[PATCH 5/7] gdb/debuginfod: Support on-demand debuginfo downloading

[Aaron Merey <amerey@redhat.com>]

At the beginning of a session, gdb may attempt to download debuginfo
for all shared libraries associated with the process or core file
being debugged.  This can be a waste of time and storage space when much
of the debuginfo ends up not being used during the session.

To reduce the gdb's startup latency and to download only the debuginfo
that is really needed, this patch adds on-demand, a.k.a lazy, downloading.

When 'set debuginfo enabled lazy' is on, gdb will attempt to download
a .gdb_index for each shared library instead of its full debuginfo.
Each debuginfo download will be deferred until gdb needs to expand symtabs
associated with the debuginfo's index.

Because these indices are significantly smaller than their corresponding
debuginfo, this generally reduces the total amount of data gdb downloads
by a large margin.  Reductions of 80%-95% have been commonly observed when
debugging large GUI programs.

    (gdb) set debuginfod enabled lazy
    (gdb) start
    Downloading section .gdb_index for /lib64/libcurl.so.4
    [...]
    1826        client->server_mhandle = curl_multi_init ();
    (gdb) step
    Downloading separate debug info for /lib64/libcurl.so.4
    Downloading separate debug info for [libcurl dwz]
    curl_multi_init () at ../../lib/multi.c:457
    Downloading source file /usr/src/debug/curl-7.85.0-6.fc37.x86_64/build-full/lib/../../lib/multi.c
    457     {
    (gdb)

Some of the key functions below include dwarf2_has_separate_index which
downloads the separate .gdb_index.  If successful, it initializes a
separate debug objfile to own the index and act as a placeholder for
the full debuginfo that might be downloaded later.

read_full_dwarf_from_debuginfod downloads the full debuginfo and
reinitializes the separate debug objfile.  It is called by
dwarf2_gdb_index::expand_symtabs_matching and
dwarf2_base_index_functions::find_pc_sect_compunit_symtab when symtab
expansion is required.
---
 gdb/dwarf2/frame.c          |  13 +++
 gdb/dwarf2/frame.h          |   4 +
 gdb/dwarf2/index-cache.c    |  33 ++++++
 gdb/dwarf2/index-cache.h    |  13 +++
 gdb/dwarf2/public.h         |   7 ++
 gdb/dwarf2/read-gdb-index.c |  89 ++++++++++++----
 gdb/dwarf2/read.c           | 197 +++++++++++++++++++++++++++++++++++-
 gdb/dwarf2/read.h           |   9 ++
 gdb/dwarf2/section.c        |   4 +-
 gdb/elfread.c               |   2 +-
 gdb/objfiles.c              |  18 ++++
 gdb/objfiles.h              |  16 +++
 gdb/symfile.c               |  76 +++++++++++++-
 gdb/symfile.h               |   7 ++
 gdb/symtab.h                |   5 +
 15 files changed, 472 insertions(+), 21 deletions(-)

diff --git a/gdb/dwarf2/frame.c b/gdb/dwarf2/frame.c
index a561aaf3100..f6d227d8b36 100644
--- a/gdb/dwarf2/frame.c
+++ b/gdb/dwarf2/frame.c
@@ -1609,6 +1609,19 @@ set_comp_unit (struct objfile *objfile, struct comp_unit *unit)
   return dwarf2_frame_bfd_data.set (abfd, unit);
 }
 
+/* See frame.h.  */
+
+void
+clear_comp_unit (struct objfile *objfile)
+{
+  bfd *abfd = objfile->obfd.get ();
+
+  if (gdb_bfd_requires_relocations (abfd))
+    dwarf2_frame_objfile_data.clear (objfile);
+  else
+    dwarf2_frame_bfd_data.clear (abfd);
+}
+
 /* Find the FDE for *PC.  Return a pointer to the FDE, and store the
    initial location associated with it into *PC.  */
 
diff --git a/gdb/dwarf2/frame.h b/gdb/dwarf2/frame.h
index 5643e557513..1851a14b67e 100644
--- a/gdb/dwarf2/frame.h
+++ b/gdb/dwarf2/frame.h
@@ -238,6 +238,10 @@ void dwarf2_append_unwinders (struct gdbarch *gdbarch);
 extern const struct frame_base *
   dwarf2_frame_base_sniffer (frame_info_ptr this_frame);
 
