[PATCH v2 07/17] Do not write the index cache from an index

[Tom Tromey <tom@tromey.com>]

The new .debug_names reader will work by creating a cooked index from
.debug_names.  This patch updates cooked_index::maybe_write_index to
avoid writing the index in this case.

However, in order to do this in a clean way, the readers are changed
so that a nullptr result from index_for_writing means "cannot be
done", and then the error message is moved into write_dwarf_index
(where it historically lived).
---
 gdb/dwarf2/cooked-index.c | 7 +++++--
 gdb/dwarf2/index-write.c  | 2 ++
 gdb/dwarf2/mapped-index.h | 4 +---
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/gdb/dwarf2/cooked-index.c b/gdb/dwarf2/cooked-index.c
index 542ec2fd69f..d63fd0ab5bc 100644
--- a/gdb/dwarf2/cooked-index.c
+++ b/gdb/dwarf2/cooked-index.c
@@ -704,8 +704,11 @@ void
 cooked_index::maybe_write_index (dwarf2_per_bfd *per_bfd,
 				 const index_cache_store_context &ctx)
 {
-  /* (maybe) store an index in the cache.  */
-  global_index_cache.store (m_per_bfd, ctx);
+  if (index_for_writing () != nullptr)
+    {
+      /* (maybe) store an index in the cache.  */
+      global_index_cache.store (m_per_bfd, ctx);
+    }
   m_state->set (cooked_state::CACHE_DONE);
 }
 
diff --git a/gdb/dwarf2/index-write.c b/gdb/dwarf2/index-write.c
index 2decee62a4c..5cd550ef2d8 100644
--- a/gdb/dwarf2/index-write.c
+++ b/gdb/dwarf2/index-write.c
@@ -1629,6 +1629,8 @@ write_dwarf_index (dwarf2_per_bfd *per_bfd, const char *dir,
   if (per_bfd->index_table == nullptr)
     error (_("No debugging symbols"));
   cooked_index *table = per_bfd->index_table->index_for_writing ();
+  if (table == nullptr)
+    error (_("Cannot use an index to create the index"));
 
   if (per_bfd->types.size () > 1)
     error (_("Cannot make an index when the file has multiple .debug_types sections"));
diff --git a/gdb/dwarf2/mapped-index.h b/gdb/dwarf2/mapped-index.h
index df91e6402dc..86a3c59e7e4 100644
--- a/gdb/dwarf2/mapped-index.h
+++ b/gdb/dwarf2/mapped-index.h
@@ -127,9 +127,7 @@ struct mapped_index_base : public dwarf_scanner_base
 				 dwarf2_per_objfile *per_objfile) const;
 
   cooked_index *index_for_writing () override
-  {
-    error (_("Cannot use an index to create the index"));
-  }
+  { return nullptr; }
 };
 
 #endif /* GDB_DWARF2_MAPPED_INDEX_H */

-- 
2.43.0