[gdb/symtab] Fix data race on index_cache::m_enabled

With gdb build with -fsanitize=thread and test-case gdb.base/index-cache.exp I
run into:
...
(gdb) file build/gdb/testsuite/outputs/gdb.base/index-cache/index-cache
Reading symbols from build/gdb/testsuite/outputs/gdb.base/index-cache/index-cache...
(gdb) show index-cache enabled
The index cache is off.
(gdb) PASS: gdb.base/index-cache.exp: test_basic_stuff: index-cache is disabled by default
set index-cache enabled on
==================
WARNING: ThreadSanitizer: data race (pid=32248)
  Write of size 1 at 0x00000321f540 by main thread:
    #0 index_cache::enable() gdb/dwarf2/index-cache.c:76 (gdb+0x82cfdd)
    #1 set_index_cache_enabled_command gdb/dwarf2/index-cache.c:270 (gdb+0x82d9af)
    #2 bool setting::set<bool>(bool const&) gdb/command.h:353 (gdb+0x6fe5f2)
    #3 do_set_command(char const*, int, cmd_list_element*) gdb/cli/cli-setshow.c:414 (gdb+0x6fcd21)
    #4 execute_command(char const*, int) gdb/top.c:567 (gdb+0xff2e64)
    #5 command_handler(char const*) gdb/event-top.c:552 (gdb+0x94acc0)
    #6 command_line_handler(std::unique_ptr<char, gdb::xfree_deleter<char> >&&) gdb/event-top.c:788 (gdb+0x94b37d)
    #7 tui_command_line_handler gdb/tui/tui-interp.c:104 (gdb+0x103467e)
    #8 gdb_rl_callback_handler gdb/event-top.c:259 (gdb+0x94a265)
    #9 rl_callback_read_char readline/readline/callback.c:290 (gdb+0x11bdd3f)
    #10 gdb_rl_callback_read_char_wrapper_noexcept gdb/event-top.c:195 (gdb+0x94a064)
    #11 gdb_rl_callback_read_char_wrapper gdb/event-top.c:234 (gdb+0x94a125)
    #12 stdin_event_handler gdb/ui.c:155 (gdb+0x1074922)
    #13 handle_file_event gdbsupport/event-loop.cc:573 (gdb+0x1d94de4)
    #14 gdb_wait_for_event gdbsupport/event-loop.cc:694 (gdb+0x1d9551c)
    #15 gdb_do_one_event(int) gdbsupport/event-loop.cc:264 (gdb+0x1d93908)
    #16 start_event_loop gdb/main.c:412 (gdb+0xb5a256)
    #17 captured_command_loop gdb/main.c:476 (gdb+0xb5a445)
    #18 captured_main gdb/main.c:1320 (gdb+0xb5c5c5)
    #19 gdb_main(captured_main_args*) gdb/main.c:1339 (gdb+0xb5c674)
    #20 main gdb/gdb.c:32 (gdb+0x416776)

