fix(questdb): disable Npgsql GSS encryption probe unless the connection string sets it (lmxopcua#758)

QuestDB speaks plain PG-wire; Npgsql's default GSS Encryption Mode=Prefer made
Linux central boots log 'Cannot load library libgssapi_krb5.so.2'. An explicit
'GSS Encryption Mode' in the connection string is kept. Not re-verified on a
Linux host here; the Postgres Sql driver is left alone (Kerberos may be wanted).

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbConnection.cs

@@ -42,6 +42,13 @@ internal sealed class QuestDbConnection : IAsyncDisposable, IDisposable
         var explicitPool = HasExplicitMaxPoolSize(options.ConnectionString);
         if (!explicitPool) csb.MaxPoolSize = DefaultMaxPoolSize;
         EffectiveMaxPoolSize = csb.MaxPoolSize;
+        // #758: QuestDB speaks plain PG-wire with no GSS support. Npgsql's default GSS Encryption Mode=Prefer probes
+        // for GSSAPI on every new physical connection, which on Linux logs "Cannot load library libgssapi_krb5.so.2"
+        // when the Kerberos client library is not installed. The probe falls back cleanly, so this only silences a
+        // misleading line; an explicit "GSS Encryption Mode" in the connection string is always kept.
+        if (!HasExplicitKey(options.ConnectionString, "GSSEncryptionMode"))
+            csb.GssEncryptionMode = GssEncryptionMode.Disable;
+        EffectiveGssEncryptionMode = csb.GssEncryptionMode;
         (log ?? NullLogger.Instance).LogInformation(
             "QuestDB historian connection pool: Maximum Pool Size = {MaxPoolSize} ({Source}).",
             EffectiveMaxPoolSize, explicitPool ? "from the connection string" : "default cap");
@@ -51,17 +58,23 @@ internal sealed class QuestDbConnection : IAsyncDisposable, IDisposable
     /// <summary>The Npgsql <c>Maximum Pool Size</c> in effect (the connection string's own value, else <see cref="DefaultMaxPoolSize"/>).</summary>
     internal int EffectiveMaxPoolSize { get; }
 
+    /// <summary>The Npgsql <c>GSS Encryption Mode</c> in effect (the connection string's own value, else <c>Disable</c>, #758).</summary>
+    internal GssEncryptionMode EffectiveGssEncryptionMode { get; }
+
     // Npgsql accepts "Maximum Pool Size" and "MaxPoolSize" (case-insensitive); a generic parse keeps the caller's own
     // keywords so "was it set" is answered by the string itself, not by the builder's default.
-    private static bool HasExplicitMaxPoolSize(string connectionString)
+    private static bool HasExplicitMaxPoolSize(string connectionString) =>
+        HasExplicitKey(connectionString, "MaxPoolSize", "MaximumPoolSize");
+
+    // Spaces are ignored in keywords ("GSS Encryption Mode" == "GSSEncryptionMode"); matching is case-insensitive.
+    private static bool HasExplicitKey(string connectionString, params string[] normalizedKeys)
     {
         var generic = new System.Data.Common.DbConnectionStringBuilder { ConnectionString = connectionString };
         foreach (string key in generic.Keys)
         {
-            var k = key.Replace(" ", "", StringComparison.Ordinal);
-            if (string.Equals(k, "MaxPoolSize", StringComparison.OrdinalIgnoreCase)
-                || string.Equals(k, "MaximumPoolSize", StringComparison.OrdinalIgnoreCase))
-                return true;
+            var k = key.Replace(" ", "", StringComparison.Ordinal).Replace("_", "", StringComparison.Ordinal);
+            foreach (var wanted in normalizedKeys)
+                if (string.Equals(k, wanted, StringComparison.OrdinalIgnoreCase)) return true;
         }
         return false;
     }

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.Tests/QuestDbConnectionTests.cs

@@ -146,6 +146,24 @@ public sealed class QuestDbConnectionTests
         return c.EffectiveMaxPoolSize;
     }
 
+    // ---- GSS probe (#758) -------------------------------------------------------------------------------------------
+
+    private static Npgsql.GssEncryptionMode GssOf(string cs)
+    {
+        using var c = new QuestDbConnection(new QuestDbHistorianOptions { ConnectionString = cs });
+        return c.EffectiveGssEncryptionMode;
+    }
+
+    [Fact]
+    public void An_unset_GSS_Encryption_Mode_is_disabled() =>
+        GssOf("Host=127.0.0.1;Port=1").ShouldBe(Npgsql.GssEncryptionMode.Disable);
+
+    [Theory]
+    [InlineData("Host=127.0.0.1;Port=1;GSS Encryption Mode=Prefer", Npgsql.GssEncryptionMode.Prefer)]
+    [InlineData("Host=127.0.0.1;Port=1;GssEncryptionMode=Require", Npgsql.GssEncryptionMode.Require)]
+    public void An_explicit_GSS_Encryption_Mode_is_kept(string cs, Npgsql.GssEncryptionMode expected) =>
+        GssOf(cs).ShouldBe(expected);
+
     [Fact]
     public void An_unset_Maximum_Pool_Size_is_capped_at_24() =>
         PoolOf("Host=127.0.0.1;Port=1").ShouldBe(24);