+/* Delete OBJFILEs comp_unit.  */
+
+extern void clear_comp_unit (struct objfile * objfile);
+
 /* Compute the DWARF CFA for a frame.  */
 
 CORE_ADDR dwarf2_frame_cfa (frame_info_ptr this_frame);
diff --git a/gdb/dwarf2/index-cache.c b/gdb/dwarf2/index-cache.c
index 79ab706ee9d..bbafcd321b2 100644
--- a/gdb/dwarf2/index-cache.c
+++ b/gdb/dwarf2/index-cache.c
@@ -216,6 +216,33 @@ index_cache::lookup_gdb_index (const bfd_build_id *build_id,
   return {};
 }
 
+/* See index-cache.h.  */
+
+gdb::array_view<const gdb_byte>
+index_cache::lookup_gdb_index_debuginfod (const char *index_path,
+					  std::unique_ptr<index_cache_resource> *resource)
+{
+  try
+    {
+      /* Try to map that file.  */
+      index_cache_resource_mmap *mmap_resource
+	= new index_cache_resource_mmap (index_path);
+
+      /* Hand the resource to the caller.  */
+      resource->reset (mmap_resource);
+
+      return gdb::array_view<const gdb_byte>
+	  ((const gdb_byte *) mmap_resource->mapping.get (),
+	   mmap_resource->mapping.size ());
+    }
+  catch (const gdb_exception_error &except)
+    {
+      warning (_("Unable to read %s: %s"), index_path, except.what ());
+    }
+
+  return {};
+}
+
 #else /* !HAVE_SYS_MMAN_H */
 
 /* See dwarf-index-cache.h.  This is a no-op on unsupported systems.  */
@@ -227,6 +254,12 @@ index_cache::lookup_gdb_index (const bfd_build_id *build_id,
   return {};
 }
 
+gdb::array_view<const gdb_byte>
+index_cache::lookup_gdb_index_debuginfod (const char *index_path,
+					  std::unique_ptr<index_cache_resource> *resource)
+{
+  return {};
+}
 #endif
 
 /* See dwarf-index-cache.h.  */
diff --git a/gdb/dwarf2/index-cache.h b/gdb/dwarf2/index-cache.h
index 1efff17049f..e400afd5123 100644
--- a/gdb/dwarf2/index-cache.h
+++ b/gdb/dwarf2/index-cache.h
@@ -67,6 +67,19 @@ class index_cache
   lookup_gdb_index (const bfd_build_id *build_id,
 		    std::unique_ptr<index_cache_resource> *resource);
 
+  /* Look for an index file located at INDEX_PATH in the debuginfod cache.
+     Unlike lookup_gdb_index, this function does not exit early if the
+     index cache has not been enabled.
+
+     If found, return the contents as an array_view and store the underlying
+     resources (allocated memory, mapped file, etc) in RESOURCE.  The returned
+     array_view is valid as long as RESOURCE is not destroyed.
+
+     If no matching index file is found, return an empty array view.  */
+  gdb::array_view<const gdb_byte>
+  lookup_gdb_index_debuginfod (const char *index_path,
+			       std::unique_ptr<index_cache_resource> *resource);
+
   /* Return the number of cache hits.  */
   unsigned int n_hits () const
   { return m_n_hits; }
diff --git a/gdb/dwarf2/public.h b/gdb/dwarf2/public.h
index 0e74857eb1a..4a44cdbc223 100644
--- a/gdb/dwarf2/public.h
+++ b/gdb/dwarf2/public.h
@@ -40,4 +40,11 @@ extern void dwarf2_initialize_objfile (struct objfile *objfile);
 
 extern void dwarf2_build_frame_info (struct objfile *);
 
+/* Query debuginfod for the .gdb_index associated with OBJFILE.  If
+   successful, create an objfile to hold the .gdb_index information
+   and act as a placeholder until the full debuginfo needs to be
+   downloaded.  */
+
+extern bool dwarf2_has_separate_index (struct objfile *);
+
 #endif /* DWARF2_PUBLIC_H */
diff --git a/gdb/dwarf2/read-gdb-index.c b/gdb/dwarf2/read-gdb-index.c
index 1006386cb2d..f066d55bd1a 100644
--- a/gdb/dwarf2/read-gdb-index.c
+++ b/gdb/dwarf2/read-gdb-index.c
@@ -143,6 +143,9 @@ struct dwarf2_gdb_index : public dwarf2_base_index_functions
      int global,
      symbol_compare_ftype *ordered_compare) override;
 
