[PATCH 0/2] Write DWARF index cache files in background

[PATCH 0/2] Write DWARF index cache files in background

[Tom Tromey]

We were discussing the interaction between the DWARF cooked indexer
and the DWARF index cache feature on irc and I realized that gdb's
interactive behavior would be improved by moving this work to the
background.

Regression tested on x86-64 Fedora 34.

Tom

[PATCH 1/2] Only use the per-BFD object to write a DWARF index

[Tom Tromey]

The DWARF index does not need access to the objfile or per-objfile
objects when writing -- it's entirely based on the objfile-independent
per-BFD data.

This patch implements this idea by changing the entire API to only be
passed the per-BFD object.  This simplifies some lifetime reasoning
for the next patch.
---
 gdb/dwarf2/index-cache.c | 16 +++----
 gdb/dwarf2/index-cache.h |  4 +-
 gdb/dwarf2/index-write.c | 95 +++++++++++++++++++---------------------
 gdb/dwarf2/index-write.h |  2 +-
 gdb/dwarf2/read.c        |  2 +-
 5 files changed, 57 insertions(+), 62 deletions(-)

diff --git a/gdb/dwarf2/index-cache.c b/gdb/dwarf2/index-cache.c
index 18e60cbd78c..228c425eade 100644
--- a/gdb/dwarf2/index-cache.c
+++ b/gdb/dwarf2/index-cache.c
@@ -89,19 +89,17 @@ index_cache::disable ()
 /* See dwarf-index-cache.h.  */
 
 void
-index_cache::store (dwarf2_per_objfile *per_objfile)
+index_cache::store (dwarf2_per_bfd *per_bfd)
 {
-  objfile *obj = per_objfile->objfile;
-
   if (!enabled ())
     return;
 
   /* Get build id of objfile.  */
-  const bfd_build_id *build_id = build_id_bfd_get (obj->obfd);
+  const bfd_build_id *build_id = build_id_bfd_get (per_bfd->obfd);
   if (build_id == nullptr)
     {
       index_cache_debug ("objfile %s has no build id",
-			 objfile_name (obj));
+			 bfd_get_filename (per_bfd->obfd));
       return;
     }
 
@@ -109,7 +107,7 @@ index_cache::store (dwarf2_per_objfile *per_objfile)
 
   /* Get build id of dwz file, if present.  */
   gdb::optional<std::string> dwz_build_id_str;
-  const dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd);
+  const dwz_file *dwz = dwarf2_get_dwz_file (per_bfd);
   const char *dwz_build_id_ptr = NULL;
 
   if (dwz != nullptr)
@@ -144,18 +142,18 @@ index_cache::store (dwarf2_per_objfile *per_objfile)
 	}
 
       index_cache_debug ("writing index cache for objfile %s",
-			 objfile_name (obj));
+			 bfd_get_filename (per_bfd->obfd));
 
       /* Write the index itself to the directory, using the build id as the
 	 filename.  */
-      write_dwarf_index (per_objfile, m_dir.c_str (),
+      write_dwarf_index (per_bfd, m_dir.c_str (),
 			 build_id_str.c_str (), dwz_build_id_ptr,
 			 dw_index_kind::GDB_INDEX);
     }
   catch (const gdb_exception_error &except)
     {
       index_cache_debug ("couldn't store index cache for objfile %s: %s",
-			 objfile_name (obj), except.what ());
+			 bfd_get_filename (per_bfd->obfd), except.what ());
     }
 }
 
diff --git a/gdb/dwarf2/index-cache.h b/gdb/dwarf2/index-cache.h
index 6366a9a9360..a6ac0112e41 100644
--- a/gdb/dwarf2/index-cache.h
+++ b/gdb/dwarf2/index-cache.h
@@ -24,6 +24,8 @@
 #include "gdbsupport/array-view.h"
 #include "symfile.h"
 
+class dwarf2_per_bfd;
+
 /* Base of the classes used to hold the resources of the indices loaded from
    the cache (e.g. mmapped files).  */
 