  Previous read of size 1 at 0x00000321f540 by thread T12:
    #0 index_cache::enabled() const gdb/dwarf2/index-cache.h:48 (gdb+0x82e1a6)
    #1 index_cache::store(dwarf2_per_bfd*) gdb/dwarf2/index-cache.c:94 (gdb+0x82d0bc)
    #2 cooked_index::maybe_write_index(dwarf2_per_bfd*) gdb/dwarf2/cooked-index.c:638 (gdb+0x7f1b97)
    #3 operator() gdb/dwarf2/cooked-index.c:468 (gdb+0x7f0f24)
    #4 _M_invoke /usr/include/c++/7/bits/std_function.h:316 (gdb+0x7f285b)
    #5 std::function<void ()>::operator()() const /usr/include/c++/7/bits/std_function.h:706 (gdb+0x700952)
    #6 void std::__invoke_impl<void, std::function<void ()>&>(std::__invoke_other, std::function<void ()>&) /usr/include/c++/7/bits/invoke.h:60 (gdb+0x7381a0)
    #7 std::__invoke_result<std::function<void ()>&>::type std::__invoke<std::function<void ()>&>(std::function<void ()>&) /usr/include/c++/7/bits/invoke.h:95 (gdb+0x737e91)
    #8 std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run()::{lambda()#1}::operator()() const /usr/include/c++/7/future:1421 (gdb+0x737b59)
    #9 std::__future_base::_Task_setter<std::unique_ptr<std::__future_base::_Result<void>, std::__future_base::_Result_base::_Deleter>, std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run()::{lambda()#1}, void>::operator()() const /usr/include/c++/7/future:1362 (gdb+0x738660)
    #10 std::_Function_handler<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> (), std::__future_base::_Task_setter<std::unique_ptr<std::__future_base::_Result<void>, std::__future_base::_Result_base::_Deleter>, std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run()::{lambda()#1}, void> >::_M_invoke(std::_Any_data const&) /usr/include/c++/7/bits/std_function.h:302 (gdb+0x73825c)
    #11 std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>::operator()() const /usr/include/c++/7/bits/std_function.h:706 (gdb+0x733623)
    #12 std::__future_base::_State_baseV2::_M_do_set(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*) /usr/include/c++/7/future:561 (gdb+0x732bdf)
    #13 void std::__invoke_impl<void, void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::__invoke_memfun_deref, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) /usr/include/c++/7/bits/invoke.h:73 (gdb+0x734c4f)
    #14 std::__invoke_result<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>::type std::__invoke<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) /usr/include/c++/7/bits/invoke.h:95 (gdb+0x733bc5)
    #15 std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&)::{lambda()#1}::operator()() const /usr/include/c++/7/mutex:672 (gdb+0x73300d)
    #16 std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&)::{lambda()#2}::operator()() const /usr/include/c++/7/mutex:677 (gdb+0x7330b2)
    #17 std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&)::{lambda()#2}::_FUN() /usr/include/c++/7/mutex:677 (gdb+0x7330f2)
    #18 pthread_once <null> (libtsan.so.0+0x4457c)
    #19 __gthread_once /usr/include/c++/7/x86_64-suse-linux/bits/gthr-default.h:699 (gdb+0x72f5dd)
    #20 void std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) /usr/include/c++/7/mutex:684 (gdb+0x733224)
    #21 std::__future_base::_State_baseV2::_M_set_result(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>, bool) /usr/include/c++/7/future:401 (gdb+0x732852)
    #22 std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run() /usr/include/c++/7/future:1423 (gdb+0x737bef)
    #23 std::packaged_task<void ()>::operator()() /usr/include/c++/7/future:1556 (gdb+0x1dac492)
    #24 gdb::thread_pool::thread_function() gdbsupport/thread-pool.cc:242 (gdb+0x1dabdb4)
    #25 void std::__invoke_impl<void, void (gdb::thread_pool::*)(), gdb::thread_pool*>(std::__invoke_memfun_deref, void (gdb::thread_pool::*&&)(), gdb::thread_pool*&&) /usr/include/c++/7/bits/invoke.h:73 (gdb+0x1dace63)
    #26 std::__invoke_result<void (gdb::thread_pool::*)(), gdb::thread_pool*>::type std::__invoke<void (gdb::thread_pool::*)(), gdb::thread_pool*>(void (gdb::thread_pool::*&&)(), gdb::thread_pool*&&) /usr/include/c++/7/bits/invoke.h:95 (gdb+0x1dac294)
    #27 decltype (__invoke((_S_declval<0ul>)(), (_S_declval<1ul>)())) std::thread::_Invoker<std::tuple<void (gdb::thread_pool::*)(), gdb::thread_pool*> >::_M_invoke<0ul, 1ul>(std::_Index_tuple<0ul, 1ul>) /usr/include/c++/7/thread:234 (gdb+0x1daf5c6)
    #28 std::thread::_Invoker<std::tuple<void (gdb::thread_pool::*)(), gdb::thread_pool*> >::operator()() /usr/include/c++/7/thread:243 (gdb+0x1daf551)
    #29 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (gdb::thread_pool::*)(), gdb::thread_pool*> > >::_M_run() /usr/include/c++/7/thread:186 (gdb+0x1daf506)
    #30 <null> <null> (libstdc++.so.6+0xdcac2)

  Location is global 'global_index_cache' of size 48 at 0x00000321f520 (gdb+0x00000321f540)
  ...
SUMMARY: ThreadSanitizer: data race gdb/dwarf2/index-cache.c:76 in index_cache::enable()
...

The race happens when issuing a "file $exec" command followed by a
"set index-cache enabled on" command.

The race is between:
- a worker thread reading index_cache::m_enabled to determine whether an
  index-cache entry for $exec needs to be written
  (due to command "file $exec"), and
- the main thread setting index_cache::m_enabled
  (due to command "set index-cache enabled on").

