fix: /api/diagnostics 500 « dictionary changed size during iteration » (snapshot des dicts avant itération)

tests/test_api_main.py

@@ -460,6 +460,39 @@ class TestConfig:
         # Should have some diagnostic info
         assert len(data) > 0
 
+    def test_diagnostics_concurrent_index_writes(self, client):
+        """Regression: word_index mutated while the handler iterates it raised
+        "dictionary changed size during iteration" -> 500."""
+        from backend.search import get_inverted_index
+
+        inv = get_inverted_index()
+        original = inv.word_index
+
+        class RaceDict(dict):
+            """dict whose .values() grows the dict mid-iteration, exactly like
+            the indexer writing from another thread."""
+            def values(self):
+                it = iter(dict.values(self))
+                first = next(it, _SENTINEL)
+                if first is _SENTINEL:
+                    return iter(())
+                self["__injected__"] = {"__injected_doc__": 1}
+
+                def gen():
+                    yield first
+                    yield from it  # raises RuntimeError: dict changed size
+                return gen()
+
+        _SENTINEL = object()
+        inv.word_index = RaceDict(original)
+        try:
+            resp = client.get("/api/diagnostics")
+            assert resp.status_code == 200, resp.text
+            assert "memory_estimate_mb" in resp.json()["inverted_index"]
+        finally:
+            inv.word_index = original
+            original.pop("__injected__", None)
+
 
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 # Dashboard