+  /* Calls do_expand_symtabs_matching.  If this index was initialized
+     from a file downloaded from debuginfod and there is a match, attempt
+     to download debuginfo.  */
   bool expand_symtabs_matching
     (struct objfile *objfile,
      gdb::function_view<expand_symtabs_file_matcher_ftype> file_matcher,
@@ -152,6 +155,16 @@ struct dwarf2_gdb_index : public dwarf2_base_index_functions
      block_search_flags search_flags,
      domain_enum domain,
      enum search_domain kind) override;
+
+  bool do_expand_symtabs_matching
+    (struct objfile *objfile,
+     gdb::function_view<expand_symtabs_file_matcher_ftype> file_matcher,
+     const lookup_name_info *lookup_name,
+     gdb::function_view<expand_symtabs_symbol_matcher_ftype> symbol_matcher,
+     gdb::function_view<expand_symtabs_exp_notify_ftype> expansion_notify,
+     block_search_flags search_flags,
+     domain_enum domain,
+     enum search_domain kind);
 };
 
 /* This dumps minimal information about the index.
@@ -455,7 +468,7 @@ dw2_expand_marked_cus
 }
 
 bool
-dwarf2_gdb_index::expand_symtabs_matching
+dwarf2_gdb_index::do_expand_symtabs_matching
     (struct objfile *objfile,
      gdb::function_view<expand_symtabs_file_matcher_ftype> file_matcher,
      const lookup_name_info *lookup_name,
@@ -504,6 +517,44 @@ dwarf2_gdb_index::expand_symtabs_matching
   return result;
 }
 
+bool
+dwarf2_gdb_index::expand_symtabs_matching
+    (struct objfile *objfile,
+     gdb::function_view<expand_symtabs_file_matcher_ftype> file_matcher,
+     const lookup_name_info *lookup_name,
+     gdb::function_view<expand_symtabs_symbol_matcher_ftype> symbol_matcher,
+     gdb::function_view<expand_symtabs_exp_notify_ftype> expansion_notify,
+     block_search_flags search_flags,
+     domain_enum domain,
+     enum search_domain kind)
+{
+  if (objfile->flags & OBJF_READNEVER)
+    return false;
+
+  try
+    {
+      return do_expand_symtabs_matching (objfile, file_matcher, lookup_name,
+					 symbol_matcher, expansion_notify,
+					 search_flags, domain, kind);
+    }
+  catch (gdb_exception e)
+    {
+      if (!objfile->is_separate_index_stub ())
+	{
+	  exception_print (gdb_stderr, e);
+	  return false;
+	}
+
+      /* Objfile is a stub holding only index information.  Try to reinitialize
+	 objfile with the full debuginfo.  */
+      if (!read_full_dwarf_from_debuginfod (objfile))
+	return false;
+      return do_expand_symtabs_matching (objfile, file_matcher, lookup_name,
+					 symbol_matcher, expansion_notify,
+					 search_flags, domain, kind);
+    }
+}
+
 quick_symbol_functions_up
 mapped_gdb_index::make_quick_functions () const
 {
@@ -797,28 +848,32 @@ dwarf2_read_gdb_index
 
   /* If there is a .dwz file, read it so we can get its CU list as
      well.  */
-  dwz = dwarf2_get_dwz_file (per_bfd);
-  if (dwz != NULL)
+  if (get_gdb_index_contents_dwz != nullptr)
     {
       mapped_gdb_index dwz_map;
       const gdb_byte *dwz_types_ignore;
       offset_type dwz_types_elements_ignore;
+      dwz = dwarf2_get_dwz_file (per_bfd);
 