@@ -53,7 +55,7 @@ class index_cache
   void disable ();
 
   /* Store an index for the specified object file in the cache.  */
-  void store (dwarf2_per_objfile *per_objfile);
+  void store (dwarf2_per_bfd *per_bfd);
 
   /* Look for an index file matching BUILD_ID.  If found, return the contents
      as an array_view and store the underlying resources (allocated memory,
diff --git a/gdb/dwarf2/index-write.c b/gdb/dwarf2/index-write.c
index 58b0f0b98e3..b8c8b84c775 100644
--- a/gdb/dwarf2/index-write.c
+++ b/gdb/dwarf2/index-write.c
@@ -495,7 +495,7 @@ write_address_map (struct addrmap *addrmap, data_buf &addr_vec,
 class debug_names
 {
 public:
-  debug_names (dwarf2_per_objfile *per_objfile, bool is_dwarf64,
+  debug_names (dwarf2_per_bfd *per_bfd, bool is_dwarf64,
 	       bfd_endian dwarf5_byte_order)
     : m_dwarf5_byte_order (dwarf5_byte_order),
       m_dwarf32 (dwarf5_byte_order),
@@ -505,7 +505,7 @@ class debug_names
 	       : static_cast<dwarf &> (m_dwarf32)),
       m_name_table_string_offs (m_dwarf.name_table_string_offs),
       m_name_table_entry_offs (m_dwarf.name_table_entry_offs),
-      m_debugstrlookup (per_objfile)
+      m_debugstrlookup (per_bfd)
   {}
 
   int dwarf5_offset_size () const
@@ -750,23 +750,23 @@ class debug_names
   {
   public:
 
-    /* Object constructor to be called for current DWARF2_PER_OBJFILE.
+    /* Object constructor to be called for current DWARF2_PER_BFD.
        All .debug_str section strings are automatically stored.  */
-    debug_str_lookup (dwarf2_per_objfile *per_objfile)
-      : m_abfd (per_objfile->objfile->obfd),
-	m_per_objfile (per_objfile)
+    debug_str_lookup (dwarf2_per_bfd *per_bfd)
+      : m_abfd (per_bfd->obfd),
+	m_per_bfd (per_bfd)
     {
-      per_objfile->per_bfd->str.read (per_objfile->objfile);
-      if (per_objfile->per_bfd->str.buffer == NULL)
+      gdb_assert (per_bfd->str.readin);
+      if (per_bfd->str.buffer == NULL)
 	return;
-      for (const gdb_byte *data = per_objfile->per_bfd->str.buffer;
-	   data < (per_objfile->per_bfd->str.buffer
-		   + per_objfile->per_bfd->str.size);)
+      for (const gdb_byte *data = per_bfd->str.buffer;
+	   data < (per_bfd->str.buffer
+		   + per_bfd->str.size);)
 	{
 	  const char *const s = reinterpret_cast<const char *> (data);
 	  const auto insertpair
 	    = m_str_table.emplace (c_str_view (s),
-				   data - per_objfile->per_bfd->str.buffer);
+				   data - per_bfd->str.buffer);
 	  if (!insertpair.second)
 	    complaint (_("Duplicate string \"%s\" in "
 			 ".debug_str section [in module %s]"),
@@ -783,7 +783,7 @@ class debug_names
       const auto it = m_str_table.find (c_str_view (s));
       if (it != m_str_table.end ())
 	return it->second;
-      const size_t offset = (m_per_objfile->per_bfd->str.size
+      const size_t offset = (m_per_bfd->str.size
 			     + m_str_add_buf.size ());
       m_str_table.emplace (c_str_view (s), offset);
       m_str_add_buf.append_cstr0 (s);
@@ -799,7 +799,7 @@ class debug_names
   private:
     std::unordered_map<c_str_view, size_t, c_str_view_hasher> m_str_table;
     bfd *const m_abfd;
-    dwarf2_per_objfile *m_per_objfile;
+    dwarf2_per_bfd *m_per_bfd;
 
