cmd/docker: make feature detection lazy again

Commit 20ba591b7f fixed incorrect feature
detection in the CLI, but introduced a regression; previously the "ping"
would only be executed if needed (see b39739123b),
but by not inlining the call to `ServerInfo()` would now always be called.

This patch inlines the code again to only execute the "ping" conditionally,
which allows it to be executed lazily (and omitted for commands that don't
require a daemon connection).

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

cmd/docker/docker.go

@@ -416,14 +416,21 @@ func areFlagsSupported(cmd *cobra.Command, details versionDetails) error {
 func areSubcommandsSupported(cmd *cobra.Command, details versionDetails) error {
 	// Check recursively so that, e.g., `docker stack ls` returns the same output as `docker stack`
 	for curr := cmd; curr != nil; curr = curr.Parent() {
+		// Important: in the code below, calls to "details.CurrentVersion()" and
+		// "details.ServerInfo()" are deliberately executed inline to make them
+		// be executed "lazily". This is to prevent making a connection with the
+		// daemon to perform a "ping" (even for commands that do not require a
+		// daemon connection).
+		//
+		// See commit b39739123b845f872549e91be184cc583f5b387c for details.
+
 		if cmdVersion, ok := curr.Annotations["version"]; ok && versions.LessThan(details.CurrentVersion(), cmdVersion) {
 			return fmt.Errorf("%s requires API version %s, but the Docker daemon API version is %s", cmd.CommandPath(), cmdVersion, details.CurrentVersion())
 		}
-		si := details.ServerInfo()
-		if ost, ok := curr.Annotations["ostype"]; ok && si.OSType != "" && ost != si.OSType {
-			return fmt.Errorf("%s is only supported on a Docker daemon running on %s, but the Docker daemon is running on %s", cmd.CommandPath(), ost, si.OSType)
+		if ost, ok := curr.Annotations["ostype"]; ok && details.ServerInfo().OSType != "" && ost != details.ServerInfo().OSType {
+			return fmt.Errorf("%s is only supported on a Docker daemon running on %s, but the Docker daemon is running on %s", cmd.CommandPath(), ost, details.ServerInfo().OSType)
 		}
-		if _, ok := curr.Annotations["experimental"]; ok && !si.HasExperimental {
+		if _, ok := curr.Annotations["experimental"]; ok && !details.ServerInfo().HasExperimental {
 			return fmt.Errorf("%s is only supported on a Docker daemon with experimental features enabled", cmd.CommandPath())
 		}
 	}