Fix a few remaining downgrade locking errors

Fix a few remaining call sites that could generate downgrade lock
errors. Also speed up locking a little by aliasing the __enter__ and
__exit__ methods on the wrapper lock object instead of wrapping them.

src/calibre/db/cache.py

@@ -18,7 +18,7 @@ from calibre.constants import iswindows, preferred_encoding
 from calibre.customize.ui import run_plugins_on_import, run_plugins_on_postimport
 from calibre.db import SPOOL_SIZE, _get_next_series_num_for_list
 from calibre.db.categories import get_categories
-from calibre.db.locking import create_locks, DowngradeLockError
+from calibre.db.locking import create_locks, DowngradeLockError, SafeReadLock
 from calibre.db.errors import NoSuchFormat
 from calibre.db.fields import create_field, IDENTITY, InvalidLinkTable
 from calibre.db.search import Search
@@ -57,11 +57,8 @@ def wrap_simple(lock, func):
                 return func(*args, **kwargs)
         except DowngradeLockError:
             # We already have an exclusive lock, no need to acquire a shared
-            # lock. This can happen when updating the search cache in the
-            # presence of composite columns. Updating the search cache holds an
-            # exclusive lock, but searching a composite column involves
-            # reading field values via ProxyMetadata which tries to get a
-            # shared lock.
+            # lock. See the safe_read_lock properties documentation for why
+            # this is necessary.
             return func(*args, **kwargs)
     return call_func_with_lock
 
@@ -118,6 +115,17 @@ class Cache(object):
         self._search_api = Search(self, 'saved_searches', self.field_metadata.get_search_terms())
         self.initialize_dynamic()
 
+    @property
+    def safe_read_lock(self):
+        ''' A safe read lock is a lock that does nothing if the thread already
+        has a write lock, otherwise it acquires a read lock. This is necessary
+        to prevent DowngradeLockErrors, which can happen when updating the
+        search cache in the presence of composite columns. Updating the search
+        cache holds an exclusive lock, but searching a composite column
+        involves reading field values via ProxyMetadata which tries to get a
+        shared lock. There may be other scenarios that trigger this as well. '''
+        return SafeReadLock(self.read_lock)
+
     @write_api
     def initialize_dynamic(self):
         # Reconstruct the user categories, putting them into field_metadata
@@ -501,7 +509,7 @@ class Cache(object):
             x = self.format_metadata_cache[book_id].get(fmt, None)
             if x is not None:
                 return x
-        with self.read_lock:
+        with self.safe_read_lock:
             try:
                 name = self.fields['formats'].format_fname(book_id, fmt)
                 path = self._field_for('path', book_id).replace('/', os.sep)
@@ -545,7 +553,7 @@ class Cache(object):
         cover_as_data is True then as mi.cover_data.
         '''
 
-        with self.read_lock:
+        with self.safe_read_lock:
             mi = self._get_metadata(book_id, get_user_categories=get_user_categories)
 
         if get_cover:
@@ -751,7 +759,7 @@ class Cache(object):
         ext = ('.'+fmt.lower()) if fmt else ''
         if as_path:
             if preserve_filename:
-                with self.read_lock:
+                with self.safe_read_lock:
                     try:
                         fname = self.fields['formats'].format_fname(book_id, fmt)
                     except:
@@ -777,7 +785,7 @@ class Cache(object):
                         return None
                     ret = pt.name
         elif as_file:
-            with self.read_lock:
+            with self.safe_read_lock:
                 try:
                     fname = self.fields['formats'].format_fname(book_id, fmt)
                 except:
@@ -878,7 +886,7 @@ class Cache(object):
     @api
     def get_categories(self, sort='name', book_ids=None, icon_map=None, already_fixed=None):
         try:
-            with self.read_lock:
+            with self.safe_read_lock:
                 return get_categories(self, sort=sort, book_ids=book_ids, icon_map=icon_map)
         except InvalidLinkTable as err:
             bad_field = err.field_name

src/calibre/db/lazy.py

@@ -132,7 +132,7 @@ def adata_getter(field):
             author_ids, adata = cache['adata']
         except KeyError:
             db = dbref()
-            with db.read_lock:
+            with db.safe_read_lock:
                 author_ids = db._field_ids_for('authors', book_id)
                 adata = db._author_data(author_ids)
             cache['adata'] = (author_ids, adata)

src/calibre/db/legacy.py

@@ -154,7 +154,7 @@ class LibraryDatabase(object):
         return tuple(self.new_api.all_book_ids())
 
