Port dotnet/runtime#31946 to release/3.1 branch (#28014)

When string.Replace is given a target string with zero collation weight, it would enter an infinite loop. It is now changed so that the call to Replace terminates when such a condition is encountered.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/System.Private.CoreLib/shared/System/String.Manipulation.cs b/src/System.Private.CoreLib/shared/System/String.Manipulation.cs
index 564fca0dcd..daf6fc3dff 100644
--- a/src/System.Private.CoreLib/shared/System/String.Manipulation.cs
+++ b/src/System.Private.CoreLib/shared/System/String.Manipulation.cs
@@ -1020,7 +1020,11 @@ namespace System
             do
             {
                 index = ci.IndexOf(this, oldValue, startIndex, this.Length - startIndex, options, &matchLength);
-                if (index >= 0)
+
+                // There's the possibility that 'oldValue' has zero collation weight (empty string equivalent).
+                // If this is the case, we behave as if there are no more substitutions to be made.
+
+                if (index >= 0 && matchLength > 0)
                 {
                     // append the unmodified portion of string
                     result.Append(this, startIndex, index - startIndex);