feat: add --header-command flag (#9059)

This allows specifying a command to run that can output headers for
cases where users require dynamic headers (like to authenticate to their
VPN).

The primary use case is to add this flag in SSH configs created by the
VS Code plugin, although maybe config-ssh should do the same.

cli/root_test.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"runtime"
 	"strings"
 	"sync/atomic"
 	"testing"
@@ -72,20 +73,29 @@ func TestRoot(t *testing.T) {
 	t.Run("Header", func(t *testing.T) {
 		t.Parallel()
 
+		var url string
 		var called int64
 		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			atomic.AddInt64(&called, 1)
 			assert.Equal(t, "wow", r.Header.Get("X-Testing"))
 			assert.Equal(t, "Dean was Here!", r.Header.Get("Cool-Header"))
+			assert.Equal(t, "very-wow-"+url, r.Header.Get("X-Process-Testing"))
+			assert.Equal(t, "more-wow", r.Header.Get("X-Process-Testing2"))
 			w.WriteHeader(http.StatusGone)
 		}))
 		defer srv.Close()
+		url = srv.URL
 		buf := new(bytes.Buffer)
+		coderURLEnv := "$CODER_URL"
+		if runtime.GOOS == "windows" {
+			coderURLEnv = "%CODER_URL%"
+		}
 		inv, _ := clitest.New(t,
 			"--no-feature-warning",
 			"--no-version-warning",
 			"--header", "X-Testing=wow",
 			"--header", "Cool-Header=Dean was Here!",
+			"--header-command", "printf X-Process-Testing=very-wow-"+coderURLEnv+"'\\r\\n'X-Process-Testing2=more-wow",
 			"login", srv.URL,
 		)
 		inv.Stdout = buf
@@ -97,8 +107,8 @@ func TestRoot(t *testing.T) {
 	})
 }
 
-// TestDERPHeaders ensures that the client sends the global `--header`s to the
-// DERP server when connecting.
+// TestDERPHeaders ensures that the client sends the global `--header`s and
+// `--header-command` to the DERP server when connecting.
 func TestDERPHeaders(t *testing.T) {
 	t.Parallel()
 
@@ -129,8 +139,9 @@ func TestDERPHeaders(t *testing.T) {
 	// Inject custom /derp handler so we can inspect the headers.
 	var (
 		expectedHeaders = map[string]string{
-			"X-Test-Header": "test-value",
-			"Cool-Header":   "Dean was Here!",
+			"X-Test-Header":     "test-value",
+			"Cool-Header":       "Dean was Here!",
+			"X-Process-Testing": "very-wow",
 		}
 		derpCalled int64
 	)
@@ -159,9 +170,12 @@ func TestDERPHeaders(t *testing.T) {
 		"--no-version-warning",
 		"ping", workspace.Name,
 		"-n", "1",
+		"--header-command", "printf X-Process-Testing=very-wow",
 	}
 	for k, v := range expectedHeaders {
-		args = append(args, "--header", fmt.Sprintf("%s=%s", k, v))
+		if k != "X-Process-Testing" {
+			args = append(args, "--header", fmt.Sprintf("%s=%s", k, v))
+		}
 	}
 	inv, root := clitest.New(t, args...)
 	clitest.SetupConfig(t, client, root)