-      gdb::array_view<const gdb_byte> dwz_index_content
-	= get_gdb_index_contents_dwz (objfile, dwz);
-
-      if (dwz_index_content.empty ())
-	return 0;
-
-      if (!read_gdb_index_from_buffer (bfd_get_filename (dwz->dwz_bfd.get ()),
-				       1, dwz_index_content, &dwz_map,
-				       &dwz_list, &dwz_list_elements,
-				       &dwz_types_ignore,
-				       &dwz_types_elements_ignore))
+      if (dwz != nullptr)
 	{
-	  warning (_("could not read '.gdb_index' section from %s; skipping"),
-		   bfd_get_filename (dwz->dwz_bfd.get ()));
-	  return 0;
+	  gdb::array_view<const gdb_byte> dwz_index_content
+	    = get_gdb_index_contents_dwz (objfile, dwz);
+
+	  if (dwz_index_content.empty ())
+	    return 0;
+
+	  if (!read_gdb_index_from_buffer (bfd_get_filename
+					     (dwz->dwz_bfd.get ()),
+					   1, dwz_index_content, &dwz_map,
+					   &dwz_list, &dwz_list_elements,
+					   &dwz_types_ignore,
+					   &dwz_types_elements_ignore))
+	    {
+	      warning (_("could not read '.gdb_index' section from %s; skipping"),
+		       bfd_get_filename (dwz->dwz_bfd.get ()));
+		return 0;
+	    }
 	}
     }
 
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 69b2310be1f..12b0dbd1ee8 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -34,6 +34,7 @@
 #include "dwarf2/attribute.h"
 #include "dwarf2/comp-unit-head.h"
 #include "dwarf2/cu.h"
+#include "dwarf2/frame.h"
 #include "dwarf2/index-cache.h"
 #include "dwarf2/index-common.h"
 #include "dwarf2/leb.h"
@@ -95,6 +96,8 @@
 #include "split-name.h"
 #include "gdbsupport/parallel-for.h"
 #include "gdbsupport/thread-pool.h"
+#include "inferior.h"
+#include "debuginfod-support.h"
 
 /* When == 1, print basic high level tracing messages.
    When > 1, be more verbose.
@@ -3156,7 +3159,7 @@ dwarf2_base_index_functions::find_per_cu (dwarf2_per_bfd *per_bfd,
 }
 
 struct compunit_symtab *
-dwarf2_base_index_functions::find_pc_sect_compunit_symtab
+dwarf2_base_index_functions::do_find_pc_sect_compunit_symtab
      (struct objfile *objfile,
       struct bound_minimal_symbol msymbol,
       CORE_ADDR pc,
@@ -3187,6 +3190,39 @@ dwarf2_base_index_functions::find_pc_sect_compunit_symtab
   return result;
 }
 
+struct compunit_symtab *
+dwarf2_base_index_functions::find_pc_sect_compunit_symtab
+     (struct objfile *objfile,
+      struct bound_minimal_symbol msymbol,
+      CORE_ADDR pc,
+      struct obj_section *section,
+      int warn_if_readin)
+{
+  if (objfile->flags & OBJF_READNEVER)
+    return nullptr;
+
+  try
+    {
+      return do_find_pc_sect_compunit_symtab (objfile, msymbol, pc,
+					      section, warn_if_readin);
+    }
+  catch (gdb_exception e)
+    {
+      if (!objfile->is_separate_index_stub ())
+	{
+	  exception_print (gdb_stderr, e);
+	  return nullptr;
+	}
+
+      /* Objfile is a stub holding only index information.  Try to
+	 reinitialize objfile with the full debuginfo.  */
+      if (!read_full_dwarf_from_debuginfod (objfile))
+	return nullptr;
+      return do_find_pc_sect_compunit_symtab (objfile, msymbol, pc,
+					      section, warn_if_readin);
+    }
+}
+
 void
 dwarf2_base_index_functions::map_symbol_filenames
      (struct objfile *objfile,
@@ -3412,6 +3448,165 @@ dwarf2_initialize_objfile (struct objfile *objfile)
   objfile->qf.push_front (make_cooked_index_funcs ());
 }
 
