Correctly handle mutation of the watcher list while iterating on them

Fixes #1227

src/option_manager.cc

@@ -108,8 +108,13 @@ void OptionManager::on_option_changed(const Option& option)
         find_option(m_options, option.name()) != m_options.end())
         return;
 
-    for (auto watcher : m_watchers)
+    // The watcher list might get mutated during calls to on_option_changed
+    auto watchers = m_watchers;
+    for (auto* watcher : watchers)
+    {
+        if (contains(m_watchers, watcher)) // make sure this watcher is still alive
             watcher->on_option_changed(option);
+    }
 }
 
 CandidateList OptionsRegistry::complete_option_name(StringView prefix,

test/regression/1227-segfault-on-option-access/cmd

@@ -0,0 +1 @@
+:test<ret>

test/regression/1227-segfault-on-option-access/in

@@ -0,0 +1 @@
+

test/regression/1227-segfault-on-option-access/out

@@ -0,0 +1 @@
+k

test/regression/1227-segfault-on-option-access/rc

@@ -0,0 +1,10 @@
+decl str _
+
+hook global BufCreate \*test\* %{
+    hook buffer BufSetOption _=.+ 'exec "i%opt{_}"'
+}
+
+def test %{
+    edit -scratch *test*
+    set buffer=*test* _ "k"
+}