     /* Data to add at the end of .debug_str for new needed symbol names.  */
     data_buf m_str_add_buf;
@@ -1011,9 +1011,9 @@ class debug_names
    .debug_names section.  */
 
 static bool
-check_dwarf64_offsets (dwarf2_per_objfile *per_objfile)
+check_dwarf64_offsets (dwarf2_per_bfd *per_bfd)
 {
-  for (const auto &per_cu : per_objfile->per_bfd->all_comp_units)
+  for (const auto &per_cu : per_bfd->all_comp_units)
     {
       if (to_underlying (per_cu->sect_off)
 	  >= (static_cast<uint64_t> (1) << 32))
@@ -1087,12 +1087,12 @@ write_gdbindex_1 (FILE *out_file,
 /* Write the contents of the internal "cooked" index.  */
 
 static void
-write_cooked_index (dwarf2_per_objfile *per_objfile,
+write_cooked_index (dwarf2_per_bfd *per_bfd,
 		    const cu_index_map &cu_index_htab,
 		    struct mapped_symtab *symtab)
 {
   for (const cooked_index_entry *entry
-	 : per_objfile->per_bfd->cooked_index_table->all_entries ())
+	 : per_bfd->cooked_index_table->all_entries ())
     {
       const auto it = cu_index_htab.find (entry->per_cu);
       gdb_assert (it != cu_index_htab.cend ());
@@ -1123,7 +1123,7 @@ write_cooked_index (dwarf2_per_objfile *per_objfile,
    associated dwz file, DWZ_OUT_FILE must be NULL.  */
 
 static void
-write_gdbindex (dwarf2_per_objfile *per_objfile, FILE *out_file,
+write_gdbindex (dwarf2_per_bfd *per_bfd, FILE *out_file,
 		FILE *dwz_out_file)
 {
   mapped_symtab symtab;
@@ -1135,7 +1135,7 @@ write_gdbindex (dwarf2_per_objfile *per_objfile, FILE *out_file,
      in the index file).  This will later be needed to write the address
      table.  */
   cu_index_map cu_index_htab;
-  cu_index_htab.reserve (per_objfile->per_bfd->all_comp_units.size ());
+  cu_index_htab.reserve (per_bfd->all_comp_units.size ());
 
   /* Store out the .debug_type CUs, if any.  */
   data_buf types_cu_list;
@@ -1146,10 +1146,9 @@ write_gdbindex (dwarf2_per_objfile *per_objfile, FILE *out_file,
 
   int counter = 0;
   int types_counter = 0;
-  for (int i = 0; i < per_objfile->per_bfd->all_comp_units.size (); ++i)
+  for (int i = 0; i < per_bfd->all_comp_units.size (); ++i)
     {
-      dwarf2_per_cu_data *per_cu
-	= per_objfile->per_bfd->all_comp_units[i].get ();
+      dwarf2_per_cu_data *per_cu = per_bfd->all_comp_units[i].get ();
 
       int &this_counter = per_cu->is_debug_types ? types_counter : counter;
 
@@ -1178,12 +1177,12 @@ write_gdbindex (dwarf2_per_objfile *per_objfile, FILE *out_file,
       ++this_counter;
     }
 
-  write_cooked_index (per_objfile, cu_index_htab, &symtab);
+  write_cooked_index (per_bfd, cu_index_htab, &symtab);
 
   /* Dump the address map.  */
   data_buf addr_vec;
   std::vector<addrmap *> addrmaps
-    = per_objfile->per_bfd->cooked_index_table->get_addrmaps ();
+    = per_bfd->cooked_index_table->get_addrmaps ();
   for (auto map : addrmaps)
     write_address_map (map, addr_vec, cu_index_htab);
 
