如何用GDB 排查 Python 程序故障，相信很多沒(méi)有經(jīng)驗(yàn)的人對(duì)此束手無(wú)策，為此本文總結(jié)了問(wèn)題出現(xiàn)的原因和解決方法，通過(guò)這篇文章希望你能解決這個(gè)問(wèn)題。
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某Team在用Python開(kāi)發(fā)一些代碼，涉及子進(jìn)程以及設(shè)法消除僵尸進(jìn)程的需求。實(shí)踐中他們碰上Python程序非預(yù)期退出的現(xiàn)象。最初他們決定 用GDB調(diào)試Python解釋器，查看exit()的源頭。我聽(tīng)了之后，覺(jué)得這個(gè)問(wèn)題應(yīng)該用別的調(diào)試思路。幫他們排查這次程序故障時(shí)，除去原始問(wèn)題，還衍 生了其他問(wèn)題。
為了簡(jiǎn)化現(xiàn)場(chǎng)、方便調(diào)試，已將原始問(wèn)題、衍生問(wèn)題濃縮成DebugPythonWithGDB_6.py、DebugPythonWithGDB_7.py。
$ vi DebugPythonWithGDB_6.py
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 | #!/usr/bin/env python # -*- encoding: utf-8 -*- import sys, os, signal, subprocess, shlex, traceback def on_SIGCHLD ( signum, frame ) : print "[on_SIGCHLD" sys.stdout.write( "signum = %u\n" % signum ) traceback.print_stack( frame ) print os.waitpid( -1, os.WNOHANG ) """ try : print os.waitpid( -1, os.WNOHANG ) except OSError : sys.stdout.write( 'Line[%u]: OSError\n' % sys.exc_info()[2].tb_lineno ) """ print "on_SIGCHLD]" def do_more ( count ) : print '[do_more() begin %u]' % count os.system( r'printf "Child = %u\n" $$;/bin/sleep 1' ) """ # # 這里存在競(jìng)爭(zhēng)條件，可以增加觸發(fā)OSError異常的概率 # os.system( r'printf "Child = %u\n" $$;/bin/sleep 1' ) os.system( r'printf "Child = %u\n" $$;/bin/sleep 1' ) os.system( r'printf "Child = %u\n" $$;/bin/sleep 1' ) os.system( r'printf "Child = %u\n" $$;/bin/sleep 1' ) """ print '[do_more() end %u]' % count def main ( prog, args ) : if 0 == len( args ) : print 'Usage: %s ' % prog else : sys.stdout.write( "Parent = %u\n" % os.getpid() ) # # 本例中，即使有下列代碼，Ctrl-C仍然無(wú)效。 # signal.signal( signal.SIGINT, signal.SIG_DFL ) # # signal.signal( signal.SIGCHLD, signal.SIG_IGN ) # signal.signal( signal.SIGCHLD, on_SIGCHLD ) # count = 0 while True : # # 本例中父進(jìn)程只是一個(gè)調(diào)度框架，不需要與子進(jìn)程進(jìn)行通信，因此不 # 需要特別處理"stdin=None, stdout=None, stderr=None"。 # child = subprocess.Popen \ ( # # 不要直接用args[0].split()，它在處理單、雙引號(hào)時(shí)不是我們 # 期望的行為。考慮這種例子，ls -l "/tmp/non exist" # shlex.split( args[0] ), # # all file descriptors except 0, 1 and 2 will be closed # before the child process is executed # close_fds = True, cwd = "/tmp" ) sys.stdout.write( "Child = %u\n" % child.pid ) # # child.send_signal( signal.SIGTERM ) # child.terminate() # child.kill() # # child.wait() # do_more( count ) count += 1 if '__main__' == __name__ : try : main( os.path.basename( sys.argv[0] ), sys.argv[1:] ) except KeyboardInterrupt : pass |
