This is useful in situations where
query parameters are not suitable, such as when headers are explicitly
expected, or when one wants to move information off the query string
that would be at risk of possibly being exposed in logs or error
messages.

Fixes hashicorp#71.

get_http.go

@@ -41,6 +41,12 @@ type HttpGetter struct {
 	// Client is the http.Client to use for Get requests.
 	// This defaults to a cleanhttp.DefaultClient if left unset.
 	Client *http.Client
+
+	// Header contains optional request header fields that should be included
+	// with every HTTP request. Note that the zero value of this field is nil,
+	// and as such it needs to be initialized before use, via something like
+	// make(http.Header).
+	Header http.Header
 }
 
 func (g *HttpGetter) ClientMode(u *url.URL) (ClientMode, error) {
@@ -72,10 +78,17 @@ func (g *HttpGetter) Get(dst string, u *url.URL) error {
 	u.RawQuery = q.Encode()
 
 	// Get the URL
-	resp, err := g.Client.Get(u.String())
+	req, err := http.NewRequest("GET", u.String(), nil)
+	if err != nil {
+		return err
+	}
+
+	req.Header = g.Header
+	resp, err := g.Client.Do(req)
 	if err != nil {
 		return err
 	}
+
 	defer resp.Body.Close()
 	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
 		return fmt.Errorf("bad response code: %d", resp.StatusCode)
@@ -118,10 +131,17 @@ func (g *HttpGetter) GetFile(dst string, u *url.URL) error {
 		g.Client = httpClient
 	}
 
-	resp, err := g.Client.Get(u.String())
+	req, err := http.NewRequest("GET", u.String(), nil)
 	if err != nil {
 		return err
 	}
+
+	req.Header = g.Header
+	resp, err := g.Client.Do(req)
+	if err != nil {
+		return err
+	}
+
 	defer resp.Body.Close()
 	if resp.StatusCode != 200 {
 		return fmt.Errorf("bad response code: %d", resp.StatusCode)

get_http_test.go

@@ -40,6 +40,34 @@ func TestHttpGetter_header(t *testing.T) {
 	}
 }
 
+func TestHttpGetter_requestHeader(t *testing.T) {
+	ln := testHttpServer(t)
+	defer ln.Close()
+
+	g := new(HttpGetter)
+	g.Header = make(http.Header)
+	g.Header.Add("X-Foobar", "foobar")
+	dst := tempDir(t)
+	defer os.RemoveAll(dst)
+
+	var u url.URL
+	u.Scheme = "http"
+	u.Host = ln.Addr().String()
+	u.Path = "/expect-header"
+	u.RawQuery = "expected=X-Foobar"
+
+	// Get it!
+	if err := g.GetFile(dst, &u); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	// Verify the main file exists
+	if _, err := os.Stat(dst); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	assertContents(t, dst, "Hello\n")
+}
+
 func TestHttpGetter_meta(t *testing.T) {
 	ln := testHttpServer(t)
 	defer ln.Close()
@@ -255,6 +283,7 @@ func testHttpServer(t *testing.T) net.Listener {
 	}
 
 	mux := http.NewServeMux()
+	mux.HandleFunc("/expect-header", testHttpHandlerExpectHeader)
 	mux.HandleFunc("/file", testHttpHandlerFile)
 	mux.HandleFunc("/header", testHttpHandlerHeader)
 	mux.HandleFunc("/meta", testHttpHandlerMeta)
@@ -269,6 +298,17 @@ func testHttpServer(t *testing.T) net.Listener {
 	return ln
 }
 
+func testHttpHandlerExpectHeader(w http.ResponseWriter, r *http.Request) {
+	if expected, ok := r.URL.Query()["expected"]; ok {
+		if r.Header.Get(expected[0]) != "" {
+			w.Write([]byte("Hello\n"))
+			return
+		}
+	}
+
+	w.WriteHeader(400)
+}
+
 func testHttpHandlerFile(w http.ResponseWriter, r *http.Request) {
 	w.Write([]byte("Hello\n"))
 }