@@ -1214,26 +1213,24 @@ static const gdb_byte dwarf5_gdb_augmentation[] = { 'G', 'D', 'B', 0 };
    many bytes were expected to be written into OUT_FILE.  */
 
 static void
-write_debug_names (dwarf2_per_objfile *per_objfile,
+write_debug_names (dwarf2_per_bfd *per_bfd,
 		   FILE *out_file, FILE *out_file_str)
 {
-  const bool dwarf5_is_dwarf64 = check_dwarf64_offsets (per_objfile);
-  struct objfile *objfile = per_objfile->objfile;
+  const bool dwarf5_is_dwarf64 = check_dwarf64_offsets (per_bfd);
   const enum bfd_endian dwarf5_byte_order
-    = gdbarch_byte_order (objfile->arch ());
+    = bfd_big_endian (per_bfd->obfd) ? BFD_ENDIAN_BIG : BFD_ENDIAN_LITTLE;
 
   /* The CU list is already sorted, so we don't need to do additional
      work here.  Also, the debug_types entries do not appear in
      all_comp_units, but only in their own hash table.  */
   data_buf cu_list;
   data_buf types_cu_list;
-  debug_names nametable (per_objfile, dwarf5_is_dwarf64, dwarf5_byte_order);
+  debug_names nametable (per_bfd, dwarf5_is_dwarf64, dwarf5_byte_order);
   int counter = 0;
   int types_counter = 0;
-  for (int i = 0; i < per_objfile->per_bfd->all_comp_units.size (); ++i)
+  for (int i = 0; i < per_bfd->all_comp_units.size (); ++i)
     {
-      dwarf2_per_cu_data *per_cu
-	= per_objfile->per_bfd->all_comp_units[i].get ();
+      dwarf2_per_cu_data *per_cu = per_bfd->all_comp_units[i].get ();
 
       int &this_counter = per_cu->is_debug_types ? types_counter : counter;
       data_buf &this_list = per_cu->is_debug_types ? types_cu_list : cu_list;
@@ -1246,12 +1243,12 @@ write_debug_names (dwarf2_per_objfile *per_objfile,
     }
 
    /* Verify that all units are represented.  */
-  gdb_assert (counter == (per_objfile->per_bfd->all_comp_units.size ()
-			  - per_objfile->per_bfd->tu_stats.nr_tus));
-  gdb_assert (types_counter == per_objfile->per_bfd->tu_stats.nr_tus);
+  gdb_assert (counter == (per_bfd->all_comp_units.size ()
+			  - per_bfd->tu_stats.nr_tus));
+  gdb_assert (types_counter == per_bfd->tu_stats.nr_tus);
 
   for (const cooked_index_entry *entry
-	 : per_objfile->per_bfd->cooked_index_table->all_entries ())
+	 : per_bfd->cooked_index_table->all_entries ())
     nametable.insert (entry);
 
   nametable.build ();
@@ -1382,26 +1379,24 @@ struct index_wip_file
 /* See dwarf-index-write.h.  */
 
 void
-write_dwarf_index (dwarf2_per_objfile *per_objfile, const char *dir,
+write_dwarf_index (dwarf2_per_bfd *per_bfd, const char *dir,
 		   const char *basename, const char *dwz_basename,
 		   dw_index_kind index_kind)
 {
-  struct objfile *objfile = per_objfile->objfile;
-
-  if (per_objfile->per_bfd->cooked_index_table == nullptr)
+  if (per_bfd->cooked_index_table == nullptr)
     {
-      if (per_objfile->per_bfd->index_table != nullptr
-	  || per_objfile->per_bfd->debug_names_table != nullptr)
+      if (per_bfd->index_table != nullptr
+	  || per_bfd->debug_names_table != nullptr)
 	error (_("Cannot use an index to create the index"));
       error (_("No debugging symbols"));
     }
 