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 | $ python DebugPythonWithGDB_6.py 'python -c "import time;time.sleep(3600)"' Parent = 10244 Child = 10245 [do_more() begin 0] [on_SIGCHLD signum = 17 File "DebugPythonWithGDB_6.py", line 81, in main( os.path.basename( sys.argv[0] ), sys.argv[1:] ) File "DebugPythonWithGDB_6.py", line 76, in main do_more( count ) File "DebugPythonWithGDB_6.py", line 20, in do_more print '[do_more() begin %u]' % count (10245, 9) on_SIGCHLD] Child = 10246 [on_SIGCHLD signum = 17 File "DebugPythonWithGDB_6.py", line 81, in main( os.path.basename( sys.argv[0] ), sys.argv[1:] ) File "DebugPythonWithGDB_6.py", line 76, in main do_more( count ) File "DebugPythonWithGDB_6.py", line 21, in do_more os.system( r'printf "Child = %u\n" $$;/bin/sleep 1' ) Traceback (most recent call last): File "DebugPythonWithGDB_6.py", line 81, in main( os.path.basename( sys.argv[0] ), sys.argv[1:] ) File "DebugPythonWithGDB_6.py", line 76, in main do_more( count ) File "DebugPythonWithGDB_6.py", line 21, in do_more os.system( r'printf "Child = %u\n" $$;/bin/sleep 1' ) File "DebugPythonWithGDB_6.py", line 10, in on_SIGCHLD print os.waitpid( -1, os.WNOHANG ) OSError: [Errno 10] No child processes |
流程進(jìn)入on_SIGCHLD()，但os.waitpid()拋出OSError異常。幫助里寫的是，如果系統(tǒng)調(diào)用 waitpid()返回-1，就拋出異常: An OSError is raised with the value of errno when the syscall returns -1. 10245號(hào)子進(jìn)程在on_SIGCHLD()里waitpid()成功，(10245, 9)中的9表示該進(jìn)程是被SIGKILL干掉的，符合預(yù)期。 10246號(hào)子進(jìn)程是do_more()里的os.system()產(chǎn)生的shell進(jìn)程，它結(jié)束時(shí)向10244號(hào)父進(jìn)程投遞了SIGCHLD信號(hào)。 on_SIGCHLD()里waitpid()時(shí)，已經(jīng)在別處wait*()過(guò)，10246號(hào)子進(jìn)程已經(jīng)徹底消失，系統(tǒng)調(diào)用waitpid()返回 -1，Python函數(shù)os.waitpid()拋出異常。 整個(gè)過(guò)程非常復(fù)雜，用偽代碼描述如下:
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 | do_more() os.system() posix_system() // posixmodule.c __libc_system() // weak_alias (__libc_system, system) do_system() // sysdeps/posix/system.c /* * SIG_IGN * * Ctrl-C暫時(shí)失效 */ sigaction( SIGINT, &sa, &intr ) /* * 屏蔽(阻塞)SIGCHLD信號(hào) */ sigaddset( &sa.sa_mask, SIGCHLD ) sigprocmask( SIG_BLOCK, &sa.sa_mask, &omask ) fork() 子進(jìn)程(10246號(hào)子進(jìn)程) /* * 恢復(fù)原有SIGINT信號(hào)處理方式 */ sigaction( SIGINT, &intr, (struct sigaction *)NULL ) /* * 調(diào)用"sh -c ..." */ execve() [shell子進(jìn)程結(jié)束，向DebugPythonWithGDB_6.py投遞SIGCHLD] [由于SIGCHLD信號(hào)已被屏蔽(阻塞)，其保持在內(nèi)核態(tài)的未決信號(hào)鏈上] 父進(jìn)程(10244號(hào)父進(jìn)程) /* * 同步調(diào)用，會(huì)阻塞。不是在信號(hào)句柄中異步調(diào)用。 * * 10246號(hào)子進(jìn)程在此被wait*()回收后徹底消失 */ waitpid( pid, &status, 0 ) /* * 恢復(fù)原有SIGINT信號(hào)處理方式 */ sigaction( SIGINT, &intr, (struct sigaction *)NULL ) /* * 取消對(duì)SIGCHLD的屏蔽(阻塞) */ sigprocmask( SIG_SETMASK, &omask, (sigset_t *)NULL ) [SIGCHLD信號(hào)的屏蔽(阻塞)被取消] [DebugPythonWithGDB_6.py的C級(jí)信號(hào)句柄signal_handler()安排"延遲調(diào)用"后返回] [DebugPythonWithGDB_6.py的on_SIGCHLD()此時(shí)并未得到執(zhí)行，因?yàn)閎uilt-in函數(shù)os.system()尚未返回] /* * built-in函數(shù)os.system()返回后，10244號(hào)父進(jìn)程開(kāi)始處理"延遲調(diào)用"，調(diào)用 * Python級(jí)信號(hào)句柄。這個(gè)SIGCHLD信號(hào)是10246號(hào)子進(jìn)程投遞過(guò)來(lái)的。 * * DebugPythonWithGDB_6.py的on_SIGCHLD()得到執(zhí)行 */ on_SIGCHLD() /* * 調(diào)用waitpid( -1, &status, WNOHANG )，試圖處理10246號(hào)子進(jìn)程。 * * 10246號(hào)子進(jìn)程已為前述waitpid( pid, &status, 0 )所處理，此處系統(tǒng)調(diào)用 * 返回-1，導(dǎo)致os.waitpid()拋出OSError異常。 */ os.waitpid( -1, os.WNOHANG ) |
整個(gè)過(guò)程之所以如此復(fù)雜，主要是因?yàn)镻ython的信號(hào)處理機(jī)制比較復(fù)雜，讓已經(jīng)非常復(fù)雜的Linux信號(hào)機(jī)制再添變數(shù)。參看:
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1 2 | 《2.50 對(duì)Python解釋器進(jìn)行調(diào)試》 《22.0 Linux信號(hào)機(jī)制》 |
就本例而言，為了確保DebugPythonWithGDB_6.py不因OSError異常而終止，只需在on_SIGCHLD()中調(diào)用os.waitpid()時(shí)捕捉OSError異常:
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1 2 3 4 5 | def on_SIGCHLD ( signum, frame ) : try : print os.waitpid( -1, os.WNOHANG ) except OSError : sys.stdout.write( 'Line[%u]: OSError\n' % sys.exc_info()[2].tb_lineno ) |
前述觀點(diǎn)有些是動(dòng)態(tài)調(diào)試得到，有些是靜態(tài)分析得到。有人可能問(wèn)了，為什么不攔截Python進(jìn)程的C級(jí)信號(hào)句柄，查看SIGCHLD 信號(hào)源，以此確認(rèn)10246號(hào)子進(jìn)程可能被回收兩次？其實(shí)我最初也想這么干來(lái)著，但這是行不通的，因?yàn)镻ython的C級(jí)信號(hào)句柄 signal_handler()是那種最原始的單形參信號(hào)句柄，不是高大上的三形參信號(hào)句柄。 用GDB調(diào)試Python解釋器:
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 | # gdb -q -ex "b *signal_handler" -ex r --args /usr/bin/python2.7-dbg DebugPythonWithGDB_6.py '/usr/bin/python2.7-dbg -c "import time;time.sleep(3600)"' ... Breakpoint 1 at 0x8216f2d: file ../Modules/signalmodule.c, line 185. Starting program: /usr/bin/python2.7-dbg DebugPythonWithGDB_6.py /usr/bin/python2.7-dbg\ -c\ \"import\ time\;time.sleep\(3600\)\" [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/i386-linux-gnu/i686/cmov/libthread_db.so.1". Parent = 10284 Child = 10288 [do_more() begin 0] Child = 10289 Breakpoint 1, signal_handler (sig_num=17) at ../Modules/signalmodule.c:185 185 { (gdb) py-bt #10 Frame 0xb7c20034, for file DebugPythonWithGDB_6.py, line 21, in do_more (count=0) os.system( r'printf "Child = %u\n" $$;/bin/sleep 1' ) #13 Frame 0xb7cb37dc, for file DebugPythonWithGDB_6.py, line 76, in main (prog='DebugPythonWithGDB_6.py', args=['/usr/bin/python2.7-dbg -c "import time;time.sleep(3600)"'], count=0, child=) do_more( count ) #16 Frame 0xb7cbe49c, for file DebugPythonWithGDB_6.py, line 81, in () main( os.path.basename( sys.argv[0] ), sys.argv[1:] ) (gdb) bt 7 #0 signal_handler (sig_num=17) at ../Modules/signalmodule.c:185 #1 #2 0xb7fdcd3c in __kernel_vsyscall () #3 0xb7db25eb in __sigprocmask (how=how@entry=2, set=0x0, set@entry=0xbffff0d4, oset=oset@entry=0x0) at ../sysdeps/unix/sysv/linux/sigprocmask.c:57 #4 0xb7dc2084 in do_system (line=line@entry=0xb7cbf9e4 "printf \"Child = %u\\n\" $$;/bin/sleep 1") at ../sysdeps/posix/system.c:161 #5 0xb7dc2380 in __libc_system (line=line@entry=0xb7cbf9e4 "printf \"Child = %u\\n\" $$;/bin/sleep 1") at ../sysdeps/posix/system.c:184 #6 0xb7fa9bfb in system (line=0xb7cbf9e4 "printf \"Child = %u\\n\" $$;/bin/sleep 1") at pt-system.c:28 (More stack frames follow...) |
查看#4的system.c:161，這個(gè)位置已經(jīng)在waitpid( pid, &status, 0 )之后: sigprocmask( SIG_SETMASK, &omask, (sigset_t *)NULL ) 其作用是取消對(duì)SIGCHLD的屏蔽(阻塞)。 此時(shí)內(nèi)存布局如下:
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1 2 3 4 5 6 7 8 9 10 11 12 | 內(nèi)存高址方向 fpstate // ESP+0x2DC output/x *(struct _fpstate *)($esp+0x2dc) retcode // ESP+0x2D4 x/3i $esp+0x2d4 extramask // ESP+0x2D0 x/1wx $esp+0x2d0 fpstate_unused // ESP+0x60 output/x *(struct _fpstate *)($esp+0x60) sigcontext_ia32 // ESP+8 output/x *(struct sigcontext *)($esp+8) sig // ESP+4 信號(hào)值，信號(hào)句柄***形參 pretcode // ESP RetAddr=__kernel_sigreturn // hexdump $esp 0x2dc 內(nèi)存低址方向 |
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | (gdb) x/2wa $esp 0xbfffea6c: 0xb7fdcd18 0x11 (gdb) x/3i $esp+0x2d4 0xbfffed40: pop eax 0xbfffed41: mov eax,0x77 0xbfffed46: int 0x80 (gdb) output/x *(struct sigcontext *)($esp+8) { gs = 0x33, __gsh = 0x0, fs = 0x0, __fsh = 0x0, es = 0x7b, __esh = 0x0, ds = 0x7b, __dsh = 0x0, edi = 0xb7f2a000, esi = 0x8, ebp = 0x1, esp = 0xbfffeff0, ebx = 0x2, edx = 0x0, ecx = 0xbffff0d4, eax = 0x0, trapno = 0x1, err = 0x0, eip = 0xb7fdcd3c, cs = 0x73, __csh = 0x0, eflags = 0x246, esp_at_signal = 0xbfffeff0, ss = 0x7b, __ssh = 0x0, fpstate = 0xbfffed50, oldmask = 0x0, cr2 = 0x0 } |
因?yàn)槭菃涡螀⑿盘?hào)句柄，沒(méi)有siginfo，無(wú)法在用戶態(tài)獲知信號(hào)源。但我分析此時(shí)的信號(hào)源不是10289號(hào)子進(jìn)程，而是10288 號(hào)子進(jìn)程。10288產(chǎn)生SIGCHLD時(shí)，SIGCHLD信號(hào)已被屏蔽(阻塞)，只能保持在內(nèi)核態(tài)的未決信號(hào)鏈上。之后待10289產(chǎn)生SIGCHLD 時(shí)，sigpending.signal中相應(yīng)位已經(jīng)置位，10289產(chǎn)生的SIGCHLD被丟棄，不會(huì)進(jìn)入內(nèi)核態(tài)的未決信號(hào)鏈。SIGCHLD信號(hào)的屏 蔽(阻塞)被取消后，從內(nèi)核態(tài)的未決信號(hào)鏈上取出10288產(chǎn)生的SIGCHLD進(jìn)行處理。于是斷點(diǎn)***。 如果完全理解了前述實(shí)驗(yàn)結(jié)果及分析，就會(huì)發(fā)現(xiàn)DebugPythonWithGDB_6.py存在競(jìng)爭(zhēng)條件。subprocess.Popen()對(duì)應(yīng)的 子進(jìn)程投遞SIGCHLD信號(hào)時(shí)，父進(jìn)程有兩種可能:
