Bug #1927179: VL set to wildcard crashes Calibre. Note that the problem had nothing to do with wildcards. It was triggered by an unanchored search specification (one with no column), which caused all the composite columns to be evaluated.

src/calibre/db/cache.py

@@ -142,6 +142,7 @@ class Cache(object):
         self.formatter_template_cache = {}
         self.dirtied_cache = {}
         self.vls_for_books_cache = None
+        self.vls_for_books_cache_is_loading = False
         self.dirtied_sequence = 0
         self.cover_caches = set()
         self.clear_search_cache_count = 0
@@ -251,6 +252,7 @@ class Cache(object):
         self.clear_search_cache_count += 1
         self._search_api.update_or_clear(self, book_ids)
         self.vls_for_books_cache = None
+        self.vls_for_books_cache_is_loading = False
 
     @read_api
     def last_modified(self):
@@ -2213,14 +2215,35 @@ class Cache(object):
         if self.vls_for_books_cache is None:
             # Using a list is slightly faster than a set.
             c = defaultdict(list)
+            if self.vls_for_books_cache_is_loading:
+                # We get here if resolving the books in a VL triggers another VL
+                # calculation. This can be 'real' recursion, in which case the
+                # eventual answer might be wrong. It can also be a
+                # search using a  location of 'all' that causes a composite that
+                # references virtual libraries to be evaluated. If the composite
+                # isn't used in a VL then the eventual answer will be correct.
+                return c
+            self.vls_for_books_cache_is_loading = True
             libraries = self._pref('virtual_libraries', {})
             for lib, expr in libraries.items():
+                book = None
                 try:
                     for book in self._search(expr, virtual_fields=virtual_fields):
                         c[book].append(lib)
                 except Exception as e:
-                    c[book].append(_('[Error in Virtual library {0}: {1}]').format(lib, str(e)))
+                    if book:
+                        c[book].append(_('[Error in Virtual library {0}: {1}]').format(lib, str(e)))
             self.vls_for_books_cache = {b:tuple(sorted(libs, key=sort_key)) for b, libs in c.items()}
+            # Clear the proxy_metadata composite caches so that any template
+            # evaluation returned during recursion avoidance (above) will be
+            # recomputed with correct data.
+            # Note that clear_composite_caches() normally uses a write lock. That
+            # won't work here because the call chain has already obtained a read
+            # lock that we can't upgrade. It is safe to call clear_composite_caches
+            # using the read lock because multiple threads will merely clear the
+            # cache multiple times, perhaps wasting a bit of time recomputing
+            # values. The GIL prevents corruption during the clearing process.
+            self._clear_composite_caches()
         if not book_ids:
             book_ids = self._all_book_ids()
         # book_ids is usually 1 long. The loop will be faster than a comprehension