loadDaemonCliConfig: explicitly set default host

Setting the default host was implicitly handled in `normalizeHosts`; if
no hosts were provided, an empty entry was added to the list of hosts,
then passed through to `opts.ParseHost`, which handled an empty value
to set the default location (depending on TLS being enabled or XDG paths
to be used (when running with RootlessKit).

This patch makes the logic less magical:

- If no hosts are configured by the user (through CLI-flags or daemon.json),
  then we append the default location.
- The handling for TLS listeners is open-coded for now; we need to decide
  if we want to preserve this behavior (magically using TCP instead of a
  unix-socket or named pipe if TLS is enabled).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/command/daemon.go

@@ -639,6 +639,30 @@ func loadDaemonCliConfig(opts *daemonOptions) (*config.Config, error) {
 		}
 	}
 
+	// TODO(thaJeztah): consider making empty strings an error. Existing behavior allowed for empty strings to be used as default, even if explicitly set (`dockerd -H ""`).
+	conf.Hosts = slices.DeleteFunc(conf.Hosts, func(h string) bool {
+		return strings.TrimSpace(h) == ""
+	})
+	if len(conf.Hosts) == 0 {
+		// Set the default host if no hosts are configured.
+		// TODO(thaJeztah) can set defaults in config.New() instead?
+		if conf.TLS != nil && *conf.TLS {
+			// If no host is configured, but the "--tls" flag is set, we
+			// default to using a TCP connection instead of a unix-socket
+			// or named pipe.
+			//
+			// See https://github.com/moby/moby/commit/0906195fbbd6f379c163b80f23e4c5a60bcfc5f0
+			conf.Hosts = append(conf.Hosts, dopts.DefaultTLSHost)
+		} else {
+			// Otherwise use the default unix-socket (Linux) or named pipe (Windows).
+			h, err := defaultAPISocketPath(honorXDG)
+			if err != nil {
+				return nil, err
+			}
+			conf.Hosts = append(conf.Hosts, h)
+		}
+	}
+
 	if err := normalizeHosts(conf); err != nil {
 		return nil, err
 	}
@@ -718,27 +742,37 @@ func loadDaemonCliConfig(opts *daemonOptions) (*config.Config, error) {
 	return conf, nil
 }
 
+// defaultAPISocketPath returns the default path for the Unix socket (Linux)
+// or named pipe (Windows).
+//
+// When running with rootlessKit, XDG dirs should be preferred, and the
+// default is to listen on an unprivileged socket in [XDG_RUNTIME_DIR].
+//
+// [XDG_RUNTIME_DIR]: https://specifications.freedesktop.org/basedir/0.8/#variables
+func defaultAPISocketPath(honorXDG bool) (string, error) {
+	if honorXDG {
+		runtimeDir, err := homedir.GetRuntimeDir()
+		if err != nil {
+			return "", err
+		}
+		return "unix://" + filepath.Join(runtimeDir, "docker.sock"), nil
+	}
+
+	// default unix-socket (Linux) or named pipe (Windows).
+	return dopts.DefaultHost, nil
+}
+
 // normalizeHosts normalizes the configured config.Hosts and removes duplicates.
 // It returns an error if it fails to parse a host.
 func normalizeHosts(cfg *config.Config) error {
+	if len(cfg.Hosts) == 0 {
+		return errors.New("no hosts specified")
+	}
+
 	hosts := slices.Clone(cfg.Hosts)
-	if len(hosts) == 0 {
-		// if no hosts are configured, create a single entry slice, so that the
-		// default is used.
-		//
-		// TODO(thaJeztah) implement a cleaner way for this; this depends on a
-		//                 side-effect of how we parse empty/partial hosts.
-		hosts = make([]string, 1)
-	}
-
-	useTLS := DefaultTLSValue
-	if cfg.TLS != nil {
-		useTLS = *cfg.TLS
-	}
-
 	for i, h := range hosts {
 		var err error
-		hosts[i], err = dopts.ParseHost(useTLS, honorXDG, h)
+		hosts[i], err = dopts.ParseDaemonHost(h)
 		if err != nil {
 			return err
 		}