fleet: require a minimum version of the peer at the handshake [CI]

MoShiro__MinAgentVersion makes the backend refuse agents below it at hello (fleet_version_required); fleet.min_server_version or MOSHIRO_MIN_SERVER_VERSION makes the agent drop a backend below it. Either side waits five seconds and shakes hands again, because the other one may be updating.

backend/fleet/src/Fleet.Link/FleetLink.cs

@@ -52,6 +52,9 @@ public sealed class FleetLinkOptions
 
     public string NodeVersion { get; init; } = "0.0.0";
 
+    /// <summary>服务端至少要是哪个版本（每次握手重新取）；空表示不要求。不够就关掉这条连接，等 <see cref="VersionRetry"/> 再握手，服务端可能正在更新。</summary>
+    public Func<string?>? MinServerVersion { get; init; }
+
     /// <summary>框架之外的能力，例如 <c>moshiro.v1</c> 与 <c>moshiro.kinds:udp_echo,tcp_echo</c>。</summary>
     public IReadOnlyList<string> Capabilities { get; init; } = [];
 
@@ -100,7 +103,11 @@ public sealed class FleetLink
     private volatile RpcSession<LinkContext>? _session;
     private volatile bool _stopped;
 
+    /// <summary>版本不够时重新握手的间隔：固定，不退避（对方更新完马上就能连上）。</summary>
+    public static readonly TimeSpan VersionRetry = TimeSpan.FromSeconds(5);
+
     private static readonly TimeSpan s_tcpTimeout = TimeSpan.FromSeconds(4);
+    private TimeSpan? _fixedRetry;
     private int _failedConnects;
 
     public FleetLink(FleetLinkOptions options, RpcRouter<LinkContext> router, IEnumerable<IStateHandler> handlers, Outbox outbox, TimeProvider time, INodeLog log)
@@ -166,7 +173,8 @@ public sealed class FleetLink
                 return;
             }
 
-            TimeSpan delay = _options.Backoff.Next();
+            TimeSpan delay = _fixedRetry ?? _options.Backoff.Next();
+            _fixedRetry = null;
             _log.Warn("link down, retrying", ("err", reason), ("retry_ms", (long)delay.TotalMilliseconds));
             await Task.Delay(delay, _time, ct);
         }
@@ -263,7 +271,19 @@ public sealed class FleetLink
             Result<HelloResult> helloResult = await session.RequestAsync(FleetMethods.Hello, hello, FleetRpcContext.Default.HelloParams, FleetRpcContext.Default.HelloResult, ct);
             if (!helloResult.IsSuccess)
             {
-                return "hello failed: " + helloResult.Error.Code;
+                if (helloResult.Error.Code == FleetErrors.VersionRequired)
+                {
+                    _fixedRetry = VersionRetry; // 我们的版本低于服务端要求的：等自己更新，每 5 秒再试一次
+                }
+
+                return "hello failed: " + helloResult.Error.Code + (_fixedRetry is null ? "" : ": " + helloResult.Error.Message);
+            }
+
+            string? requiredServer = _options.MinServerVersion?.Invoke();
+            if (NodeVersions.IsBelow(helloResult.Value.ServerVersion, requiredServer))
+            {
+                _fixedRetry = VersionRetry; // 服务端版本低于我们要求的（它可能正在更新）：不发任何东西，5 秒后重新握手
+                return $"server version {helloResult.Value.ServerVersion} is below the required {requiredServer}";
             }
 
             Outbox.ResetSent();