mu-server: try avoiding xapian multi-threaded access

Try to avoid multi-threaded operations with Xapian.

This remove the thread workers during indexing, and avoids the indexing
background thread. So, mu4e has to wait once again during indexing.

We can improve upon that, but first we need to know if it avoids the
problem of issue #2756.

lib/mu-indexer.cc

@@ -105,6 +105,7 @@ struct Indexer::Private {
 	bool handler(const std::string& fullpath, struct stat* statbuf, Scanner::HandleType htype);
 
 	void maybe_start_worker();
+
 	void item_worker();
 	void scan_worker();
 
@@ -135,6 +136,8 @@ struct Indexer::Private {
 		Type type;
 	};
 
+	void handle_item(WorkItem&& item);
+
 	AsyncQueue<WorkItem> todos_;
 
 	Progress   progress_{};
@@ -193,7 +196,11 @@ Indexer::Private::handler(const std::string& fullpath, struct stat* statbuf,
 		return true;
 	}
 	case Scanner::HandleType::LeaveDir: {
+#ifdef XAPIAN_SINGLE_THREADED
+		handle_item({fullpath, WorkItem::Type::Dir});
+#else
 		todos_.push({fullpath, WorkItem::Type::Dir});
+#endif /*XAPIAN_SINGLE_THREADED*/
 		return true;
 	}
 
@@ -210,9 +217,13 @@ Indexer::Private::handler(const std::string& fullpath, struct stat* statbuf,
 		if (statbuf->st_ctime <= dirstamp_ && store_.contains_message(fullpath))
 			return false;
 
+#ifdef XAPIAN_SINGLE_THREADED
+		handle_item({fullpath, WorkItem::Type::File});
+#else
 		// push the remaining messages to our "todo" queue for
 		// (re)parsing and adding/updating to the database.
 		todos_.push({fullpath, WorkItem::Type::File});
+#endif
 		return true;
 	}
 	default:
@@ -260,16 +271,10 @@ Indexer::Private::add_message(const std::string& path)
 	return true;
 }
 
+
 void
-Indexer::Private::item_worker()
+Indexer::Private::handle_item(WorkItem&& item)
 {
-	WorkItem item;
-
-	mu_debug("started worker");
-
-	while (state_ == IndexState::Scanning) {
-		if (!todos_.pop(item, 250ms))
-			continue;
 	try {
 		switch (item.type) {
 		case WorkItem::Type::File: {
@@ -286,6 +291,22 @@ Indexer::Private::item_worker()
 	} catch (const Mu::Error& er) {
 		mu_warning("error adding message @ {}: {}", item.full_path, er.what());
 	}
+}
+
+
+
+void
+Indexer::Private::item_worker()
+{
+	WorkItem item;
+
+	mu_debug("started worker");
+
+	while (state_ == IndexState::Scanning) {
+		if (!todos_.pop(item, 250ms))
+			continue;
+
+		handle_item(std::move(item));
 
 		maybe_start_worker();
 		std::this_thread::yield();

lib/mu-server.cc

@@ -149,6 +149,7 @@ struct Server::Private {
 	Store&            store() { return store_; }
 	const Store&      store() const { return store_; }
 	Indexer&          indexer() { return store().indexer(); }
+	void              do_index(const Indexer::Config& conf);
 	//CommandMap&       command_map() const { return command_map_; }
 
 	//
@@ -762,20 +763,8 @@ get_stats(const Indexer::Progress& stats, const std::string& state)
 }
 