+/* Query debuginfod for the .gdb_index matching OBJFILE's build-id.  Return the
+   contents if successful.  */
+
+static gdb::array_view<const gdb_byte>
+get_gdb_index_contents_from_debuginfod (objfile *objfile, dwarf2_per_bfd *per_bfd)
+{
+  const bfd_build_id *build_id = build_id_bfd_get (objfile->obfd.get ());
+  if (build_id == nullptr)
+    return {};
+
+  gdb::unique_xmalloc_ptr<char> index_path;
+  scoped_fd fd = debuginfod_section_query (build_id->data, build_id->size,
+					   bfd_get_filename
+					     (objfile->obfd.get ()),
+					   ".gdb_index",
+					   &index_path);
+  if (fd.get () < 0)
+    return {};
+
+  return global_index_cache.lookup_gdb_index_debuginfod
+    (index_path.get (), &per_bfd->index_cache_res);
+}
+
+/* See read.h.  */
+
+bool
+read_full_dwarf_from_debuginfod (struct objfile *objfile)
+{
+  gdb_assert (objfile->is_separate_index_stub ());
+
+  const struct bfd_build_id *build_id = build_id_bfd_get (objfile->obfd.get ());
+  if (build_id == nullptr)
+    return false;
+
+  const char *filename = bfd_get_filename (objfile->obfd.get ());
+  gdb_bfd_ref_ptr debug_bfd;
+  gdb::unique_xmalloc_ptr<char> symfile_path;
+
+  scoped_fd fd (debuginfod_debuginfo_query (build_id->data,
+					    build_id->size,
+					    filename,
+					    &symfile_path));
+
+  if (fd.get () < 0)
+    return false;
+
+  /* File successfully retrieved from server.  */
+  debug_bfd = symfile_bfd_open (symfile_path.get ());
+  if (debug_bfd == nullptr
+      || !build_id_verify (debug_bfd.get (), build_id->size, build_id->data))
+    {
+      warning (_("File \"%s\" from debuginfod cannot be opened as bfd"),
+	       filename);
+      return false;
+    }
+
+  /* Fill in objfile's missing information using the debuginfo.  This
+     may also download dwz, if available.  */
+  if (!objfile->reinit (debug_bfd.release ()))
+    return false;
+
+  struct objfile *parent = objfile->separate_debug_objfile_backlink;
+  gdb_assert (parent != nullptr);
+
+  /* Clear the parent objfile's frame comp_unit information so it can be
+     recalculated with DWARF.  */
+  clear_comp_unit (parent);
+
+  return true;
+}
+
+/* See public.h.  */
+
+bool
+dwarf2_has_separate_index (struct objfile *objfile)
+{
+  if (objfile->flags & OBJF_MAINLINE
+      || objfile->separate_debug_objfile_backlink != nullptr)
+    return 0;
+  if (objfile->is_separate_index_stub ())
+    return 1;
+  if (!IS_DIR_SEPARATOR (*objfile_filename (objfile)))
+    return 0;
+
+  gdb::unique_xmalloc_ptr<char> index_path;
+  const bfd_build_id *build_id = build_id_bfd_get (objfile->obfd.get ());
+  scoped_fd fd = debuginfod_section_query (build_id->data,
+					   build_id->size,
+					   bfd_get_filename
+					     (objfile->obfd.get ()),
+					   ".gdb_index",
+					   &index_path);
+  if (fd.get () >= 0)
+    {
+      /* We found a separate .gdb_index file so a separate debuginfo file
+	 should exist, but don't want to read it until we really have to.
+	 Create an objfile to own the index information and act as a
+	 placeholder for the debuginfo that we have the option of aquiring
+	 later.  */
+      gdb_bfd_ref_ptr abfd (gdb_bfd_open (objfile_filename (objfile), gnutarget));
+      if (abfd == nullptr)
+	return false;
+
+      dwarf2_per_bfd_objfile_data_key.clear (objfile);
+      dwarf2_objfile_data_key.clear (objfile);
+
+      /* Initialize the new objfile using a bfd opened from the parent's
+	 filename.  This objfile will be reinitialized if/when we download
+	 the full debuginfo.  */
+      symbol_file_add_from_index
+	(abfd, current_inferior ()->symfile_flags | SYMFILE_NO_READ, objfile);
+
+      dwarf2_per_bfd *per_bfd;
+      dwarf2_per_objfile *per_objfile = get_dwarf2_per_objfile (objfile);
+
+      /* Perform a limited initialization based to dwarf2_has_info and
+	 dwarf2_initialize_objfile.  */
+      if (per_objfile == nullptr)
+	{
+	  per_bfd = dwarf2_per_bfd_objfile_data_key.get (objfile);
+	  if (per_bfd == nullptr)
+	    {
+	      per_bfd = new dwarf2_per_bfd (objfile->obfd.get (), nullptr, false);
+	      dwarf2_per_bfd_objfile_data_key.set (objfile, per_bfd);
+	    }
+	  per_objfile = dwarf2_objfile_data_key.emplace (objfile, objfile, per_bfd);
+	}
+
+      struct objfile *stub = objfile->separate_debug_objfile;
+      per_objfile = get_dwarf2_per_objfile (stub);
+      if (per_objfile == nullptr)
+	{
+	  per_bfd = dwarf2_per_bfd_objfile_data_key.get (stub);
+	  if (per_bfd == nullptr)
+	    {
+	      per_bfd = new dwarf2_per_bfd (stub->obfd.get (), nullptr, false);
+	      dwarf2_per_bfd_objfile_data_key.set (stub, per_bfd);
+	    }
+	  per_objfile = dwarf2_objfile_data_key.emplace (stub, stub, per_bfd);
+	}
+
+      /* Do not include a dwz index callback, otherwise we could download a dwz
+	 file. We don't want to do this until the full debuginfo is needed.  */
+      if (dwarf2_read_gdb_index (per_objfile,
+				 get_gdb_index_contents_from_debuginfod,
+				 nullptr))
+	{
+	  dwarf_read_debug_printf ("found .gdb_index from debuginfod");
+	  stub->qf.push_front (per_bfd->index_table->make_quick_functions ());
+	  return 1;
+	}
+
+      /* Unable to read the index.  */
+      stub->unlink ();
+    }
+
+  return 0;
+}
+
 \f
 
