Ignore comments in SQL composability check

And rewrite without regex

Fixes #18412

src/EFCore.Relational/Query/QuerySqlGenerator.cs

@@ -6,6 +6,7 @@
 using System.Linq;
 using System.Linq.Expressions;
 using System.Text.RegularExpressions;
+using System.Threading;
 using JetBrains.Annotations;
 using Microsoft.EntityFrameworkCore.Diagnostics;
 using Microsoft.EntityFrameworkCore.Query.SqlExpressions;
@@ -26,9 +27,6 @@ namespace Microsoft.EntityFrameworkCore.Query
     /// </summary>
     public class QuerySqlGenerator : SqlExpressionVisitor
     {
-        private static readonly Regex _composableSql
-            = new Regex(@"^\s*?SELECT\b", RegexOptions.IgnoreCase, TimeSpan.FromMilliseconds(value: 1000.0));
-
         private readonly IRelationalCommandBuilderFactory _relationalCommandBuilderFactory;
         private readonly ISqlGenerationHelper _sqlGenerationHelper;
         private IRelationalCommandBuilder _relationalCommandBuilder;
@@ -437,10 +435,53 @@ protected virtual void CheckComposableSql([NotNull] string sql)
         {
             Check.NotNull(sql, nameof(sql));
 
-            if (!_composableSql.IsMatch(sql))
+            var pos = -1;
+            char c;
+
+            do
             {
+                c = NextChar();
+
+                if (char.IsWhiteSpace(c))
+                {
+                    continue;
+                }
+
+                if (c == '-')
+                {
+                    if (NextChar() != '-')
+                    {
+                        throw new InvalidOperationException(RelationalStrings.FromSqlNonComposable);
+                    }
+
+                    while (NextChar() != '\n') { }
+
+                    continue;
+                }
+
+                if (char.ToLowerInvariant(c) == 's' &&
+                    char.ToLowerInvariant(NextChar()) == 'e' &&
+                    char.ToLowerInvariant(NextChar()) == 'l' &&
+                    char.ToLowerInvariant(NextChar()) == 'e' &&
+                    char.ToLowerInvariant(NextChar()) == 'c' &&
+                    char.ToLowerInvariant(NextChar()) == 't')
+                {
+                    c = NextChar();
+                    if (char.IsWhiteSpace(c)
+                        || c == '-' && NextChar() == '-')
+                    {
+                        return;
+                    }
+                }
+
                 throw new InvalidOperationException(RelationalStrings.FromSqlNonComposable);
             }
+            while (true);
+
+            char NextChar()
+                => ++pos < sql.Length
+                    ? sql[pos]
+                    : throw new InvalidOperationException(RelationalStrings.FromSqlNonComposable);
         }
 
         /// <inheritdoc />

test/EFCore.Relational.Tests/Query/Internal/QuerySqlGeneratorTest.cs

@@ -0,0 +1,60 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using JetBrains.Annotations;
+using Microsoft.EntityFrameworkCore.Diagnostics;
+using Microsoft.EntityFrameworkCore.Storage;
+using Microsoft.EntityFrameworkCore.TestUtilities;
+using Xunit;
+
+namespace Microsoft.EntityFrameworkCore.Query.Internal
+{
+    public class QuerySqlGeneratorTest
+    {
+        [Theory]
+        [InlineData("INSERT something")]
+        [InlineData("SELECTANDSOMEOTHERSTUFF")]
+        [InlineData("SELECT")]
+        [InlineData("SELEC")]
+        [InlineData("- bad comment\nSELECT something")]
+        [InlineData("SELECT-\n1")]
+        [InlineData("")]
+        public void CheckComposableSql_throws(string sql)
+            => Assert.Equal(
+                RelationalStrings.FromSqlNonComposable,
+                Assert.Throws<InvalidOperationException>(
+                    () => CreateDummyQuerySqlGenerator().CheckComposableSql(sql)).Message);
+
+        [Theory]
+        [InlineData("SELECT something")]
+        [InlineData("   SELECT something")]
+        [InlineData("-- comment\n SELECT something")]
+        [InlineData("-- comment1\r\n --\t\rcomment2\r\nSELECT something")]
+        [InlineData("SELECT--\n1")]
+        public void CheckComposableSql_does_not_throw(string sql)
+            => CreateDummyQuerySqlGenerator().CheckComposableSql(sql);
+
+        private DummyQuerySqlGenerator CreateDummyQuerySqlGenerator()
+            => new DummyQuerySqlGenerator(
+                new QuerySqlGeneratorDependencies(
+                    new RelationalCommandBuilderFactory(
+                        new RelationalCommandBuilderDependencies(
+                            new TestRelationalTypeMappingSource(
+                                TestServiceFactory.Instance.Create<TypeMappingSourceDependencies>(),
+                                TestServiceFactory.Instance.Create<RelationalTypeMappingSourceDependencies>()))),
+                    new RelationalSqlGenerationHelper(
+                        new RelationalSqlGenerationHelperDependencies())));
+
+        class DummyQuerySqlGenerator : QuerySqlGenerator
+        {
+            public DummyQuerySqlGenerator([NotNull] QuerySqlGeneratorDependencies dependencies)
+                : base(dependencies)
+            {
+            }
+
+            public new void CheckComposableSql(string sql)
+                => base.CheckComposableSql(sql);
+        }
+    }
+}