Make beam_lib:cmp/2 stricter

The beam_lib:cmp/2 function only compares the executable parts
of the BEAM files, not attributes or abstract code. Since the
types and specs (used by Dialyzer) are contained in the abstract
code, beam_lib:cmp/2 will return 'ok' if the only difference
between two BEAM file are in the types or specs.

If an Erlang/OTP system is installed in a revision control
system, and beam_lib:cmp/2 is used to avoid committing unchanged
but newly compiled BEAM files, BEAM files with no other changes
than in types or specs may not get updated, which can problems
if Dialyzer or the debugger is run.

To avoid that problem, change beam_lib:cmp/2 to compare all
chunks *except* for the "CInf" chunk. The "CInf" chunk contains
the compilation time, compiler options, and compiler version.

2 files changed, 21 insertions, 8 deletions

diff --git a/lib/stdlib/doc/src/beam_lib.xml b/lib/stdlib/doc/src/beam_lib.xml
index b9286f1402..27308e02f3 100644
--- a/lib/stdlib/doc/src/beam_lib.xml
+++ b/lib/stdlib/doc/src/beam_lib.xml
@@ -347,9 +347,10 @@ chunkref() = chunkname() | chunkid()</code>
       </type>
       <desc>
         <p>Compares the contents of two BEAM files. If the module names
-          are the same, and the chunks with the identifiers
-          <c>"Code"</c>, <c>"ExpT"</c>, <c>"ImpT"</c>, <c>"StrT"</c>,
-          and <c>"Atom"</c> have the same contents in both files,
+          are the same, and all chunks except for the <c>"CInf"</c> chunk
+	  (the chunk containing the compilation information which is
+	  returned by <c>Module:module_info(compile)</c>)
+	  have the same contents in both files,
           <c>ok</c> is returned. Otherwise an error message is returned.</p>
       </desc>
     </func>
diff --git a/lib/stdlib/src/beam_lib.erl b/lib/stdlib/src/beam_lib.erl
index 820afd3739..0f853daafc 100644
--- a/lib/stdlib/src/beam_lib.erl
+++ b/lib/stdlib/src/beam_lib.erl
@@ -331,13 +331,11 @@ beam_files(Dir) ->
 
 %% -> ok | throw(Error)
 cmp_files(File1, File2) ->
-    {ok, {M1, L1}} = read_significant_chunks(File1),
-    {ok, {M2, L2}} = read_significant_chunks(File2),
+    {ok, {M1, L1}} = read_all_but_useless_chunks(File1),
+    {ok, {M2, L2}} = read_all_but_useless_chunks(File2),
     if
 	M1 =:= M2 ->
-	    List1 = filter_funtab(L1),
-	    List2 = filter_funtab(L2),
-	    cmp_lists(List1, List2);
+	    cmp_lists(L1, L2);
 	true ->
 	    error({modules_different, M1, M2})
     end.
@@ -408,6 +406,20 @@ pad(Size) ->
     end.
 
 %% -> {ok, {Module, Chunks}} | throw(Error)
+read_all_but_useless_chunks(File0) when is_atom(File0);
+					is_list(File0);
+					is_binary(File0) ->
+    File = beam_filename(File0),
+    {ok, Module, ChunkIds0} = scan_beam(File, info),
+    ChunkIds = [Name || {Name,_,_} <- ChunkIds0,
+			not is_useless_chunk(Name)],
+    {ok, Module, Chunks} = scan_beam(File, ChunkIds),
+    {ok, {Module, lists:reverse(Chunks)}}.
+
+is_useless_chunk("CInf") -> true;
+is_useless_chunk(_) -> false.
+
+%% -> {ok, {Module, Chunks}} | throw(Error)
 read_significant_chunks(File) ->
     case read_chunk_data(File, significant_chunks(), [allow_missing_chunks]) of
 	{ok, {Module, Chunks0}} ->