     def is_empty(self):
-        with self.new_api.read_lock:
+        with self.new_api.safe_read_lock:
             return not bool(self.new_api.fields['title'].table.book_col_map)
 
     def get_usage_count_by_id(self, field):
@@ -363,7 +363,7 @@ class LibraryDatabase(object):
 
     def authors_with_sort_strings(self, index, index_is_id=False):
         book_id = index if index_is_id else self.id(index)
-        with self.new_api.read_lock:
+        with self.new_api.safe_read_lock:
             authors = self.new_api._field_ids_for('authors', book_id)
             adata = self.new_api._author_data(authors)
             return [(aid, adata[aid]['name'], adata[aid]['sort'], adata[aid]['link']) for aid in authors]
@@ -379,7 +379,7 @@ class LibraryDatabase(object):
             self.notify('metadata', list(changed_books))
 
     def book_on_device(self, book_id):
-        with self.new_api.read_lock:
+        with self.new_api.safe_read_lock:
             return self.new_api.fields['ondevice'].book_on_device(book_id)
 
     def book_on_device_string(self, book_id):
@@ -393,7 +393,7 @@ class LibraryDatabase(object):
         return self.new_api.fields['ondevice'].book_on_device_func
 
     def books_in_series(self, series_id):
-        with self.new_api.read_lock:
+        with self.new_api.safe_read_lock:
             book_ids = self.new_api._books_for_field('series', series_id)
             ff = self.new_api._field_for
             return sorted(book_ids, key=lambda x:ff('series_index', x))

src/calibre/db/locking.py

@@ -213,16 +213,15 @@ class RWLockWrapper(object):
         self._shlock = shlock
         self._is_shared = is_shared
 
-    def __enter__(self):
+    def acquire(self):
         self._shlock.acquire(shared=self._is_shared)
-        return self
 
-    def __exit__(self, *args):
-        self.release()
-
-    def release(self):
+    def release(self, *args):
         self._shlock.release()
 
+    __enter__ = acquire
+    __exit__ = release
+
     def owns_lock(self):
         return self._shlock.owns_lock()
 
@@ -231,11 +230,11 @@ class DebugRWLockWrapper(RWLockWrapper):
     def __init__(self, *args, **kwargs):
         RWLockWrapper.__init__(self, *args, **kwargs)
 
-    def __enter__(self):
+    def acquire(self):
         print ('#' * 120, file=sys.stderr)
         print ('acquire called: thread id:', current_thread(), 'shared:', self._is_shared, file=sys.stderr)
         traceback.print_stack()
-        RWLockWrapper.__enter__(self)
+        RWLockWrapper.acquire(self)
         print ('acquire done: thread id:', current_thread(), file=sys.stderr)
         print ('_' * 120, file=sys.stderr)
 
@@ -247,4 +246,28 @@ class DebugRWLockWrapper(RWLockWrapper):
         print ('release done: thread id:', current_thread(), 'is_shared:', self._shlock.is_shared, 'is_exclusive:', self._shlock.is_exclusive, file=sys.stderr)
         print ('_' * 120, file=sys.stderr)
 
+    __enter__ = acquire
+    __exit__ = release
 
+class SafeReadLock(object):
+
+    def __init__(self, read_lock):
+        self.read_lock = read_lock
+        self.acquired = False
+
+    def acquire(self):
+        try:
+            self.read_lock.acquire()
+        except DowngradeLockError:
+            pass
+        else:
+            self.acquired = True
+        return self
+
+    def release(self, *args):
+        if self.acquired:
+            self.read_lock.release()
+        self.acquired = False
+
+    __enter__ = acquire
+    __exit__  = release

src/calibre/db/view.py

@@ -205,7 +205,7 @@ class View(object):
 
     def get_series_sort(self, idx, index_is_id=True, default_value=''):
         book_id = idx if index_is_id else self.index_to_id(idx)
-        with self.cache.read_lock:
+        with self.cache.safe_read_lock:
             lang_map = self.cache.fields['languages'].book_value_map
             lang = lang_map.get(book_id, None) or None
             if lang:
@@ -223,7 +223,7 @@ class View(object):
 
     def get_author_data(self, idx, index_is_id=True, default_value=None):
         id_ = idx if index_is_id else self.index_to_id(idx)
-        with self.cache.read_lock:
+        with self.cache.safe_read_lock:
             ids = self.cache._field_ids_for('authors', id_)
             adata = self.cache._author_data(ids)
             ans = [':::'.join((adata[aid]['name'], adata[aid]['sort'], adata[aid]['link'])) for aid in ids if aid in adata]