Use X-Request-Id as canonical request identifier (if available)

If `X-Request-Id` is available in an HTTP request made against the
CA server, it'll be used as the identifier for the request. This
slightly changes the existing behavior, which relied on the custom
`X-Smallstep-Id` header, but usage of that header is currently not
very widespread, and `X-Request-Id` is more generally known for
the use case `X-Smallstep-Id` is used for.

`X-Smallstep-Id` is currently still considered, but it'll only be
used if `X-Request-Id` is not set.

logging/context.go

@@ -21,14 +21,27 @@ func NewRequestID() string {
 	return xid.New().String()
 }
 
-// RequestID returns a new middleware that gets the given header and sets it
-// in the context so it can be written in the logger. If the header does not
-// exists or it's the empty string, it uses github.com/rs/xid to create a new
-// one.
+// defaultRequestIDHeader is the header name used for propagating
+// request IDs. If available in an HTTP request, it'll be used instead
+// of the X-Smallstep-Id header.
+const defaultRequestIDHeader = "X-Request-Id"
+
+// RequestID returns a new middleware that obtains the current request ID
+// and sets it in the context. It first tries to read the request ID from
+// the "X-Request-Id" header. If that's not set, it tries to read it from
+// the provided header name. If the header does not exist or its value is
+// the empty string, it uses github.com/rs/xid to create a new one.
 func RequestID(headerName string) func(next http.Handler) http.Handler {
+	if headerName == "" {
+		headerName = defaultTraceIDHeader
+	}
 	return func(next http.Handler) http.Handler {
 		fn := func(w http.ResponseWriter, req *http.Request) {
-			requestID := req.Header.Get(headerName)
+			requestID := req.Header.Get(defaultRequestIDHeader)
+			if requestID == "" {
+				requestID = req.Header.Get(headerName)
+			}
+
 			if requestID == "" {
 				requestID = NewRequestID()
 				req.Header.Set(headerName, requestID)

logging/context_test.go

@@ -0,0 +1,94 @@
+package logging
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func newRequest(t *testing.T) *http.Request {
+	r, err := http.NewRequest(http.MethodGet, "https://example.com", http.NoBody)
+	require.NoError(t, err)
+	return r
+}
+
+func TestRequestID(t *testing.T) {
+	requestWithID := newRequest(t)
+	requestWithID.Header.Set("X-Request-Id", "reqID")
+	requestWithoutID := newRequest(t)
+	requestWithEmptyHeader := newRequest(t)
+	requestWithEmptyHeader.Header.Set("X-Request-Id", "")
+	requestWithSmallstepID := newRequest(t)
+	requestWithSmallstepID.Header.Set("X-Smallstep-Id", "smallstepID")
+
+	tests := []struct {
+		name       string
+		headerName string
+		handler    http.HandlerFunc
+		req        *http.Request
+	}{
+		{
+			name:       "default-request-id",
+			headerName: defaultTraceIDHeader,
+			handler: func(_ http.ResponseWriter, r *http.Request) {
+				assert.Empty(t, r.Header.Get("X-Smallstep-Id"))
+				assert.Equal(t, "reqID", r.Header.Get("X-Request-Id"))
+				reqID, ok := GetRequestID(r.Context())
+				if assert.True(t, ok) {
+					assert.Equal(t, "reqID", reqID)
+				}
+			},
+			req: requestWithID,
+		},
+		{
+			name:       "no-request-id",
+			headerName: "X-Request-Id",
+			handler: func(_ http.ResponseWriter, r *http.Request) {
+				assert.Empty(t, r.Header.Get("X-Smallstep-Id"))
+				value := r.Header.Get("X-Request-Id")
+				assert.NotEmpty(t, value)
+				reqID, ok := GetRequestID(r.Context())
+				if assert.True(t, ok) {
+					assert.Equal(t, value, reqID)
+				}
+			},
+			req: requestWithoutID,
+		},
+		{
+			name:       "empty-header-name",
+			headerName: "",
+			handler: func(_ http.ResponseWriter, r *http.Request) {
+				assert.Empty(t, r.Header.Get("X-Request-Id"))
+				value := r.Header.Get("X-Smallstep-Id")
+				assert.NotEmpty(t, value)
+				reqID, ok := GetRequestID(r.Context())
+				if assert.True(t, ok) {
+					assert.Equal(t, value, reqID)
+				}
+			},
+			req: requestWithEmptyHeader,
+		},
+		{
+			name:       "fallback-header-name",
+			headerName: defaultTraceIDHeader,
+			handler: func(_ http.ResponseWriter, r *http.Request) {
+				assert.Empty(t, r.Header.Get("X-Request-Id"))
+				assert.Equal(t, "smallstepID", r.Header.Get("X-Smallstep-Id"))
+				reqID, ok := GetRequestID(r.Context())
+				if assert.True(t, ok) {
+					assert.Equal(t, "smallstepID", reqID)
+				}
+			},
+			req: requestWithSmallstepID,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			h := RequestID(tt.headerName)
+			h(tt.handler).ServeHTTP(httptest.NewRecorder(), tt.req)
+		})
+	}
+}