IndexSearcher fix for virtual calls from constructor

src/Lucene.Net/Search/IndexSearcher.cs

@@ -1,12 +1,11 @@
 using Lucene.Net.Diagnostics;
-using Lucene.Net.Support;
 using Lucene.Net.Support.Threading;
 using Lucene.Net.Util;
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
-using System.Threading;
 using System.Threading.Tasks;
 
 namespace Lucene.Net.Search
@@ -118,7 +117,7 @@ public IndexSearcher(IndexReader r)
         /// @lucene.experimental
         /// </summary>
         public IndexSearcher(IndexReader r, TaskScheduler executor)
-            : this(r.Context, executor)
+            : this(r?.Context, executor)
         {
         }
 
@@ -135,13 +134,44 @@ public IndexSearcher(IndexReader r, TaskScheduler executor)
         /// <seealso cref="IndexReaderContext"/>
         /// <seealso cref="IndexReader.Context"/>
         public IndexSearcher(IndexReaderContext context, TaskScheduler executor)
+            : this(context, executor, allocateLeafSlices: executor is not null)
         {
-            if (Debugging.AssertsEnabled) Debugging.Assert(context.IsTopLevel,"IndexSearcher's ReaderContext must be topLevel for reader {0}", context.Reader);
+        }
+
+        /// <summary>
+        /// LUCENENET specific constructor that can be used by the subclasses to
+        /// control whether the leaf slices are allocated in the base class or subclass.
+        /// </summary>
+        /// <remarks>
+        /// If executor is non-<c>null</c> and you choose to skip allocating the leaf slices
+        /// (i.e. <paramref name="allocateLeafSlices"/> == <c>false</c>), you must
+        /// set the <see cref="m_leafSlices"/> field in your subclass constructor.
+        /// This is commonly done by calling <see cref="GetSlices(IList{AtomicReaderContext})"/>
+        /// and using the result to set <see cref="m_leafSlices"/>. You may wish to do this if you
+        /// have state to pass into your constructor and need to set it prior to the call to
+        /// <see cref="GetSlices(IList{AtomicReaderContext})"/> so it is available for use
+        /// as a member field or property inside a custom override of
+        /// <see cref="GetSlices(IList{AtomicReaderContext})"/>.
+        /// </remarks>
+        [SuppressMessage("CodeQuality", "IDE0079:Remove unnecessary suppression", Justification = "This is a SonarCloud issue")]
+        [SuppressMessage("CodeQuality", "S1699:Constructors should only call non-overridable methods", Justification = "Required for continuity with Lucene's design")]
+        protected IndexSearcher(IndexReaderContext context, TaskScheduler executor, bool allocateLeafSlices)
+        {
+            if (context is null)
+                throw new ArgumentNullException(nameof(context));
+
+            if (Debugging.AssertsEnabled)
+                Debugging.Assert(context.IsTopLevel,"IndexSearcher's ReaderContext must be topLevel for reader {0}", context.Reader);
+
             reader = context.Reader;
             this.executor = executor;
             this.m_readerContext = context;
             m_leafContexts = context.Leaves;
-            this.m_leafSlices = executor is null ? null : GetSlicesInternal(m_leafContexts);
+
+            if (allocateLeafSlices)
+            {
+                this.m_leafSlices = GetSlices(m_leafContexts);
+            }
         }
 
         /// <summary>
@@ -160,18 +190,8 @@ public IndexSearcher(IndexReaderContext context)
         /// Expert: Creates an array of leaf slices each holding a subset of the given leaves.
         /// Each <see cref="LeafSlice"/> is executed in a single thread. By default there
         /// will be one <see cref="LeafSlice"/> per leaf (<see cref="AtomicReaderContext"/>).
-        /// 
-        /// NOTE: When overriding this method, be aware that the constructor of this class calls 
-        /// a private method and not this virtual method. So if you need to override
-        /// the behavior during the initialization, call your own private method from the constructor
-        /// with whatever custom behavior you need.
         /// </summary>
-        // LUCENENET specific - renamed to GetSlices to better indicate the purpose of this method
         protected virtual LeafSlice[] GetSlices(IList<AtomicReaderContext> leaves)
-            => GetSlicesInternal(leaves);
-
-        // LUCENENET specific - creating this so that we can call it from the constructor
-        protected LeafSlice[] GetSlicesInternal(IList<AtomicReaderContext> leaves)
         {
             LeafSlice[] slices = new LeafSlice[leaves.Count];
             for (int i = 0; i < slices.Length; i++)
@@ -464,6 +484,9 @@ protected virtual TopDocs Search(Weight weight, ScoreDoc after, int nDocs)
                 ReentrantLock @lock = new ReentrantLock();
                 ExecutionHelper<TopDocs> runner = new ExecutionHelper<TopDocs>(executor);
 
+                if (m_leafSlices is null)
+                    throw new InvalidOperationException($"When the constructor is passed a non-null {nameof(TaskScheduler)}, {nameof(m_leafSlices)} must also be set to a non-null value in the constructor.");
+
                 for (int i = 0; i < m_leafSlices.Length; i++) // search each sub
                 {
                     runner.Submit(new SearcherCallableNoSort(@lock, this, m_leafSlices[i], weight, after, nDocs, hq).Call);
@@ -560,6 +583,9 @@ protected virtual TopFieldDocs Search(Weight weight, FieldDoc after, int nDocs,
                 ReentrantLock @lock = new ReentrantLock();
                 ExecutionHelper<TopFieldDocs> runner = new ExecutionHelper<TopFieldDocs>(executor);
 
+                if (m_leafSlices is null)
+                    throw new InvalidOperationException($"When the constructor is passed a non-null {nameof(TaskScheduler)}, {nameof(m_leafSlices)} must also be set to a non-null value in the constructor.");
+
                 for (int i = 0; i < m_leafSlices.Length; i++) // search each leaf slice
                 {
                     runner.Submit(new SearcherCallableWithSort(@lock, this, m_leafSlices[i], weight, after, nDocs, topCollector, sort, doDocScores, doMaxScore).Call);