[PATCH 5/5] Avoid race when writing to index cache

[Tom Tromey <tom@tromey.com>]

The background DWARF reader changes introduced a race when writing to
the index cache.  The problem here is that constructing the
index_cache_store_context object should only happen on the main
thread, to ensure that the various value captures do not race.

This patch adds an assert to the construct to that effect, and then
arranges for this object to be constructed by the cooked_index_worker
constructor -- which is only invoked on the main thread.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31262
---
 gdb/dwarf2/cooked-index.c | 27 ++++++++++++---------------
 gdb/dwarf2/cooked-index.h | 15 ++++++++++-----
 gdb/dwarf2/index-cache.c  |  4 ++++
 3 files changed, 26 insertions(+), 20 deletions(-)

diff --git a/gdb/dwarf2/cooked-index.c b/gdb/dwarf2/cooked-index.c
index ad9fe871552..25f3f132989 100644
--- a/gdb/dwarf2/cooked-index.c
+++ b/gdb/dwarf2/cooked-index.c
@@ -566,6 +566,15 @@ cooked_index_worker::set (cooked_state desired_state)
 #endif /* CXX_STD_THREAD */
 }
 
+/* See cooked-index.h.  */
+
+void
+cooked_index_worker::write_to_cache (const cooked_index *idx) const
+{
+  if (idx != nullptr)
+    m_cache_store.store ();
+}
+
 cooked_index::cooked_index (dwarf2_per_objfile *per_objfile,
 			    std::unique_ptr<cooked_index_worker> &&worker)
   : m_state (std::move (worker)),
@@ -609,17 +618,16 @@ cooked_index::set_contents (vec_type &&vec)
 
   m_state->set (cooked_state::MAIN_AVAILABLE);
 
-  index_cache_store_context ctx (global_index_cache, m_per_bfd);
-
   /* This is run after finalization is done -- but not before.  If
      this task were submitted earlier, it would have to wait for
      finalization.  However, that would take a slot in the global
      thread pool, and if enough such tasks were submitted at once, it
      would cause a livelock.  */
-  gdb::task_group finalizers ([this, ctx = std::move (ctx)] ()
+  gdb::task_group finalizers ([this] ()
   {
     m_state->set (cooked_state::FINALIZED);
-    maybe_write_index (ctx);
+    m_state->write_to_cache (index_for_writing ());
+    m_state->set (cooked_state::CACHE_DONE);
   });
 
   for (auto &idx : m_vector)
@@ -824,17 +832,6 @@ cooked_index::dump (gdbarch *arch)
     }
 }
 
-void
-cooked_index::maybe_write_index (const index_cache_store_context &ctx)
-{
-  if (index_for_writing () != nullptr)
-    {
-      /* (maybe) store an index in the cache.  */
-      ctx.store ();
-    }
-  m_state->set (cooked_state::CACHE_DONE);
-}
-
 /* Wait for all the index cache entries to be written before gdb
    exits.  */
 static void
diff --git a/gdb/dwarf2/cooked-index.h b/gdb/dwarf2/cooked-index.h
index fd7686205d5..a5fe479df14 100644
--- a/gdb/dwarf2/cooked-index.h
+++ b/gdb/dwarf2/cooked-index.h
@@ -511,7 +511,8 @@ class cooked_index_worker
 public:
 
   explicit cooked_index_worker (dwarf2_per_objfile *per_objfile)
-    : m_per_objfile (per_objfile)
+    : m_per_objfile (per_objfile),
+      m_cache_store (global_index_cache, per_objfile->per_bfd)
   { }
   virtual ~cooked_index_worker ()
   { }
@@ -530,10 +531,15 @@ class cooked_index_worker
 
 protected:
 
-  /* Let cooked_index call the 'set' method.  */
+  /* Let cooked_index call the 'set' and 'write_to_cache' methods.  */
   friend class cooked_index;
+
+  /* Set the current state.  */
   void set (cooked_state desired_state);
 
+  /* Write to the index cache.  */
+  void write_to_cache (const cooked_index *idx) const;
+
   /* Helper function that does the work of reading.  This must be able
      to be run in a worker thread without problems.  */
   virtual void do_reading () = 0;
@@ -578,6 +584,8 @@ class cooked_index_worker
      scanning is stopped and this exception will later be reported by
      the 'wait' method.  */
   std::optional<gdb_exception> m_failed;
+  /* An object used to write to the index cache.  */
+  index_cache_store_context m_cache_store;
 };
 
 /* The main index of DIEs.
@@ -715,9 +723,6 @@ class cooked_index : public dwarf_scanner_base
 
 private:
 
-  /* Maybe write the index to the index cache.  */
-  void maybe_write_index (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.  */
   vec_type m_vector;
diff --git a/gdb/dwarf2/index-cache.c b/gdb/dwarf2/index-cache.c
index 780d2c4f200..d9047ef46bb 100644
--- a/gdb/dwarf2/index-cache.c
+++ b/gdb/dwarf2/index-cache.c
@@ -33,6 +33,7 @@
 #include "gdbsupport/selftest.h"
 #include <string>
 #include <stdlib.h>
+#include "run-on-main-thread.h"
 
 /* When set to true, show debug messages about the index cache.  */
 static bool debug_index_cache = false;
@@ -94,6 +95,9 @@ index_cache_store_context::index_cache_store_context (const index_cache &ic,
      m_dir (ic.m_dir),
      m_per_bfd (per_bfd)
 {
+  /* Capturing globals may only be done on the main thread.  */
+  gdb_assert (is_main_thread ());
+
   if (!m_enabled)
     return;
 

-- 
2.43.0