 /* Build a partial symbol table.  */
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index 0e6853ea2f4..4e6c35cba5e 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -866,6 +866,10 @@ struct dwarf2_base_index_functions : public quick_symbol_functions
      CORE_ADDR pc, struct obj_section *section, int warn_if_readin)
        override final;
 
+  struct compunit_symtab *do_find_pc_sect_compunit_symtab
+    (struct objfile *objfile, struct bound_minimal_symbol msymbol,
+     CORE_ADDR pc, struct obj_section *section, int warn_if_readin);
+
   struct compunit_symtab *find_compunit_symtab_by_address
     (struct objfile *objfile, CORE_ADDR address) override
   {
@@ -942,4 +946,9 @@ extern bool read_addrmap_from_aranges (dwarf2_per_objfile *per_objfile,
 				       dwarf2_section_info *section,
 				       addrmap *mutable_map);
 
+/* If OBJFILE is a stub holding only information from a .gdb_index, then attempt
+   to download the full debuginfo and reinitialize OBJFILE with it.  */
+
+extern bool read_full_dwarf_from_debuginfod (struct objfile *objfile);
+
 #endif /* DWARF2READ_H */
diff --git a/gdb/dwarf2/section.c b/gdb/dwarf2/section.c
index c9ef41893ee..ff8ef527c29 100644
--- a/gdb/dwarf2/section.c
+++ b/gdb/dwarf2/section.c
@@ -54,7 +54,9 @@ dwarf2_section_info::get_bfd_owner () const
       section = get_containing_section ();
       gdb_assert (!section->is_virtual);
     }
-  gdb_assert (section->s.section != nullptr);
+  if (section->s.section == nullptr)
+    throw_error (NOT_FOUND_ERROR,
+		 _("Can't find owner of DWARF section."));
   return section->s.section->owner;
 }
 
diff --git a/gdb/elfread.c b/gdb/elfread.c
index ca684aab57e..6c3e384eb8d 100644
--- a/gdb/elfread.c
+++ b/gdb/elfread.c
@@ -1229,7 +1229,7 @@ elf_symfile_read_dwarf2 (struct objfile *objfile,
 	  symbol_file_add_separate (debug_bfd, debugfile.c_str (),
 				    symfile_flags, objfile);
 	}
-      else
+      else if (!dwarf2_has_separate_index (objfile))
 	{
 	  has_dwarf2 = false;
 	  const struct bfd_build_id *build_id
diff --git a/gdb/objfiles.c b/gdb/objfiles.c
index ed29131d528..fa8ab4a02b6 100644
--- a/gdb/objfiles.c
+++ b/gdb/objfiles.c
@@ -52,6 +52,7 @@
 #include "gdb_bfd.h"
 #include "btrace.h"
 #include "gdbsupport/pathstuff.h"
+#include "symfile.h"
 