-  if (per_objfile->per_bfd->types.size () > 1)
+  if (per_bfd->types.size () > 1)
     error (_("Cannot make an index when the file has multiple .debug_types sections"));
 
   struct stat st;
-  if (stat (objfile_name (objfile), &st) < 0)
-    perror_with_name (objfile_name (objfile));
+  if (stat (bfd_get_filename (per_bfd->obfd), &st) < 0)
+    perror_with_name (bfd_get_filename (per_bfd->obfd));
 
   const char *index_suffix = (index_kind == dw_index_kind::DEBUG_NAMES
 			      ? INDEX5_SUFFIX : INDEX4_SUFFIX);
@@ -1416,13 +1411,13 @@ write_dwarf_index (dwarf2_per_objfile *per_objfile, const char *dir,
     {
       index_wip_file str_wip_file (dir, basename, DEBUG_STR_SUFFIX);
 
-      write_debug_names (per_objfile, objfile_index_wip.out_file.get (),
+      write_debug_names (per_bfd, objfile_index_wip.out_file.get (),
 			 str_wip_file.out_file.get ());
 
       str_wip_file.finalize ();
     }
   else
-    write_gdbindex (per_objfile, objfile_index_wip.out_file.get (),
+    write_gdbindex (per_bfd, objfile_index_wip.out_file.get (),
 		    (dwz_index_wip.has_value ()
 		     ? dwz_index_wip->out_file.get () : NULL));
 
@@ -1479,8 +1474,8 @@ save_gdb_index_command (const char *arg, int from_tty)
 	      if (dwz != NULL)
 		dwz_basename = lbasename (dwz->filename ());
 
-	      write_dwarf_index (per_objfile, arg, basename, dwz_basename,
-				 index_kind);
+	      write_dwarf_index (per_objfile->per_bfd, arg, basename,
+				 dwz_basename, index_kind);
 	    }
 	  catch (const gdb_exception_error &except)
 	    {
diff --git a/gdb/dwarf2/index-write.h b/gdb/dwarf2/index-write.h
index b8855dcae7c..3a9c1432c7f 100644
--- a/gdb/dwarf2/index-write.h
+++ b/gdb/dwarf2/index-write.h
@@ -33,7 +33,7 @@
    same, but for the dwz file's index.  */
 
 extern void write_dwarf_index
-  (dwarf2_per_objfile *per_objfile, const char *dir, const char *basename,
+  (dwarf2_per_bfd *per_bfd, const char *dir, const char *basename,
    const char *dwz_basename, dw_index_kind index_kind);
 
 #endif /* DWARF_INDEX_WRITE_H */
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 698720276a9..954ef11b19e 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -5419,7 +5419,7 @@ dwarf2_build_psymtabs (struct objfile *objfile, bool already_attached)
       dwarf2_build_psymtabs_hard (per_objfile);
 
       /* (maybe) store an index in the cache.  */
-      global_index_cache.store (per_objfile);
+      global_index_cache.store (per_objfile->per_bfd);
     }
   catch (const gdb_exception_error &except)
     {
-- 
2.34.1

[PATCH 2/2] Write the DWARF index in the background

[Tom Tromey]

The new DWARF cooked indexer interacts poorly with the DWARF index
cache.  In particular, the cache will require gdb to wait for the
cooked index to be finalized.  As this happens in the foreground, it
means that users with this setting enabled will see a slowdown.

This patch changes gdb to write the cache entry a worker thread.  (As
usual, in the absence of threads, this work is simply done immediately
in the main thread.)

Some care is taken to ensure that this can't crash, and that gdb will
not exit before the task is complete.

To avoid use-after-free problems, the DWARF per-BFD object explicitly
waits for the index cache task to complete.

To avoid gdb exiting early, an exit observer is used to wait for all
such pending tasks.

