New chunk should be equal/larger than the one it replaces (dotnet#17219)

* The new chunk should be equal or larger than the one it is replacing. Fixes #17205

* Adding .exe.stackdump to .gitignore

Author: maryamariyan

.gitignore

@@ -236,6 +236,7 @@ $RECYCLE.BIN/
 
 # Common binary extensions on Windows
 *.exe
+*.exe.stackdump
 *.dll
 *.lib
 

src/mscorlib/shared/System/Text/StringBuilder.Debug.cs

@@ -2,9 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System.Collections.Generic;
 using System.Diagnostics;
-using System.Runtime.Serialization;
 
 namespace System.Text
 {

src/mscorlib/shared/System/Text/StringBuilder.cs

@@ -480,24 +480,32 @@ public int Length
                     StringBuilder chunk = FindChunkForIndex(value);
                     if (chunk != this)
                     {
-                        // We crossed a chunk boundary when reducing the Length. We must replace this middle-chunk with a new larger chunk,
-                        // to ensure the original capacity is preserved.
-
                         // Avoid possible infinite capacity growth.  See https://github.com/dotnet/coreclr/pull/16926
                         int capacityToPreserve = Math.Min(Capacity, Math.Max(Length * 6 / 5, m_ChunkChars.Length));
                         int newLen = capacityToPreserve - chunk.m_ChunkOffset;
-                        char[] newArray = new char[newLen];
-
-                        Debug.Assert(newLen > chunk.m_ChunkChars.Length, "The new chunk should be larger than the one it is replacing.");
-                        Array.Copy(chunk.m_ChunkChars, 0, newArray, 0, chunk.m_ChunkLength);
+                        if (newLen > chunk.m_ChunkChars.Length)
+                        {
+                            // We crossed a chunk boundary when reducing the Length. We must replace this middle-chunk with a new larger chunk,
+                            // to ensure the capacity we want is preserved.
+                            char[] newArray = new char[newLen];
+                            Array.Copy(chunk.m_ChunkChars, 0, newArray, 0, chunk.m_ChunkLength);
+                            m_ChunkChars = newArray;
+                        }
+                        else
+                        {
+                            // Special case where the capacity we want to keep corresponds exactly to the size of the content.
+                            // Just take ownership of the array.
+                            Debug.Assert(newLen == chunk.m_ChunkChars.Length, "The new chunk should be larger or equal to the one it is replacing.");
+                            m_ChunkChars = chunk.m_ChunkChars;
+                        }
 
-                        m_ChunkChars = newArray;
                         m_ChunkPrevious = chunk.m_ChunkPrevious;
                         m_ChunkOffset = chunk.m_ChunkOffset;
                     }
                     m_ChunkLength = value - chunk.m_ChunkOffset;
                     AssertInvariants();
                 }
+                Debug.Assert(Length == value, "Something went wrong setting Length.");
             }
         }