Prefer start module option to heuristics

Useful for defining a start module that doesn't have start/0 (but main/1).

Signed-off-by: Paul Guyot <[email protected]>

Author: pguyot

src/packbeam_api.erl

@@ -216,7 +216,7 @@ create(OutputPath, InputPaths, ApplicationModule, Prune, StartModule) ->
 list(InputPath) ->
     case file_type(InputPath) of
         avm ->
-            parse_file(InputPath, false);
+            parse_file(InputPath, undefined, false);
         _ ->
             throw(io_lib:format("Expected AVM file: ~p", [InputPath]))
     end.
@@ -244,7 +244,7 @@ list(InputPath) ->
 extract(InputPath, AVMElementNames, OutputDir) ->
     case file_type(InputPath) of
         avm ->
-            ParsedFiles = parse_file(InputPath, false),
+            ParsedFiles = parse_file(InputPath, undefined, false),
             write_files(filter_names(AVMElementNames, ParsedFiles), OutputDir);
         _ ->
             throw(io_lib:format("Expected AVM file: ~p", [InputPath]))
@@ -271,7 +271,7 @@ extract(InputPath, AVMElementNames, OutputDir) ->
 delete(OutputPath, InputPath, AVMElementNames) ->
     case file_type(InputPath) of
         avm ->
-            ParsedFiles = parse_file(InputPath, false),
+            ParsedFiles = parse_file(InputPath, undefined, false),
             write_packbeam(OutputPath, remove_names(AVMElementNames, ParsedFiles));
         _ ->
             throw(io_lib:format("Expected AVM file: ~p", [InputPath]))
@@ -341,7 +341,7 @@ get_flags(AVMElement) ->
 parse_files(InputPaths, StartModule, IncludeLines) ->
     Files = lists:foldl(
         fun(InputPath, Accum) ->
-            Accum ++ parse_file(InputPath, IncludeLines)
+            Accum ++ parse_file(InputPath, StartModule, IncludeLines)
         end,
         [],
         InputPaths
@@ -354,10 +354,10 @@ parse_files(InputPaths, StartModule, IncludeLines) ->
     end.
 
 %% @private
-parse_file({InputPath, ModuleName}, IncludeLines) ->
-    parse_file(file_type(InputPath), ModuleName, load_file(InputPath), IncludeLines);
-parse_file(InputPath, IncludeLines) ->
-    parse_file(file_type(InputPath), InputPath, load_file(InputPath), IncludeLines).
+parse_file({InputPath, ModuleName}, StartModule, IncludeLines) ->
+    parse_file(file_type(InputPath), ModuleName, StartModule, load_file(InputPath), IncludeLines);
+parse_file(InputPath, StartModule, IncludeLines) ->
+    parse_file(file_type(InputPath), InputPath, StartModule, load_file(InputPath), IncludeLines).
 
 %% @private
 file_type(InputPath) ->
@@ -595,18 +595,25 @@ filter_modules(Modules, ParsedFiles) ->
     ).
 
 %% @private
-parse_file(beam, _ModuleName, Data, IncludeLines) ->
+parse_file(beam, _ModuleName, StartModule, Data, IncludeLines) ->
     {ok, Module, Chunks} = beam_lib:all_chunks(Data),
     {UncompressedChunks, UncompressedLiterals} = maybe_uncompress_literals(Chunks),
     FilteredChunks = filter_chunks(UncompressedChunks, IncludeLines),
     {ok, Binary} = beam_lib:build_module(FilteredChunks),
     {ok, {Module, ChunkRefs}} = beam_lib:chunks(Data, [imports, exports, atoms]),
     Exports = proplists:get_value(exports, ChunkRefs),
     Flags =
-        case lists:member({start, 0}, Exports) of
-            true ->
+        if
+            StartModule =:= Module ->
                 ?BEAM_CODE_FLAG bor ?BEAM_START_FLAG;
-            _ ->
+            StartModule =:= undefined ->
+                case lists:member({start, 0}, Exports) of
+                    true ->
+                        ?BEAM_CODE_FLAG bor ?BEAM_START_FLAG;
+                    _ ->
+                        ?BEAM_CODE_FLAG
+                end;
+            true ->
                 ?BEAM_CODE_FLAG
         end,
     [
@@ -619,14 +626,14 @@ parse_file(beam, _ModuleName, Data, IncludeLines) ->
             {uncompressed_literals, UncompressedLiterals}
         ]
     ];
-parse_file(avm, ModuleName, Data, _IncludeLines) ->
+parse_file(avm, ModuleName, _StartModule, Data, _IncludeLines) ->
     case Data of
         <<?AVM_HEADER, AVMData/binary>> ->
             parse_avm_data(AVMData);
         _ ->
             throw(io_lib:format("Invalid AVM header: ~p", [ModuleName]))
     end;
-parse_file(normal, ModuleName, Data, _IncludeLines) ->
+parse_file(normal, ModuleName, _StartModule, Data, _IncludeLines) ->
     DataSize = byte_size(Data),
     Blob = <<DataSize:32, Data/binary>>,
     [[{element_name, ModuleName}, {flags, ?NORMAL_FILE_FLAG}, {data, Blob}]].

test/g.erl

@@ -0,0 +1,9 @@
+% Copyright 2025 Paul Guyot <[email protected]>
+% SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later
+
+-module(g).
+
+-export([main/1]).
+
+main(_Args) ->
+    c:test().

test/test_packbeam.erl

@@ -112,6 +112,40 @@ packbeam_create_start_test() ->
 
     ok.
 
+packbeam_create_start_main_test() ->
+    AVMFile = dest_dir("packbeam_create_start_main_test.avm"),
+    ?assertMatch(
+        ok,
+        packbeam_api:create(
+            AVMFile,
+            [
+                test_beam_path("c.beam"),
+                test_beam_path("f.beam"),
+                test_beam_path("g.beam")
+            ],
+            false,
+            g
+        )
+    ),
+
+    ParsedFiles = packbeam_api:list(AVMFile),
+
+    ?assert(is_list(ParsedFiles)),
+    ?assertEqual(length(ParsedFiles), 3),
+
+    [GFile | _] = ParsedFiles,
+
+    % io:format(user, "~p~n", [ParsedFiles]),
+
+    ?assertMatch(g, get_module(GFile)),
+    ?assertMatch("g.beam", get_module_name(GFile)),
+    ?assert(is_beam(GFile)),
+    ?assert(is_start(GFile)),
+    ?assert(lists:member({main, 1}, get_exports(GFile))),
+    ?assert(not lists:member({start, 0}, get_exports(GFile))),
+
+    ok.
+
 packbeam_create_prune_test() ->
     AVMFile = dest_dir("packbeam_create_prune_test.avm"),
     ?assertMatch(