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

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;
+}
+
 
 
 /* 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.  */