[corlib] Don't decrement ConditionalWeakTable::size on remove. (#10477)

We can't decrement the size on removal as we use tombstones.
When using tombstones, the invariant to keep is on the number of null entries as those
are used as search stop-marks.

Without this change, it's possible that a long series of add/remove operations could leave the array with no null entries,
just tombstones and filled entries. In that case, search would degenerate into a linear search that would look into all array elements.

This change fixes worst-case perf of not-found searches to be bound to the load factor.

Additionally, clear the array to null entries, otherwise we fall into the above trap even more easily.

1 files changed, 1 insertions, 2 deletions

diff --git a/mcs/class/corlib/System.Runtime.CompilerServices/ConditionalWeakTable.cs b/mcs/class/corlib/System.Runtime.CompilerServices/ConditionalWeakTable.cs
index 17474df4f97..97bab6e7961 100644
--- a/mcs/class/corlib/System.Runtime.CompilerServices/ConditionalWeakTable.cs
+++ b/mcs/class/corlib/System.Runtime.CompilerServices/ConditionalWeakTable.cs
@@ -225,7 +225,6 @@ namespace System.Runtime.CompilerServices
 					if (k == key) {
 						data [idx].key = GC.EPHEMERON_TOMBSTONE;
 						data [idx].value = null;
-						--size;
 						return true;
 					}
 					if (k == null)
@@ -325,7 +324,7 @@ namespace System.Runtime.CompilerServices
 			{
 				for (int i = 0; i < data.Length; i++)
 				{
-					data[i].key = GC.EPHEMERON_TOMBSTONE;
+					data[i].key = null;
 					data[i].value = null;
 				}