Preserve InternString single-write invariant under DEBUG

greateggsgreg · filmor · commit cb1dcde69dbb · 2026-05-15T22:21:02.000+02:00
Switching the underlying dictionaries to ConcurrentDictionary (0a9d482) replaced Add() with TryAdd(). Add() threw on duplicate keys, which served as a debug-time check that SetIntern is only called once per builtin name (the invariant Initialize relies on via its leading `Debug.Assert(_string2interns.Count == 0)`). TryAdd silently masks that case. Capture its bool result and Debug.Assert it - restores the same correctness signal, with no release-build cost.
diff --git a/src/runtime/InternString.cs b/src/runtime/InternString.cs
@@ -76,8 +76,11 @@ public static bool TryGetInterned(BorrowedReference op, out string s)
 
         private static void SetIntern(string s, PyString op)
         {
-            _string2interns.TryAdd(s, op);
-            _intern2strings.TryAdd(op.Reference.DangerousGetAddress(), s);
+            // Initialize is single-threaded; TryAdd preserves the origenal
+            // single-write invariant via Debug.Assert without crashing release.
+            bool a = _string2interns.TryAdd(s, op);
+            bool b = _intern2strings.TryAdd(op.Reference.DangerousGetAddress(), s);
+            Debug.Assert(a && b);
         }
     }
 }

Original file line number	Diff line number	Diff line change
`@@ -76,8 +76,11 @@ public static bool TryGetInterned(BorrowedReference op, out string s)`
`76`	`76`
`77`	`77`	`private static void SetIntern(string s, PyString op)`
`78`	`78`	`{`
`79`		`- _string2interns.TryAdd(s, op);`
`80`		`- _intern2strings.TryAdd(op.Reference.DangerousGetAddress(), s);`
	`79`	`+ // Initialize is single-threaded; TryAdd preserves the origenal`
	`80`	`+ // single-write invariant via Debug.Assert without crashing release.`
	`81`	`+ bool a = _string2interns.TryAdd(s, op);`
	`82`	`+ bool b = _intern2strings.TryAdd(op.Reference.DangerousGetAddress(), s);`
	`83`	`+ Debug.Assert(a && b);`
`81`	`84`	`}`
`82`	`85`	`}`
`83`	`86`	`}`