Fix this by capturing the value of index_cache::m_enabled in the main thread,
and using the captured value in the worker thread.

Tested on x86_64-linux.

PR symtab/30392
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30392

Author: vries

gdb/dwarf2/cooked-index.c

@@ -460,12 +460,15 @@ cooked_index::cooked_index (vec_type &&vec)
 void
 cooked_index::start_writing_index (dwarf2_per_bfd *per_bfd)
 {
+  index_cache_store_context ctx (global_index_cache);
+
   /* This must be set after all the finalization tasks have been
      started, because it may call 'wait'.  */
   m_write_future
-    = gdb::thread_pool::g_thread_pool->post_task ([this, per_bfd] ()
+    = gdb::thread_pool::g_thread_pool->post_task ([this, per_bfd,
+						   ctx = std::move (ctx)] ()
 	{
-	  maybe_write_index (per_bfd);
+	  maybe_write_index (per_bfd, ctx);
 	});
 }
 
@@ -629,13 +632,14 @@ cooked_index::dump (gdbarch *arch) const
 }
 
 void
-cooked_index::maybe_write_index (dwarf2_per_bfd *per_bfd)
+cooked_index::maybe_write_index (dwarf2_per_bfd *per_bfd,
+				 const index_cache_store_context &ctx)
 {
   /* Wait for finalization.  */
   wait ();
 
   /* (maybe) store an index in the cache.  */
-  global_index_cache.store (per_bfd);
+  global_index_cache.store (per_bfd, ctx);
 }
 
 /* Wait for all the index cache entries to be written before gdb

gdb/dwarf2/cooked-index.h

@@ -37,6 +37,7 @@
 
 struct dwarf2_per_cu_data;
 struct dwarf2_per_bfd;
+struct index_cache_store_context;
 
 /* Flags that describe an entry in the index.  */
 enum cooked_index_flag_enum : unsigned char
@@ -435,7 +436,8 @@ class cooked_index : public dwarf_scanner_base
 private:
 
   /* Maybe write the index to the index cache.  */
-  void maybe_write_index (dwarf2_per_bfd *per_bfd);
+  void maybe_write_index (dwarf2_per_bfd *per_bfd,
+			  const index_cache_store_context &);
 
   /* The vector of cooked_index objects.  This is stored because the
      entries are stored on the obstacks in those objects.  */

gdb/dwarf2/index-cache.c

@@ -86,12 +86,20 @@ index_cache::disable ()
   m_enabled = false;
 }
 
+ /* See index-cache.h.  */
+
+index_cache_store_context::index_cache_store_context (const index_cache &ic)
+  :  m_enabled (ic.enabled ())
+{
+}
+
 /* See dwarf-index-cache.h.  */
 
 void
-index_cache::store (dwarf2_per_bfd *per_bfd)
+index_cache::store (dwarf2_per_bfd *per_bfd,
+		    const index_cache_store_context &ctx)
 {
-  if (!enabled ())
+  if (!ctx.m_enabled)
     return;
 
   /* Get build id of objfile.  */

gdb/dwarf2/index-cache.h

@@ -25,6 +25,7 @@
 #include "symfile.h"
 
 class dwarf2_per_bfd;
+class index_cache;
 
 /* Base of the classes used to hold the resources of the indices loaded from
    the cache (e.g. mmapped files).  */
@@ -34,6 +35,20 @@ struct index_cache_resource
   virtual ~index_cache_resource () = 0;
 };
 
+/* Information to be captured in the main thread, and to be used by worker
+   threads during store ().  */
+
+struct index_cache_store_context
+{
+  friend class index_cache;
+
+  explicit index_cache_store_context (const index_cache &ic);
+
+private:
+  /* Captured value of enabled ().  */
+  bool m_enabled;
+};
+
 /* Class to manage the access to the DWARF index cache.  */
 
 class index_cache
@@ -55,7 +70,8 @@ class index_cache
   void disable ();
 
   /* Store an index for the specified object file in the cache.  */
-  void store (dwarf2_per_bfd *per_bfd);
+  void store (dwarf2_per_bfd *per_bfd,
+	      const index_cache_store_context &);
 
   /* Look for an index file matching BUILD_ID.  If found, return the contents
      as an array_view and store the underlying resources (allocated memory,