[#24] Client: Implement pool part1

first iteration - base classes and methods

Signed-off-by: Pavel Gross <p.gross@yadro.com>

src/FrostFS.SDK.ClientV2/Poll/ClientWrapper.cs

@@ -0,0 +1,137 @@
+using System.Threading.Tasks;
+
+namespace FrostFS.SDK.ClientV2;
+
+// clientWrapper is used by default, alternative implementations are intended for testing purposes only.
+public class ClientWrapper
+{
+    private readonly object _lock = new();
+
+    public ClientWrapper(WrapperPrm wrapperPrm)
+    {
+        WrapperPrm = wrapperPrm;
+        StatusMonitor = new ClientStatusMonitor(wrapperPrm.Logger, wrapperPrm.Address, wrapperPrm.ErrorThreshold);
+
+        try
+        {
+            Client = new FrostFSClient(WrapperPrm);
+            StatusMonitor.SetHealthy();
+        }
+        catch (FrostFsException)
+        {
+        }
+    }
+
+    internal FrostFSClient? Client { get; private set; }
+
+    internal WrapperPrm WrapperPrm { get; }
+
+    internal ClientStatusMonitor StatusMonitor { get; }
+
+    internal FrostFSClient? GetClient()
+    {
+        lock (_lock)
+        {
+            if (StatusMonitor.IsHealthy())
+            {
+                return Client;
+            }
+
+            return null;
+        }
+    }
+
+    // dial establishes a connection to the server from the FrostFS network.
+    // Returns an error describing failure reason. If failed, the client
+    // SHOULD NOT be used.
+    internal async Task<string?> Dial(CallContext ctx)
+    {
+        var client = GetClient();
+
+        if (client == null)
+            return "pool client unhealthy";
+
+        var result = await client.Dial(ctx).ConfigureAwait(false);
+        if (!string.IsNullOrEmpty(result))
+        {
+            StatusMonitor.SetUnhealthyOnDial();
+            return result;
+        }
+
+        return null;
+    }
+
+    private async Task ScheduleGracefulClose()
+    {
+        if (Client == null)
+            return;
+
+        await Task.Delay((int)WrapperPrm.GracefulCloseOnSwitchTimeout).ConfigureAwait(false);
+
+        Client.Close();
+    }
+
+    // restartIfUnhealthy checks healthy status of client and recreate it if status is unhealthy.
+    // Indicating if status was changed by this function call and returns error that caused unhealthy status.
+    internal async Task<bool> RestartIfUnhealthy(CallContext ctx)
+    {
+        bool wasHealthy;
+
+        try
+        {
+            var prmNodeInfo = new PrmNodeInfo { Context = ctx };
+            var response = await Client!.GetNodeInfoAsync(prmNodeInfo).ConfigureAwait(false);
+            return false;
+        }
+        catch (FrostFsException)
+        {
+            wasHealthy = true;
+        }
+
+        // if connection is dialed before, to avoid routine/connection leak,
+        // pool has to close it and then initialize once again.
+        if (StatusMonitor.IsDialed())
+        {
+            await ScheduleGracefulClose().ConfigureAwait(false);
+        }
+
+#pragma warning disable CA2000 // Dispose objects before losing scope: will be disposed manually
+        FrostFSClient client = new(WrapperPrm);
+#pragma warning restore CA2000
+
+        //TODO: set additioanl params          
+        var error = await client.Dial(ctx).ConfigureAwait(false);
+        if (!string.IsNullOrEmpty(error))
+        {
+            StatusMonitor.SetUnhealthyOnDial();
+            return wasHealthy;
+        }
+
+        lock (_lock)
+        {
+            Client = client;
+        }
+
+        try
+        {
+            var prmNodeInfo = new PrmNodeInfo { Context = ctx };
+            var res = await client.GetNodeInfoAsync(prmNodeInfo).ConfigureAwait(false);
+        }
+        catch (FrostFsException)
+        {
+            StatusMonitor.SetUnhealthy();
+            return wasHealthy;
+        }
+
+        StatusMonitor.SetHealthy();
+        return !wasHealthy;
+    }
+
+    internal void IncRequests(ulong elapsed, MethodIndex method)
+    {
+        var methodStat = StatusMonitor.Methods[(int)method];
+
+        methodStat.IncRequests(elapsed);
+    }
+}
+