代码束文件在哪里？

Question

我打印了所有加载的代码如下，为什么 {lager_default_tracer,[]},束文件在哪里?

(lager_test_1@macbook.local)10> io:format("~p",[code:all_loaded()]).
[{io,"/usr/local/lib/erlang/lib/stdlib-2.3/ebin/io.beam"},
 {erl_distribution,"/usr/local/lib/erlang/lib/kernel-3.1/ebin/erl_distribution.beam"},
 {edlin,"/usr/local/lib/erlang/lib/stdlib-2.3/ebin/edlin.beam"},
 {beam_clean,"/usr/local/lib/erlang/lib/compiler-5.0.3/ebin/beam_clean.beam"},
 {v3_core,"/usr/local/lib/erlang/lib/compiler-5.0.3/ebin/v3_core.beam"},
 {erl_epmd,"/usr/local/lib/erlang/lib/kernel-3.1/ebin/erl_epmd.beam"},
 {love_misc,"/usr/local/bin/lager_test/lib/hanoch-0.0.1.6/ebin/love_misc.beam"},
 {zlib,preloaded},
 {error_handler,"/usr/local/lib/erlang/lib/kernel-3.1/ebin/error_handler.beam"},
 {io_lib,"/usr/local/lib/erlang/lib/stdlib-2.3/ebin/io_lib.beam"},
 {lib,"/usr/local/lib/erlang/lib/stdlib-2.3/ebin/lib.beam"},
 {mnesia,"/usr/local/lib/erlang/lib/mnesia-4.12.4/ebin/mnesia.beam"},
 {lager_test_app,"/usr/local/bin/lager_test/lib/lager_test-0.0.1.0/ebin/lager_test_app.beam"},
 {beam_jump,"/usr/local/lib/erlang/lib/compiler-5.0.3/ebin/beam_jump.beam"},
 {v3_codegen,"/usr/local/lib/erlang/lib/compiler-5.0.3/ebin/v3_codegen.beam"},
 {beam_flatten,"/usr/local/lib/erlang/lib/compiler-5.0.3/ebin/beam_flatten.beam"},
 {mnesia_tm,"/usr/local/lib/erlang/lib/mnesia-4.12.4/ebin/mnesia_tm.beam"},
 {prim_eval,preloaded},
 {beam_bool,"/usr/local/lib/erlang/lib/compiler-5.0.3/ebin/beam_bool.beam"},
 {error_logger_lager_h,"/usr/local/bin/lager_test/lib/lager-2.0.0/ebin/error_logger_lager_h.beam"},
 {lager_msg,"/usr/local/bin/lager_test/lib/lager-2.0.0/ebin/lager_msg.beam"},
 {mnesia_frag,"/usr/local/lib/erlang/lib/mnesia-4.12.4/ebin/mnesia_frag.beam"},
 {filename,"/usr/local/lib/erlang/lib/stdlib-2.3/ebin/filename.beam"},
 {lager_default_tracer,[]},
 {lager_default_formatter,"/usr/local/bin/lager_test/lib/lager-2.0.0/ebin/lager_default_formatter.beam"},
 {mnesia_locker,"/usr/local/lib/erlang/lib/mnesia-4.12.4/ebin/mnesia_locker.beam"},
 {mnesia_recover,"/usr/local/lib/erlang/lib/mnesia-4.12.4/ebin/mnesia_recover.beam"},
 {mnesia_dumper,"/usr/local/lib/erlang/lib/mnesia-4.12.4/ebin/mnesia_dumper.beam"},
 {mnesia_kernel_sup,"/usr/local/lib/erlang/lib/mnesia-4.12.4/ebin/mnesia_kernel_sup.beam"},
 {mnesia_sp,"/usr/local/lib/erlang/lib/mnesia-4.12.4/ebin/mnesia_sp.beam"},
 {erts_internal,preloaded},
 {unicode,"/usr/local/lib/erlang/lib/stdlib-2.3/ebin/unicode.beam"},
 {lager_backend_throttle,"/usr/local/bin/lager_test/lib/lager-2.0.0/ebin/lager_backend_throttle.beam"},
 {beam_type,"/usr/local/lib/erlang/lib/compiler-5.0.3/ebin/beam_type.beam"},
 {orddict,"/usr/local/lib/erlang/lib/stdlib-2.3/ebin/orddict.beam"},
 {gb_sets,"/usr/local/lib/erlang/lib/stdlib-2.3/ebin/gb_sets.beam"},
 {sofs,"/usr/local/lib/erlang/lib/stdlib-2.3/ebin/sofs.beam"},
 {inet_db,"/usr/local/lib/erlang/lib/kernel-3.1/ebin/inet_db.beam"},
 {lager_test_a,"/usr/local/bin/lager_test/lib/lager_test-0.0.1.0/ebin/lager_test_a.beam"},
 {inet,"/usr/local/lib/erlang/lib/kernel-3.1/ebin/inet.beam"},