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1 2 | 1) os.system()調(diào)用sigprocmask( SIG_BLOCK, &sa.sa_mask, &omask )之前 2) os.system()調(diào)用sigprocmask( SIG_BLOCK, &sa.sa_mask, &omask )之后 |
情況1)會(huì)觸發(fā)OSError異常，情況2)不會(huì)觸發(fā)OSError異常。執(zhí)行: $ python DebugPythonWithGDB_6.py ‘python -c “import time;time.sleep(3600)”‘ 有時(shí)會(huì)因OSError異常而終止，有時(shí)就一直循環(huán)執(zhí)行下去。出現(xiàn)這種差異，正是競(jìng)爭(zhēng)環(huán)境的表征。 小結(jié)一下: 假設(shè)針對(duì)SIGCHLD安裝了Python級(jí)信號(hào)句柄，其調(diào)用os.waitpid( -1, os.WNOHANG )回收子進(jìn)程。如果別處會(huì)調(diào)用os.system()，則必須在os.waitpid()外側(cè)捕捉OSError異常。不建議這種方式的混用。 對(duì)waitpid()的分析到此就結(jié)束了，說(shuō)點(diǎn)調(diào)試過(guò)程中出現(xiàn)的其他問(wèn)題。 意外地發(fā)現(xiàn)Ctrl-C無(wú)法終止情況2)，而我已經(jīng)調(diào)用: signal.signal( signal.SIGINT, signal.SIG_DFL ) 這是因?yàn)閐o_system()中一上來(lái)就調(diào)用了:
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1 2 | sa.sa_handler = SIG_IGN; sigaction( SIGINT, &sa, &intr ); |
導(dǎo)致Ctrl-C暫時(shí)失效，直至do_system()結(jié)束。假設(shè)DebugPythonWithGDB_6.py已經(jīng)出 現(xiàn)情況2)，查看它的信號(hào)處理方式:
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 | # ps auwx | grep python root 10355 0.0 0.5 8116 5812 pts/0 S+ 15:57 0:00 python DebugPythonWithGDB_6.py python -c "import time;time.sleep(3600)" root 10389 0.0 0.0 0 0 pts/0 Z+ 15:57 0:00 [python] root 10393 0.0 0.0 2936 852 pts/1 R+ 15:57 0:00 grep python # stap -DMAXACTION=10000 -g /usr/share/doc/systemtap-doc/examples/process/psig.stp -x 10355 10355: python HUP default INT ignored // 不是預(yù)期的default QUIT ignored ILL default TRAP default ABRT default BUS default FPE default KILL default USR1 default SEGV default USR2 default PIPE ignored ALRM default TERM default STKFLT default CHLD blocked,caught 0x818a480 0 ... |
上面顯示SIGINT的處理方式是ignored，其實(shí)是ignored、default交叉出現(xiàn)，但我們 基本上不可能看到default。 $ vi DebugPythonWithGDB_7.py
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 | #!/usr/bin/env python # -*- encoding: utf-8 -*- import sys, os, subprocess, shlex, traceback def do_more ( count ) : print '[do_more() begin %u]' % count os.system( r'printf "Child = %u\n" $$;/bin/sleep 1' ) print '[do_more() end %u]' % count def main ( prog, args ) : if 0 == len( args ) : print 'Usage: %s ' % prog else : sys.stdout.write( "Parent = %u\n" % os.getpid() ) count = 0 while True : child = subprocess.Popen \ ( shlex.split( args[0] ), close_fds = True, cwd = "/tmp" ) sys.stdout.write( "Child = %u\n" % child.pid ) child.kill() do_more( count ) count += 1 if '__main__' == __name__ : try : main( os.path.basename( sys.argv[0] ), sys.argv[1:] ) except KeyboardInterrupt : pass |