In normal use, neither of these waits will be very visible.  For users
using "-batch" to pre-generate the index, though, it would be.
However I don't think there is much to be done about this, as it was
the status quo ante.
---
 gdb/dwarf2/cooked-index.c | 71 +++++++++++++++++++++++++++++++++++++--
 gdb/dwarf2/cooked-index.h | 30 +++++++++++------
 gdb/dwarf2/read.c         | 10 +++---
 3 files changed, 94 insertions(+), 17 deletions(-)

diff --git a/gdb/dwarf2/cooked-index.c b/gdb/dwarf2/cooked-index.c
index b66ef5a1c64..7084ad18c20 100644
--- a/gdb/dwarf2/cooked-index.c
+++ b/gdb/dwarf2/cooked-index.c
@@ -20,10 +20,19 @@
 #include "defs.h"
 #include "dwarf2/cooked-index.h"
 #include "dwarf2/read.h"
+#include "dwarf2/index-cache.h"
 #include "cp-support.h"
 #include "ada-lang.h"
 #include "split-name.h"
+#include "observable.h"
+#include "run-on-main-thread.h"
 #include <algorithm>
+#include <unordered_set>
+
+/* We don't want gdb to exit while it is in the process of writing to
+   the index cache.  So, all live cooked index vectors are stored
+   here, and then these are all waited for before exit proceeds.  */
+static std::unordered_set<cooked_index_vector *> active_vectors;
 
 /* Hash function for cooked_index_entry.  */
 
@@ -113,14 +122,45 @@ cooked_index::add (sect_offset die_offset, enum dwarf_tag tag,
   return result;
 }
 
-cooked_index_vector::cooked_index_vector (vec_type &&vec)
+cooked_index_vector::cooked_index_vector (vec_type &&vec,
+					  dwarf2_per_bfd *per_bfd)
   : m_vector (std::move (vec)),
     m_future (gdb::thread_pool::g_thread_pool->post_task
 	      ([this] ()
 	      {
 		finalize ();
-	      }))
+	      })),
+    m_write_future (gdb::thread_pool::g_thread_pool->post_task
+		    ([this, per_bfd] ()
+		    {
+		      maybe_write_index (per_bfd);
+		    }))
 {
+  /* ACTIVE_VECTORS is not locked, and this assert ensures that this
+     will be caught if ever moved to the background.  */
+  gdb_assert (is_main_thread ());
+  active_vectors.insert (this);
+}
+
+cooked_index_vector::~cooked_index_vector ()
+{
+  /* The 'finalize' method may be run in a different thread.  If
+     this object is destroyed before this completes, then the method
+     will end up writing to freed memory.  Waiting for this to
+     complete avoids this problem; and the cost seems ignorable
+     because creating and immediately destroying the debug info is a
+     relatively rare thing to do.  */
+  m_future.wait ();
+
+  /* Likewise for the index-creating future, though this one must also
+     waited for by the per-BFD object to ensure the required data
+     remains live.  */
+  m_write_future.wait ();
+
+  /* Remove our entry from the global list.  See the assert in the
+     constructor to understand this.  */
+  gdb_assert (is_main_thread ());
+  active_vectors.erase (this);
 }
 
 /* See cooked-index.h.  */
@@ -340,3 +380,30 @@ cooked_index_vector::finalize ()
 	       return *a < *b;
 	     });
 }
+
+void
+cooked_index_vector::maybe_write_index (dwarf2_per_bfd *per_bfd)
+{
+  /* Wait for finalization.  */
+  m_future.wait ();
+
+  /* (maybe) store an index in the cache.  */
+  global_index_cache.store (per_bfd);
+}
+
+/* Wait for all the index cache entries to be written before gdb
+   exits.  */
+static void
+wait_for_index_cache (int)
+{
+  gdb_assert (is_main_thread ());
+  for (cooked_index_vector *item : active_vectors)
+    item->wait ();
+}
+
+void _initialize_cooked_index ();
+void
+_initialize_cooked_index ()
+{
+  gdb::observers::gdb_exiting.attach (wait_for_index_cache, "cooked-index");
+}
diff --git a/gdb/dwarf2/cooked-index.h b/gdb/dwarf2/cooked-index.h
index 661664d9f84..940f0a79b20 100644
--- a/gdb/dwarf2/cooked-index.h
+++ b/gdb/dwarf2/cooked-index.h
@@ -33,6 +33,7 @@
 #include "gdbsupport/thread-pool.h"
 
 struct dwarf2_per_cu_data;
+struct dwarf2_per_bfd;
 