 void
-Server::Private::index_handler(const Command& cmd)
+Server::Private::do_index(const Indexer::Config& conf)
 {
-	Mu::Indexer::Config conf{};
-	conf.cleanup    = cmd.boolean_arg(":cleanup");
-	conf.lazy_check = cmd.boolean_arg(":lazy-check");
-	// ignore .noupdate with an empty store.
-	conf.ignore_noupdate = store().empty();
-
-	indexer().stop();
-	if (index_thread_.joinable())
-		index_thread_.join();
-
-	// start a background track.
-	index_thread_ = std::thread([this, conf = std::move(conf)] {
 	StopWatch sw{"indexing"};
 	indexer().start(conf);
 	while (indexer().is_running()) {
@@ -785,7 +774,34 @@ Server::Private::index_handler(const Command& cmd)
 	}
 	output_sexp(get_stats(indexer().progress(), "complete"),
 		    Server::OutputFlags::Flush);
+}
+
+void
+Server::Private::index_handler(const Command& cmd)
+{
+	Mu::Indexer::Config conf{};
+	conf.cleanup    = cmd.boolean_arg(":cleanup");
+	conf.lazy_check = cmd.boolean_arg(":lazy-check");
+	// ignore .noupdate with an empty store.
+	conf.ignore_noupdate = store().empty();
+
+#ifdef XAPIAN_SINGLE_THREADED
+	// nothing to do
+	if (indexer().is_running()) {
+		throw Error{Error::Code::Xapian, "indexer is already running"};
+	}
+	do_index(conf);
+#else
+	indexer().stop();
+	if (index_thread_.joinable())
+		index_thread_.join();
+
+	// start a background track.
+	index_thread_ = std::thread([this, conf = std::move(conf)] {
+		do_index(conf);
 	});
+#endif /*XAPIAN_SINGLE_THREADED */
+
 }
 
 void
@@ -959,6 +975,9 @@ Server::Private::ping_handler(const Command& cmd)
 				       ":personal-addresses", std::move(addrs),
 				       ":database-path", store().path(),
 				       ":root-maildir", store().root_maildir(),
+#ifdef XAPIAN_SINGLE_THREADED
+				       ":xapian-single-threaded", Sexp::t_sym,
+#endif /*XAPIAN_SINGLE_THREADED*/
 				       ":doccount", storecount)));
 }
 

meson.build

@@ -133,7 +133,13 @@ add_project_arguments(['-DHAVE_CONFIG_H'], language: 'cpp')
 config_h_dep=declare_dependency(
   include_directories: include_directories(['.']))
 
-
+#
+# single-threaded Xapian access?
+#
+if get_option('xapian-single-threaded')
+  config_h_data.set('XAPIAN_SINGLE_THREADED', true)
+  message('use Xapian only in a single thread')
+endif
 #
 # d_type, d_ino are not available universally, so let's check
 # (we use them for optimizations in mu-scanner
@@ -322,6 +328,8 @@ if gmime_dep.version() == '3.2.13'
   warning('See: https://github.com/jstedfast/gmime/issues/133')
 endif
 
+
+
 # Local Variables:
 # indent-tabs-mode: nil
 # End:

meson_options.txt

@@ -14,36 +14,56 @@
 ## along with this program; if not, write to the Free Software Foundation,
 ## Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 
-option('tests',
+
+option('cld2',
        type : 'feature',
        value: 'auto',
-       description: 'build unit tests')
+       description: 'Add support for language-detection through cld2')
+
+#
+# emacs
+#
+
+option('emacs',
+       type: 'string',
+       value: 'emacs',
+       description: 'name/path of the emacs executable (for byte-compilation)')
+
+option('lispdir',
+       type: 'string',
+       description: 'path under which to install emacs-lisp files')
+
+
+#
+# guile
+#
 
 option('guile',
        type : 'feature',
        value: 'auto',
        description: 'build the guile scripting support (requires guile-3.x)')
 
-option('cld2',
-       type : 'feature',
-       value: 'auto',
-       description: 'Compact Language Detector2')
-
 # by default, this uses guile_dep.get_variable(pkgconfig: 'extensiondir')
 option('guile-extension-dir',
        type: 'string',
        description: 'custom install path for the guile extension module')
 
+
+#
+# misc
+#
+
+option('tests',
+       type : 'feature',
+       value: 'auto',
+       description: 'build unit tests')
+
+option('xapian-single-threaded',
+       type : 'boolean',
+       value: true,
+       description: 'only use Xapian from a single thread')
+
 option('readline',
        type: 'feature',
        value: 'auto',
        description: 'enable readline support for the mu4e repl')
-
-option('emacs',
-       type: 'string',
-       value: 'emacs',
-       description: 'name/path of the emacs executable')
-
-option('lispdir',
-       type: 'string',
-       description: 'path under which to install emacs-lisp files')