macOS 下编译集成 Python 3 的 GDB

Known Rabbit included in tools

2017-09-12 2561 words 6 minutes

Contents

2017-10-31更新：brew维护者明确表示需要上游合并，而从文末来看明显不可能合并了。我的PR在那放着也不会丢，从这个链接下载patch并自行apply即可。

Python是信仰。版本也是信仰的一部分。

开玩笑的啦。之所以非要自己编译，是因为gdb的某个插件不支持Py2.x。更何况，3比2在某些细节上更合我的口味。而brew的formula只支持可选链接brew版的python2，并没有3，只好自己研究一下咯。

其实让我最惊讶的是，主流发行版（如Ubuntu）有的已经把gdb链接到python3了，然而都几年了没人研究也没人提，网上也找不到——什么时候让mac也可以这么diao呀？

撰写本文时所使用的版本是GDB 8.0.1。

查错

不想看可以直接跳过到下一章节如何修复报错并成功编译。

先用brew edit gdb 查看了一下formula，发现通过它指定--with-python=/usr/local/bin/python实现链接到brew的python，我试着在后面加了个3。报这样的错误。

 checking whether to use python... /usr/local/opt/python3
 checking for python2.7... no
 checking for python2.6... no
 checking for python2.5... no
 checking for python2.4... no
 configure: error: no usable python found at /usr/local/opt/python3
 make[1]: *** [configure-gdb] Error 1
 make: *** [all] Error 2

告诉我说，py3不可用。这个报错是在configure的过程中产生的，而configure是用autoconf¹生成的。于是我就在configure.ac脚本里搜 “no usable python”，定位到如下位置。

  if test "${have_libpython}" = no; then
    case "${with_python}" in
    yes)
      AC_MSG_ERROR([python is missing or unusable])
      ;;
    auto)
      AC_MSG_WARN([python is missing or unusable; some features may be unavailable.])
      ;;
    *)
      AC_MSG_ERROR([no usable python found at ${with_python}])
      ;;
    esac

既然with_python是我们刚指定的参数，那么have_libpython就是系统检测的可用python路径咯。

再往上翻一点，发现设置这个变量的位置在895行，作为测试从python-config获得的编译参数是否可用的测试结果，

    case "${python_version}" in
    python*)
      AC_TRY_LIBPYTHON(${python_version}, have_libpython,
                       ${python_includes}, ${python_libs})
      ;;

而编译参数的来源位于configure.ac的838行，

    python_includes=`${python_prog} ${srcdir}/python/python-config.py --includes`
    python_libs=`${python_prog} ${srcdir}/python/python-config.py --ldflags`
    python_prefix=`${python_prog} ${srcdir}/python/python-config.py --exec-prefix`

把它的结果和原装flags比较一下，还是有一点不一样的。

$ python3 python/python-config.py --ldflags
-L/usr/local/opt/python3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/config-3.6m-darwin -ldl -framework CoreFoundation -lpython3.6m -Wl,-stack_size,1000000 -framework CoreFoundation Python.framework/Versions/3.6/Python

$ python3-config --ldflags
-L/usr/local/opt/python3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/config-3.6m-darwin -lpython3.6m -ldl -framework CoreFoundation
# 下面是ubuntu的输出
~$ python3-config --ldflags
-L/usr/lib/python3.5/config-3.5m-x86_64-linux-gnu -L/usr/lib -lpython3.5m -lpthread -ldl  -lutil -lm  -Xlinker -export-dynamic -Wl,-O1 -Wl,-Bsymbolic-functions

强行把这个python-config换成了python自带的，编译通过，就这么给gdb的formula交了个PR。后来又对比一下gdb自己写的和python官方的python-config，除了细节大部分都大同小异。只有一句话。

@@ -72,7 +72,7 @@ for opt in opt_flags:
                     libs.insert(0, '-L' + getvar('LIBPL'))
                 elif os.name == 'nt':
                     libs.insert(0, '-L' + sysconfig.PREFIX + '/libs')
-            if getvar('LINKFORSHARED') is not None:
+            if not getvar('PYTHONFRAMEWORK'):
                 libs.extend(getvar('LINKFORSHARED').split())
         print (to_unix_path(' '.join(libs)))

在网上还有一个类似²的patch。直接把这10和11行删掉了。不按官方的来，我觉得不妥。

溯源

其实我更感兴趣的是，为啥实现同样的功能非要两份冗余的代码？为啥不信任官方的工具呢？我早就听说，许多发行版早就发布了集成python3的gdb。brew维护者说：要等上游更新。然而这么多年了，默认还是python2。

于是我从ppa下来ubuntu版gdb的源码研究，发现它除了src外，还有个文件夹patch。里面赫然有一个python-config.patch，写法与我的patch大同小异。（惊！历史的开拓者！）我又往下翻changelog³，看是哪个版本加入的python3的支持。