 #include <algorithm>
 #include <vector>
@@ -298,6 +299,23 @@ build_objfile_section_table (struct objfile *objfile)
 			   objfile, 1);
 }
 
+bool
+objfile::reinit (struct bfd *abfd)
+{
+  try
+  {
+    deferred_read_symbols (this, abfd);
+    return true;
+  }
+  catch (const gdb_exception &except)
+  {
+    exception_print (gdb_stderr, except);
+    this->flags |= OBJF_READNEVER;
+  }
+
+  return false;
+}
+
 /* Given a pointer to an initialized bfd (ABFD) and some flag bits,
    initialize the new objfile as best we can and link it into the list
    of all known objfiles.
diff --git a/gdb/objfiles.h b/gdb/objfiles.h
index 342aa09ac6a..55c9ad1f987 100644
--- a/gdb/objfiles.h
+++ b/gdb/objfiles.h
@@ -375,6 +375,8 @@ class separate_debug_iterator
   struct objfile *m_parent;
 };
 
+#define SEPARATE_INDEX_FILENAME "<separate index stub>"
+
 /* A range adapter wrapping separate_debug_iterator.  */
 
 typedef iterator_range<separate_debug_iterator> separate_debug_range;
@@ -497,6 +499,12 @@ struct objfile
   /* See quick_symbol_functions.  */
   struct symtab *find_last_source_symtab ();
 
+  /* Reinitialize this objfile using ABFD.  Return true if successful.
+     Objfile should have been originally initialized using a separate
+     index from ABFD.  Updates this objfile with ABFD's symbols and
+     section information.  */
+  bool reinit (struct bfd *abfd);
+
   /* See quick_symbol_functions.  */
   void forget_cached_source_info ();
 
@@ -609,6 +617,14 @@ struct objfile
     this->section_offsets[idx] = offset;
   }
 
+  /* Return true if this objfile holds .gdb_index data and represents
+     a debuginfo file whose download has been deferred.  */
+
+  bool is_separate_index_stub ()
+  {
+    return strcmp (original_name, SEPARATE_INDEX_FILENAME) == 0;
+  }
+
 private:
 
   /* Ensure that partial symbols have been read and return the "quick" (aka
diff --git a/gdb/symfile.c b/gdb/symfile.c
index 373f5592107..1e05dd6af6e 100644
--- a/gdb/symfile.c
+++ b/gdb/symfile.c
@@ -1035,7 +1035,8 @@ static struct objfile *
 symbol_file_add_with_addrs (const gdb_bfd_ref_ptr &abfd, const char *name,
 			    symfile_add_flags add_flags,
 			    section_addr_info *addrs,
-			    objfile_flags flags, struct objfile *parent)
+			    objfile_flags flags, struct objfile *parent,
+			    bool skip_syms = false)
 {
   struct objfile *objfile;
   const int from_tty = add_flags & SYMFILE_VERBOSE;
@@ -1073,6 +1074,15 @@ symbol_file_add_with_addrs (const gdb_bfd_ref_ptr &abfd, const char *name,
     flags |= OBJF_MAINLINE;
   objfile = objfile::make (abfd, name, flags, parent);
 
+  if (skip_syms)
+    {
+      /* objfile was initialized only using a separate index so don't
+	 try to read symbols yet.  */
+      objfile->section_offsets = parent->section_offsets;
+      gdb::observers::new_objfile.notify (objfile);
+      return objfile;
+    }
+
   /* We either created a new mapped symbol table, mapped an existing
      symbol table file which has not had initial symbol reading
      performed, or need to read an unmapped symbol table.  */
@@ -1134,6 +1144,27 @@ symbol_file_add_with_addrs (const gdb_bfd_ref_ptr &abfd, const char *name,
   return (objfile);
 }
 