$ python DebugPythonWithGDB_7.py ‘python -c “import time;time.sleep(3600)”‘ DebugPythonWithGDB_7.py沒(méi)有顯式調(diào)用wait()，它一直循環(huán)執(zhí)行下去。我以為subprocess.Popen()會(huì)生成一堆僵尸進(jìn)程。從另一個(gè)終端查看相關(guān)進(jìn)程，發(fā)現(xiàn)始終只有一個(gè)僵尸進(jìn)程，很快就被回收了。這個(gè)現(xiàn)象挺奇特，只能假設(shè)有隱式wait()存在。
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | # gdb -q -ex "b *waitpid" -ex r --args /usr/bin/python2.7-dbg DebugPythonWithGDB_7.py '/usr/bin/python2.7-dbg -c "import time;time.sleep(3600)"' ... Parent = 14508 Child = 14512 [do_more() begin 0] Child = 14513 [do_more() end 0] Breakpoint 1, waitpid () at ../sysdeps/unix/syscall-template.S:81 81 T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS) (gdb) py-bt #4 Frame 0xb7c21034, for file /usr/lib/python2.7/subprocess.py, line 1363, in _internal_poll (self=, _deadstate=2147483647, _waitpid=, _WNOHANG=1, _os_error=, _ECHILD=10) pid, sts = _waitpid(self.pid, _WNOHANG) #8 Frame 0xb7c6549c, for file /usr/lib/python2.7/subprocess.py, line 762, in __del__ (self=, _maxint=2147483647) self._internal_poll(_deadstate=_maxint) #18 Frame 0xb7cb37dc, for file DebugPythonWithGDB_7.py, line 22, in main (prog='DebugPythonWithGDB_7.py', args=['/usr/bin/python2.7-dbg -c "import time;time.sleep(3600)"'], count=1, child=) cwd = "/tmp" #21 Frame 0xb7cbe49c, for file DebugPythonWithGDB_7.py, line 31, in () main( os.path.basename( sys.argv[0] ), sys.argv[1:] ) (gdb) bt 9 #0 waitpid () at ../sysdeps/unix/syscall-template.S:81 #1 0x081f80a3 in posix_waitpid (self=0x0, args=(14512, 1)) at ../Modules/posixmodule.c:6207 #2 0x080bc300 in PyCFunction_Call (func=, arg=(14512, 1), kw=0x0) at ../Objects/methodobject.c:81 #3 0x08149d0b in call_function (pp_stack=0xbfffebd4, oparg=2) at ../Python/ceval.c:4033 #4 0x081454ec in PyEval_EvalFrameEx (f=Frame 0xb7c21034, for file /usr/lib/python2.7/subprocess.py, line 1363, in _internal_poll (self=, _deadstate=2147483647, _waitpid=, _WNOHANG=1, _os_error=, _ECHILD=10), throwflag=0) at ../Python/ceval.c:2679 #5 0x08147a77 in PyEval_EvalCodeEx (co=0xb7c60448, globals={'STDOUT': -2, '_has_poll': True, 'gc': , 'check_call': , 'mswindows': False, 'select': , 'list2cmdline': , '__all__': ['Popen', 'PIPE', 'STDOUT', 'call', 'check_call', 'check_output', 'CalledProcessError'], 'errno': , '_demo_posix': , '__package__': None, 'PIPE': -1, '_cleanup': , '_eintr_retry_call': , 'call': , '__doc__': 'subprocess - Subprocesses with accessible I/O streams\n\nThis module allows you to spawn processes, connect to their\ninput/output/error pipes, and obtain their return codes. This module\nintends to replace several older modules and functions:\n\nos.system\nos.spawn*\nos.popen*\npopen2.