+  * debian/patches: Update for new upstream version
+    - Remove patches subsumed by new upstream version/tarball:
+      + ppc64le.diff
+      + hurd-new-RPC-reply-stub-functions.patch
+      + hurd-adapt-to-changed-MIG-output.patch
+      + hurd-adjust-to-startup-with-shell-changes.patch
+      + hurd-make-MIG-output-parsing-more-robust.patch
+      + kfreebsd_bug752390.diff
+      + restore-run.1.patch
+    - Adjust patches to work with new upstream version:
+      + gdb-fortran-main.patch: Update for upstream changes
+      + python-config.patch: Remove parts applied upstream
+
+  [ Héctor Orón Martínez ]
+  * Switch gdb to python3 (Closes: #752581)
+    + Add gdb-python2 package for use with python2-only scripts
+  * Enable babeltrace on supported arches
+
+  [ Samuel Bronson ]
+  * Make gdb-python2 provide gdb.
+  * Totally rewrite the NEWS entry about the Python changes.
+    + And split out most of the new text to README.python_switch.
+
+ -- Samuel Bronson <naesten@gmail.com>  Tue, 26 Aug 2014 15:34:03 -0400
+
 gdb (7.8-0ubuntu2) utopic; urgency=medium

版本7.8-0ubuntu2。备注还是：修改了被上游合并的patch。可上游合并了为啥还是.patch呢？一看开头：debian/patches。😯……敢情你们浓眉大眼的，代码还是三手的？

再去看debian的代码，它们专门有个Debpatches服务，从那里可以找到所有debian贡献者们写的patch。所以说嘛，linux老大不是什么ubuntu，最后不还是得看debian脸色？在这里我只找到了几个release版本所含的patch，最早加入这个补丁的版本是7.6.2-1.1(jessie)，可以看到时间：2013-08-14。比Ubuntu还早了一年。看来Ubuntu也不是很快很bleeding-edge嘛！

那这些年gdb开发者们都干啥吃的？其实他们知道的比香港记者都快。早在2012年12月，圣诞节前五天，就有一个ubuntu的开发者在邮件列表里为gdb-7.5提交了类似的patch。然鹅！为啥这样都没有被合并呢？看看邮件列表⁴里的对话：

From: Matthias Klose <doko at ubuntu dot com>
Date: Thu, 20 Dec 2012 17:13:14 +0100

Am 20.12.2012 16:59, schrieb Joel Brobecker:
> Can you tell us what problem you are trying to solve?

cross building in a multiarch environment, where the host libraries are
installed, and a <host>-python-config is shipped as part of the system python.
This <host>-python-config is a backport of http://bugs.python.org/issue16235
(not yet applied upstream).

==========
From: Joel Brobecker <brobecker at adacore dot com>
Date: Fri, 21 Dec 2012 11:57:21 +0400
I think that this was the plan, yes; but Doug Evans would probably
know more, as he is the main author of this code (I only fiddled
with it).

==========
From: Doug Evans <dje at google dot com>
Date: Fri, 21 Dec 2012 09:30:47 -0800

The patch is fine with me.
[The plan wasn't to shift and call the "real" python-config ... at the
time there was no real python-config for cross builds.]

==========
From: Joel Brobecker <brobecker at adacore dot com>
Date: Fri, 21 Dec 2012 21:50:59 +0400
Can we make this new behavior optional? The "real" python-config
is broken for several platforms, and having our own that we can
control was very convenient.

==========
From: Doug Evans <dje at google dot com>
Date: Sat, 22 Dec 2012 11:33:08 -0800
I certainly don't want to break your use-case.

I need to look at the patch again (but I probably won't get to it
until next week).

他缩，等老子周一上班再看。注意时间：Sat, 22 Dec。嗯，周六。我掐指一算，23号，周日。24！25！26！圣诞节，三天，小，长，假！

于是在一片欢乐祥和的气氛中，bug就这么被遗忘了(´･_･`)

修复

对了开头好像有一句没说……本文遇到的bug及修复都仅针对mac系统（亲测10.12.6），如果你一直都用apt-get装软件的话呢，它已经帮你把屁股擦干净了。更何况，mac的python-config是一个python脚本，而ubuntu下的是一个shell脚本，代码生成的，所以即便真的要改的话也不是一个方案啦。

将下面的patch放到一个文本文件（比如py.patch）里，然后把这个文件放到gdb的源码目录中，运行git apply py.patch。然后正常编译即可。这个文件已经好多版本没改啦，所以大概都能用。

diff --git a/gdb/python/python-config.py b/gdb/python/python-config.py
index c2b2969..554dac9 100644
--- a/gdb/python/python-config.py
+++ b/gdb/python/python-config.py
@@ -72,7 +72,7 @@ for opt in opt_flags:
                     libs.insert(0, '-L' + getvar('LIBPL'))
                 elif os.name == 'nt':
                     libs.insert(0, '-L' + sysconfig.PREFIX + '/libs')
-            if getvar('LINKFORSHARED') is not None:
+            if not getvar('PYTHONFRAMEWORK'):
                 libs.extend(getvar('LINKFORSHARED').split())
         print (to_unix_path(' '.join(libs)))

Contents

macOS 下编译集成 Python 3 的 GDB

查错

溯源

修复

参考资料