client: don't downgrade when failing to negotiate

Historically, the client would downgrade to API v1.24 when failing
to negotiate as this was the API version from before API-version
negotiation was introduced.

Given that those daemons are EOL and those API versions no longer
supported, we should not fall back to an older API version, and
just continue using the latest / current version.

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

client/client.go

@@ -313,19 +313,12 @@ func (cli *Client) ClientVersion() string {
 }
 
 // negotiateAPIVersion updates the version to match the API version from
-// the ping response. It falls back to the lowest version supported if the
-// API version is empty.
+// the ping response.
 //
 // It returns an error if version is invalid, or lower than the minimum
 // supported API version in which case the client's API version is not
 // updated, and negotiation is not marked as completed.
 func (cli *Client) negotiateAPIVersion(pingVersion string) error {
-	pingVersion = strings.TrimPrefix(pingVersion, "v")
-	if pingVersion == "" {
-		// TODO(thaJeztah): consider returning an error on empty value or not falling back; see https://github.com/moby/moby/pull/51119#discussion_r2413148487
-		pingVersion = MinAPIVersion
-	}
-
 	var err error
 	pingVersion, err = parseAPIVersion(pingVersion)
 	if err != nil {

client/client_test.go

@@ -300,12 +300,10 @@ func TestNegotiateAPIVersion(t *testing.T) {
 			expectedVersion: "1.51",
 		},
 		{
-			// client should downgrade to the last version before version
-			// negotiation was added (1.24) if the daemon does not report
-			// a version.
+			// client should not downgrade if the daemon didn't report a version.
 			doc:             "downgrade legacy",
 			pingVersion:     "",
-			expectedVersion: MinAPIVersion,
+			expectedVersion: MaxAPIVersion,
 		},
 		{
 			// client should not downgrade to the version reported by the daemon

client/ping.go

@@ -97,6 +97,11 @@ func (cli *Client) Ping(ctx context.Context, options PingOptions) (PingResult, e
 		return ping, nil
 	}
 
+	if ping.APIVersion == "" {
+		cli.setAPIVersion(MaxAPIVersion)
+		return ping, nil
+	}
+
 	return ping, cli.negotiateAPIVersion(ping.APIVersion)
 }