api: only set url field in config if previously unset

In #16872 we added support for configuring the API client with a unix domain
socket. In order to set the host correctly, we parse the address before mutating
the Address field in the configuration. But this prevents the configuration from
being reused across multiple clients, as the next time we parse the address it
will no longer be pointing to the socket. This breaks consumers like the
autoscaler, which reuse the API config between plugins.

Update the `NewClient` constructor to only override the `url` field if it hasn't
already been parsed. Include a test demonstrating safe reuse with a unix domain
socket.

Ref: hashicorp/nomad-autoscaler#944
Ref: hashicorp/nomad-autoscaler#945

.changelog/23785.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+api: Fixed a bug where an `api.Config` targeting a unix domain socket could not be reused between clients
+```

api/api.go

@@ -509,9 +509,13 @@ func NewClient(config *Config) (*Client, error) {
 	}
 
 	// we have to test the address that comes from DefaultConfig, because it
-	// could be the value of NOMAD_ADDR which is applied without testing
-	if config.url, err = url.Parse(config.Address); err != nil {
-		return nil, fmt.Errorf("invalid address '%s': %v", config.Address, err)
+	// could be the value of NOMAD_ADDR which is applied without testing. But
+	// only on the first use of this Config, otherwise we'll have mutated the
+	// address
+	if config.url == nil {
+		if config.url, err = url.Parse(config.Address); err != nil {
+			return nil, fmt.Errorf("invalid address '%s': %v", config.Address, err)
+		}
 	}
 
 	httpClient := config.HttpClient

api/api_test.go

@@ -11,9 +11,12 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"os"
+	"path"
 	"strings"
 	"testing"
 	"time"
@@ -595,3 +598,44 @@ func TestClient_autoUnzip(t *testing.T) {
 		Body:   io.NopCloser(&b),
 	}, nil)
 }
+
+func TestUnixSocketConfig(t *testing.T) {
+
+	td := os.TempDir() // testing.TempDir() on macOS makes paths that are too long
+	socketPath := path.Join(td, t.Name()+".sock")
+	os.Remove(socketPath) // git rid of stale ones now.
+
+	t.Cleanup(func() { os.Remove(socketPath) })
+
+	ts := httptest.NewUnstartedServer(
+		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Write([]byte(`"10.1.1.1"`))
+		}))
+
+	// Create a Unix domain socket and listen for incoming connections.
+	socket, err := net.Listen("unix", socketPath)
+	must.NoError(t, err)
+	t.Cleanup(func() { socket.Close() })
+
+	ts.Listener = socket
+	ts.Start()
+	defer ts.Close()
+
+	cfg := &Config{Address: "unix://" + socketPath}
+
+	client1, err := NewClient(cfg)
+	must.NoError(t, err)
+	t.Cleanup(client1.Close)
+
+	resp, err := client1.Status().Leader()
+	must.NoError(t, err)
+	must.Eq(t, "10.1.1.1", resp)
+
+	client2, err := NewClient(cfg)
+	must.NoError(t, err)
+	t.Cleanup(client2.Close)
+
+	_, err = client2.Status().Leader()
+	must.NoError(t, err)
+	must.Eq(t, "10.1.1.1", resp)
+}