*\ncommands.*\n\nInformation about how the subprocess module can be used to replace these\nmodules and functions can be found below.\n\...(truncated), locals=0x0, args=0xb7c655e4, argcount=1, kws=0xb7c655e8, kwcount=1, defs=0xb7c73a20, defcount=5, closure=0x0) at ../Python/ceval.c:3265 #6 0x0814a1e5 in fast_function (func=, pp_stack=0xbfffeef4, n=3, na=1, nk=1) at ../Python/ceval.c:4129 #7 0x08149e93 in call_function (pp_stack=0xbfffeef4, oparg=256) at ../Python/ceval.c:4054 #8 0x081454ec in PyEval_EvalFrameEx (f=Frame 0xb7c6549c, for file /usr/lib/python2.7/subprocess.py, line 762, in __del__ (self=, _maxint=2147483647), throwflag=0) at ../Python/ceval.c:2679 (More stack frames follow...) |
調(diào)用?；厮荼砻?
查看: /usr/lib/python2.7/subprocess.py:1363
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 | try : _waitpid( self.pid, _WNOHANG ) ... except _os_error as e : ... if e.errno == _ECHILD : # # This happens if SIGCLD is set to be ignored or waiting for child # processes has otherwise been disabled for our process. This # child is dead, we can't get the status. # # http://bugs.python.org/issue15756 # ... |
考慮這種情形，調(diào)用subprocess.Popen()之前做過(guò)如下動(dòng)作之一:
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1 2 | signal.signal( signal.SIGCHLD, signal.SIG_IGN ) signal.signal( signal.SIGCHLD, on_SIGCHLD ) |
_internal_poll()調(diào)用_waitpid()時(shí)，在外側(cè)捕捉了_os_error異常，就是對(duì)付上述可能。 subprocess.Popen()不是built-in函數(shù)，對(duì)應(yīng)很多條PVM指令，不像os.system()是built-in函數(shù)，對(duì)應(yīng)單條 PVM指令。前者執(zhí)行過(guò)程中，Python級(jí)信號(hào)句柄on_SIGCHLD()有很大機(jī)會(huì)得到執(zhí)行，_internal_poll()調(diào)用 _waitpid()，遭遇_os_error異常的可能性不低。 Popen()對(duì)象的析構(gòu)函數(shù)自動(dòng)調(diào)用wait*()，于是Popen()對(duì)象離開(kāi)作用域時(shí)自動(dòng)回收子進(jìn)程。 Python能夠改變變量作用域的是def、class、lamda、global。下面這些并不涉及作用域變更:
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1 2 3 | if/elif/else try/except/finally for/while |
DebugPythonWithGDB_7.py中Popen()對(duì)象的作用域是main()。盡管沒(méi)有離開(kāi)作用域，但對(duì)child 變量的重新賦值會(huì)觸發(fā)對(duì)前一個(gè)Popen()對(duì)象的析構(gòu)。這就解釋了為什么始終只有一個(gè)僵尸進(jìn)程。 回顧原始問(wèn)題與衍生問(wèn)題，再次表明，掌握對(duì)Python解釋器的調(diào)試技術(shù)可以快速排查眾多看似神秘實(shí)則基礎(chǔ)的程序故障。
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