 /* Flags that describe an entry in the index.  */
 enum cooked_index_flag_enum : unsigned char
@@ -251,19 +252,10 @@ class cooked_index_vector
      object.  */
   typedef std::vector<std::unique_ptr<cooked_index>> vec_type;
 
-  explicit cooked_index_vector (vec_type &&vec);
+  explicit cooked_index_vector (vec_type &&vec, dwarf2_per_bfd *per_bfd);
   DISABLE_COPY_AND_ASSIGN (cooked_index_vector);
 
-  ~cooked_index_vector ()
-  {
-    /* The 'finalize' method may be run in a different thread.  If
-       this object is destroyed before this completes, then the method
-       will end up writing to freed memory.  Waiting for this to
-       complete avoids this problem; and the cost seems ignorable
-       because creating and immediately destroying the debug info is a
-       relatively rare thing to do.  */
-    m_future.wait ();
-  }
+  ~cooked_index_vector ();
 
   /* A simple range over part of m_entries.  */
   typedef iterator_range<std::vector<cooked_index_entry *>::iterator> range;
@@ -293,6 +285,16 @@ class cooked_index_vector
      "main".  This will return NULL if no such entry is available.  */
   const cooked_index_entry *get_main () const;
 
+  /* Wait for the index to be completely finished.  For ordinary uses,
+     the index code ensures this itself -- e.g., 'all_entries' will
+     wait on the 'finalize' future.  However, on destruction, if an
+     index is being written, it's also necessary to wait for that to
+     complete.  */
+  void wait ()
+  {
+    m_write_future.wait ();
+  }
+
 private:
 
   /* GNAT only emits mangled ("encoded") names in the DWARF, and does
@@ -308,6 +310,9 @@ class cooked_index_vector
      into the internal hash table.  */
   void finalize ();
 
+  /* Maybe write the index to the index cache.  */
+  void maybe_write_index (dwarf2_per_bfd *per_bfd);
+
   /* The vector of cooked_index objects.  This is stored because the
      entries are stored on the obstacks in those objects.  */
   vec_type m_vector;
@@ -322,6 +327,9 @@ class cooked_index_vector
      that the 'get' method is never called on this future, only
      'wait'.  */
   std::future<void> m_future;
+
+  /* A future that tracks when the 'index_write' method is done.  */
+  std::future<void> m_write_future;
 };
 
 #endif /* GDB_DWARF2_COOKED_INDEX_H */
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 954ef11b19e..5f98cb31c84 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -1523,6 +1523,11 @@ dwarf2_per_bfd::dwarf2_per_bfd (bfd *obfd, const dwarf2_debug_sections *names,
 
 dwarf2_per_bfd::~dwarf2_per_bfd ()
 {
+  /* Data from the per-BFD may be needed when finalizing the cooked
+     index table, so wait here while this happens.  */
+  if (cooked_index_table != nullptr)
+    cooked_index_table->wait ();
+
   for (auto &per_cu : all_comp_units)
     {
       per_cu->imported_symtabs_free ();
@@ -5417,9 +5422,6 @@ dwarf2_build_psymtabs (struct objfile *objfile, bool already_attached)
   try
     {
       dwarf2_build_psymtabs_hard (per_objfile);
-
-      /* (maybe) store an index in the cache.  */
-      global_index_cache.store (per_objfile->per_bfd);
     }
   catch (const gdb_exception_error &except)
     {
@@ -7189,7 +7191,7 @@ dwarf2_build_psymtabs_hard (dwarf2_per_objfile *per_objfile)
      indexes.end ());
   indexes.shrink_to_fit ();
   per_bfd->cooked_index_table.reset
-    (new cooked_index_vector (std::move (indexes)));
+    (new cooked_index_vector (std::move (indexes), per_bfd));
 
   const cooked_index_entry *main_entry
     = per_bfd->cooked_index_table->get_main ();
-- 
2.34.1