Respect max name length (#5463)

Fixes #5460

(cherry picked from commit 5ba1592)

src/Npgsql/Internal/Postgres/DataTypeName.cs

@@ -16,7 +16,7 @@ namespace Npgsql.Internal.Postgres;
     /// We need to respect this to get to valid names when deriving them (for multirange/arrays etc).
     /// This does not include the namespace.
     /// </remarks>
-    const int NAMEDATALEN = 64 - 1; // Minus null terminator.
+    internal const int NAMEDATALEN = 64 - 1; // Minus null terminator.
 
     readonly string _value;
 
@@ -32,10 +32,11 @@ namespace Npgsql.Internal.Postgres;
             if (fullyQualifiedDataTypeName.AsSpan(schemaEndIndex).EndsWith("[]".AsSpan()))
                 fullyQualifiedDataTypeName = NormalizeName(fullyQualifiedDataTypeName);
 
-            var typeNameLength = fullyQualifiedDataTypeName.Length - schemaEndIndex + 1;
+            var typeNameLength = fullyQualifiedDataTypeName.Length - (schemaEndIndex + 1);
             if (typeNameLength > NAMEDATALEN)
                 throw new ArgumentException(
-                    $"Name is too long and would be truncated to: {fullyQualifiedDataTypeName.Substring(0, fullyQualifiedDataTypeName.Length - typeNameLength + NAMEDATALEN)}");
+                    $"Name is too long and would be truncated to: {fullyQualifiedDataTypeName.Substring(0,
+                        fullyQualifiedDataTypeName.Length - typeNameLength + NAMEDATALEN)}");
         }
 
         _value = fullyQualifiedDataTypeName;

test/Npgsql.Tests/DataTypeNameTests.cs

@@ -0,0 +1,26 @@
+using System;
+using Npgsql.Internal.Postgres;
+using NUnit.Framework;
+
+namespace Npgsql.Tests;
+
+public class DataTypeNameTests
+{
+    [Test]
+    public void MaxLengthDataTypeName()
+    {
+        var name = new string('a', DataTypeName.NAMEDATALEN);
+        var fullyQualifiedDataTypeName= $"public.{name}";
+        Assert.DoesNotThrow(() => new DataTypeName(fullyQualifiedDataTypeName));
+        Assert.AreEqual(new DataTypeName(fullyQualifiedDataTypeName).Value, fullyQualifiedDataTypeName);
+    }
+
+    [Test]
+    public void TooLongDataTypeName()
+    {
+        var name = new string('a', DataTypeName.NAMEDATALEN + 1);
+        var fullyQualifiedDataTypeName= $"public.{name}";
+        var exception = Assert.Throws<ArgumentException>(() => new DataTypeName(fullyQualifiedDataTypeName));
+        Assert.That(exception!.Message, Does.EndWith($": public.{new string('a', DataTypeName.NAMEDATALEN)}"));
+    }
+}