[6/7] gdb: generate gdb-index identically regardless of work thread count

diff --git a/gdb/dwarf2/index-write.c b/gdb/dwarf2/index-write.c
index e1c343e8790..bc07bcb8f4c 100644
--- a/gdb/dwarf2/index-write.c
+++ b/gdb/dwarf2/index-write.c
@@ -212,6 +212,13 @@  struct mapped_symtab
   void add_index_entry (const char *name, int is_static,
 			gdb_index_symbol_kind kind, offset_type cu_index);
 
+  /* When entries are originally added into the data hash the order will
+     vary based on the number of worker threads GDB is configured to use.
+     This function will rebuild the hash such that the final layout will be
+     deterministic regardless of the number of worker threads used.  */
+
+  void sort ();
+
   /* Access the obstack.  */
   auto_obstack *obstack ()
   { return &m_string_obstack; }
@@ -298,6 +305,65 @@  mapped_symtab::hash_expand ()
       }
 }
 
+/* See mapped_symtab class declaration.  */
+
+void mapped_symtab::sort ()
+{
+  /* Move contents out of this->data vector.  */
+  std::vector<symtab_index_entry> original_data = std::move (m_data);
+
+  /* Restore the size of m_data, this will avoid having to expand the hash
+     table (and rehash all elements) when we reinsert after sorting.
+     However, we do reset the element count, this allows for some sanity
+     checking asserts during the reinsert phase.  */
+  gdb_assert (m_data.size () == 0);
+  m_data.resize (original_data.size ());
+  m_element_count = 0;
+
+  /* Remove empty entries from ORIGINAL_DATA, this makes sorting quicker.  */
+  auto it = std::remove_if (original_data.begin (), original_data.end (),
+			    [] (const symtab_index_entry &entry) -> bool
+			    {
+			      return entry.name == nullptr;
+			    });
+  original_data.erase (it, original_data.end ());
+
+  /* Sort the existing contents.  */
+  std::sort (original_data.begin (), original_data.end (),
+	     [] (const symtab_index_entry &a,
+		 const symtab_index_entry &b) -> bool
+	     {
+	       /* Return true if A is before B.  */
+	       gdb_assert (a.name != nullptr);
+	       gdb_assert (b.name != nullptr);
+
+	       return strcmp (a.name, b.name) < 0;
+	     });
+
+  /* Re-insert each item from the sorted list.  */
+  for (auto &entry : original_data)
+    {
+      /* We know that ORIGINAL_DATA contains no duplicates, this data was
+	 taken from a hash table that de-duplicated entries for us, so
+	 count this as a new item.
+
+	 As we retained the original size of m_data (see above) then we
+	 should never need to grow m_data_ during this re-insertion phase,
+	 assert that now.  */
+      ++m_element_count;
+      gdb_assert (!this->hash_needs_expanding ());
+
+      /* Lookup a slot.  */
+      symtab_index_entry &slot = this->find_slot (entry.name);
+
+      /* As discussed above, we should not find duplicates.  */
+      gdb_assert (slot.name == nullptr);
+
+      /* Move this item into the slot we found.  */
+      slot = std::move (entry);
+    }
+}
+
 /* See class definition.  */
 
 void
@@ -1346,6 +1412,9 @@  write_gdbindex (dwarf2_per_bfd *per_bfd, cooked_index *table,
   for (auto map : table->get_addrmaps ())
     write_address_map (map, addr_vec, cu_index_htab);
 
+  /* Ensure symbol hash is built domestically.  */
+  symtab.sort ();
+
   /* Now that we've processed all symbols we can shrink their cu_indices
      lists.  */
   symtab.minimize ();
diff --git a/gdb/testsuite/gdb.gdb/index-file.exp b/gdb/testsuite/gdb.gdb/index-file.exp
index c6edd286fb9..08415920061 100644
--- a/gdb/testsuite/gdb.gdb/index-file.exp
+++ b/gdb/testsuite/gdb.gdb/index-file.exp
@@ -35,6 +35,9 @@  with_timeout_factor $timeout_factor {
     clean_restart $filename
 }
 
+# Record how many worker threads GDB is using.
+set worker_threads [gdb_get_worker_threads]
+
 # Generate an index file.
 set dir1 [standard_output_file "index_1"]
 remote_exec host "mkdir -p ${dir1}"
@@ -113,3 +116,41 @@  proc check_symbol_table_usage { filename } {
 
 set index_filename_base [file tail $filename]
 check_symbol_table_usage "$dir1/${index_filename_base}.gdb-index"
+
+# If GDB is using more than 1 worker thread then reduce the number of
+# worker threads, regenerate the index, and check that we get the same
+# index file back.  At one point the layout of the index would vary
+# based on the number of worker threads used.
+if { $worker_threads > 1 } {
+    # Start GDB, but don't load a file yet.
+    clean_restart
+
+    # Adjust the number of threads to use.
+    set reduced_threads [expr $worker_threads / 2]
+    gdb_test_no_output "maint set worker-threads $reduced_threads"
+
+    with_timeout_factor $timeout_factor {
+	# Now load the test binary.
+	gdb_file_cmd $filename
+    }
+
+    # Generate an index file.
+    set dir2 [standard_output_file "index_2"]
+    remote_exec host "mkdir -p ${dir2}"
+    with_timeout_factor $timeout_factor {
+	gdb_test_no_output "save gdb-index $dir2" \
+	    "create second gdb-index file"
+    }
+
+    # Close GDB.
+    gdb_exit
+
+    # Now check that the index files are identical.
+    foreach suffix { gdb-index  } {
+	set result \
+	    [remote_exec host \
+		 "cmp -s \"$dir1/${index_filename_base}.${suffix}\" \"$dir2/${index_filename_base}.${suffix}\""]
+	gdb_assert { [lindex $result 0] == 0 } \
+	    "$suffix files are identical"
+    }
+}
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 63885860795..b534a61d066 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -10026,6 +10026,21 @@  proc is_target_non_stop { {testname ""} } {
     return $is_non_stop
 }
 
+# Return the number of worker threads that GDB is currently using.
+
+proc gdb_get_worker_threads { {testname ""} } {
+    set worker_threads "UNKNOWN"
+    gdb_test_multiple "maintenance show worker-threads" $testname {
+	-wrap -re "^The number of worker threads GDB can use is unlimited \\(currently ($::decimal)\\)\\." {
+	    set worker_threads $expect_out(1,string)
+	}
+	-wrap -re "The number of worker threads GDB can use is ($::decimal)\\." {
+	    set worker_threads $expect_out(1,string)
+	}
+    }
+    return $worker_threads
+}
+
 # Check if the compiler emits epilogue information associated
 # with the closing brace or with the last statement line.
 #