+/* See symfile.h.  */
+
+void
+symbol_file_add_from_index (const gdb_bfd_ref_ptr &bfd,
+			    symfile_add_flags symfile_flags,
+			    struct objfile *parent)
+{
+  section_addr_info sap = build_section_addr_info_from_objfile (parent);
+
+  symbol_file_add_with_addrs
+    (bfd, SEPARATE_INDEX_FILENAME, symfile_flags, &sap,
+     (parent->flags & (OBJF_REORDERED | OBJF_SHARED | OBJF_READNOW
+		      | OBJF_USERLOADED | OBJF_MAINLINE | OBJF_PSYMTABS_READ))
+		  | OBJF_NOT_FILENAME, parent, true);
+
+  objfile *result = parent->separate_debug_objfile;
+  init_objfile_sect_indices (result);
+
+  return;
+}
+
 /* Add BFD as a separate debug file for OBJFILE.  For NAME description
    see the objfile constructor.  */
 
@@ -2440,6 +2471,49 @@ remove_symbol_file_command (const char *args, int from_tty)
   clear_symtab_users (0);
 }
 
+/* See symtab.h.  */
+
+void
+deferred_read_symbols (struct objfile *objfile, struct bfd *abfd)
+{
+  const char *obfd_filename;
+
+  obfd_filename = bfd_get_filename (abfd);
+  gdb_bfd_ref_ptr temp (gdb_bfd_open (obfd_filename, gnutarget));
+
+  if (temp == nullptr)
+    error (_("Can't open %s to read symbols."), obfd_filename);
+
+  if (!bfd_check_format (temp.get (), bfd_object))
+    error (_("Can't read symbols from %s: %s."), obfd_filename,
+	   bfd_errmsg (bfd_get_error ()));
+
+  objfile->obfd = std::move (temp);
+
+  /* Nuke all the state that we will re-read.  */
+  objfile->registry_fields.clear_registry ();
+  objfile->sections = NULL;
+  objfile->sect_index_bss = -1;
+  objfile->sect_index_data = -1;
+  objfile->sect_index_rodata = -1;
+  objfile->sect_index_text = -1;
+  objfile->compunit_symtabs = NULL;
+  objfile->template_symbols = NULL;
+
+  objfile_set_sym_fns (objfile, find_sym_fns (objfile->obfd.get ()));
+  build_objfile_section_table (objfile);
+  (*objfile->sf->sym_init) (objfile);
+  init_objfile_sect_indices (objfile);
+
+  objfile->section_offsets.resize (gdb_bfd_count_sections
+				     (objfile->obfd.get ()));
+  read_symbols (objfile, 0);
+
+  objfile->mtime = bfd_get_mtime (objfile->obfd.get ());
+  objfile->original_name
+    = obstack_strdup (&objfile->objfile_obstack, obfd_filename);
+}
+
 /* Re-read symbols if a symbol-file has changed.  */
 
 void
diff --git a/gdb/symfile.h b/gdb/symfile.h
index b433e2be31a..353cf0cab42 100644
--- a/gdb/symfile.h
+++ b/gdb/symfile.h
@@ -241,6 +241,13 @@ extern struct objfile *symbol_file_add_from_bfd (const gdb_bfd_ref_ptr &,
 extern void symbol_file_add_separate (const gdb_bfd_ref_ptr &, const char *,
 				      symfile_add_flags, struct objfile *);
 
+/* Initialize an objfile acting as a placeholder for separate debuginfo
+   that has not yet been opened or read.  Infer some of its properties
+   from PARENT.  */
+
+extern void symbol_file_add_from_index (const gdb_bfd_ref_ptr &,
+					symfile_add_flags, struct objfile *);
+
 /* Find separate debuginfo for OBJFILE (using .gnu_debuglink section).
    Returns pathname, or an empty string.
 
diff --git a/gdb/symtab.h b/gdb/symtab.h
index 17d2746fd48..304482f81dd 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -2206,6 +2206,11 @@ extern bool find_pc_line_pc_range (CORE_ADDR, CORE_ADDR *, CORE_ADDR *);
 
 extern void reread_symbols (int from_tty);
 
+/* Reread a separate debug OBJFILE that was originally initialized
+   using an index.  ABFD will be used as OBJFILE's new bfd.  */
+
+extern void deferred_read_symbols (struct objfile *objfile, struct bfd *abfd);
+
 /* Look up a type named NAME in STRUCT_DOMAIN in the current language.
    The type returned must not be opaque -- i.e., must have at least one field
    defined.  */
-- 
2.39.2