当我调用module_info()时，结果如下：

(lager_test_1@macbook.local)11> lager_default_tracer:module_info().
[{exports,[{table,1},
           {handle,1},
           {module_info,0},
           {module_info,1},
           {info,1}]},
 {imports,[]},
 {attributes,[{vsn,[203040246025344403396962742072895880482]}]},
 {compile,[{options,[]},
           {version,"5.0.3"},
           {time,{2017,8,27,5,43,32}},
           {source,"/private/tmp/lager_test-0.0.1.0"}]}]

当我调用lager_default_tracer:table(111)时，结果如下：

(lager_test_1@macbook.local)12> lager_default_tracer:table(aaa).
** exception error: bad argument
     in function  lager_default_tracer:table/1

Answer 1

lager_default_tracer 模块没有关联的光束文件，因为它是在运行时间创建的。 lager应用使用goldrush，其中使用运行时间码编译加载

如果你翻阅goldrush sources you'll see that it calls the standard compile:forms/2 for dynamic compilation. By tracing that call, we can reconstruct the source code for lager_default_tracer using the steps below, which assume you've already cloned the lager source repository，编译成功。在某些步骤中，Erlang shell 命令的输出太大而无法放在这里，因此这些部分使用省略号缩写。

在您已经编译的克隆 lager 回购目录中，运行 rebar shell 启动一个 Erlang shell，确保所有必要的目录都是在 Erlang 加载路径中。

在shell中，打开dbg tracing并跟踪compile:forms/2:

1> dbg:tracer(), dbg:p(all, call).
{ok,[{matched,nonode@nohost,34}]}
2> dbg:tpl(compile, forms, []).
{ok,[{matched,nonode@nohost,2}]}

启动 lager 应用程序及其依赖项：
```
3> application:ensure_all_started(lager).
```
这将导致 dbg 跟踪产生一些冗长的输出，如下所示：

(<0.94.0>) call compile:forms([{attribute,0,module,lager_default_tracer}, ... {tuple,0,[{integer,0,2},{integer,0,1}]}]}]}]}],[nowarn_unused_vars]) {ok,[syntax_tools,compiler,goldrush,lager]} 08:29:21.478 [info] Application lager started on node nonode@nohost

此处，输出被缩写为仅显示第一行和最后几行。最后两行是 application:ensure_all_started/1 的结果。这些行上方的所有内容都是 lager_default_tracer 模块的 abstract format。
复制跟踪输出，从输出跟踪第一行的 [{attribute,0,module,lager_default_tracer} 一直到输出跟踪最后一行的 {tuple,0,[{integer,0,2},{integer,0,1}]}]}]}]}]。不要在复制的内容中包含结尾文本 [nowarn_unused_vars])。
粘贴复制的数据以将其分配给 Erlang 中名为 M 的变量 shell:

4> M = [{attribute,0,module,lager_default_tracer}, ... {tuple,0,[{integer,0,2},{integer,0,1}]}]}]}]}].

不要忘记添加最后一个句点（又名句号）来完成表达式。
关闭dbg 跟踪：

5> dbg:stop_clear(). ok
编译摘要格式：

6> {ok, _, B} = compile:forms(M, [no_unused_vars, debug_info]). {ok,lager_default_tracer, <<70,79,82,49,0,0,6,164,66,69,65,77,65,116,85,56,0,0,1,9, 0,0,0,23,20,108,...>>}

变量 B 现在以二进制形式保存 lager_default_tracer 模块的编译代码。
从存储在B:
中的已编译二进制文件中提取抽象代码块
7> {ok,{_,[{abstract_code,{_,AC}}]}} = beam_lib:chunks(B,[abstract_code]). {ok,{lager_default_tracer, [{abstract_code, ... [{clause,0,[{var,0,'Event'}],[],[{call,...}]}]}]}}]}}

变量AC现在保存抽象代码。
使用抽象代码打印 lager_default_tracer 模块的重构源代码：

8> io:fwrite("~s~n", [erl_prettypr:format(erl_syntax:form_list(AC))]). -module(lager_default_tracer). -export([info/1, reset_counters/1, table/1, handle/1]). ... handle_(Event) -> gr_counter:update_counter(table(counters), filter, {2, 1}). ok

最后一步和之前的一步取自 "Reconstructing Source Code" 下的 beam_lib documentation。

毫不奇怪，重构的源代码显示了其他 goldrush 函数的调用，因此如果您想尝试理解重构的代码，您需要访问 goldrush sources。

代码束文件在哪里？

where is code beam file?

erlang

lager