之前就觉得我的 Emacs 启动好慢，查看启动日志会发现启动到一般的时候会有一个比较长时间的卡顿。 之前一直没有理会它，今天花了点时间探索了一下，发现罪魁祸首居然是 exec-path-from-shell 这个包。

现将探索的过程记录如下： 由于使用了 spacemacs 的配置，配置上比较复杂，不太想通过实验缩减配置的方式来摸索出问题的地方。刚好最近在学习使用 strace 工具，因此决定使用 strace 来看看 Emacs 到底卡在哪里。

strace emacs --fg-daemon

输出的内容特别多，这里只截取卡顿前的部分内容

readlinkat(AT_FDCWD, "/home", 0x7ffd1d3abb50, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972", 0x7ffd1d3abf00, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972/.emacs.d", 0x7ffd1d3ac2b0, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972/.emacs.d/elpa", 0x7ffd1d3ac660, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972/.emacs.d/elpa/exec-path-from-shell-20180323.1904", 0x7ffd1d3aca10, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972/.emacs.d/elpa/exec-path-from-shell-20180323.1904/exec-path-from-shell.elc", 0x7ffd1d3acdc0, 1024) = -1 EINVAL (无效的参数)
lseek(7, -2655, SEEK_CUR)               = 1441
read(7, "\n(defvar exec-path-from-shell-de"..., 4096) = 4096
lseek(7, 5537, SEEK_SET)                = 5537
lseek(7, 5537, SEEK_SET)                = 5537
lseek(7, 5537, SEEK_SET)                = 5537
lseek(7, 5537, SEEK_SET)                = 5537
lseek(7, 5537, SEEK_SET)                = 5537
lseek(7, 5537, SEEK_SET)                = 5537
brk(0x7507000)                          = 0x7507000
lseek(7, 5537, SEEK_SET)                = 5537
lseek(7, 5537, SEEK_SET)                = 5537
lseek(7, 5537, SEEK_SET)                = 5537
read(7, "230\\205\26\0\t\22\\307\\310\t!\vC\\\"\\211\24\\2"..., 4096) = 2430
lseek(7, 7967, SEEK_SET)                = 7967
lseek(7, 7967, SEEK_SET)                = 7967
lseek(7, 7967, SEEK_SET)                = 7967
lseek(7, 7967, SEEK_SET)                = 7967
read(7, "", 4096)                       = 0
close(7)                                = 0
getpid()                                = 10818
faccessat(AT_FDCWD, "/home/lujun9972/bin/printf", X_OK) = -1 ENOENT (没有那个文件或目录)
faccessat(AT_FDCWD, "/usr/local/sbin/printf", X_OK) = -1 ENOENT (没有那个文件或目录)
faccessat(AT_FDCWD, "/usr/local/bin/printf", X_OK) = -1 ENOENT (没有那个文件或目录)
faccessat(AT_FDCWD, "/usr/bin/printf", X_OK) = 0
stat("/usr/bin/printf", {st_mode=S_IFREG|0755, st_size=51176, ...}) = 0
openat(AT_FDCWD, "/dev/null", O_RDONLY|O_CLOEXEC) = 7
faccessat(AT_FDCWD, "/proc/5070/fd/.", F_OK) = 0
faccessat(AT_FDCWD, "/proc/5070/fd/.", F_OK) = 0
faccessat(AT_FDCWD, "/bin/bash", X_OK)  = 0
stat("/bin/bash", {st_mode=S_IFREG|0755, st_size=903440, ...}) = 0
pipe2([8, 9], O_CLOEXEC)                = 0
rt_sigprocmask(SIG_BLOCK, [INT CHLD], [], 8) = 0
vfork()                                 = 10949
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
close(9)                                = 0
close(7)                                = 0
read(8, "bash: \346\227\240\346\263\225\350\256\276\345\256\232\347\273\210\347\253\257\350\277\233\347\250\213\347\273"..., 16384) = 74
read(8, "bash: \346\255\244 shell \344\270\255\346\227\240\344\273\273\345\212\241\346\216\247\345"..., 16310) = 35
read(8, "setterm: \347\273\210\347\253\257 xterm-256color \344"..., 16275) = 51
read(8, "Couldn't get a file descriptor r"..., 16224) = 56
read(8, "bash: [: \357\274\232\351\234\200\350\246\201\346\225\264\346\225\260\350\241\250\350\276\276\345\274"..., 16168) = 34
read(8, "Your display number is 0\n", 16134) = 25
read(8, "Test whether fcitx is running co"..., 16109) = 53
read(8, "Fcitx is running correctly.\n\n==="..., 16056) = 87
read(8, "Launch fbterm...\n", 15969)    = 17
read(8, "stdin isn't a tty!\n", 15952)  = 19
read(8, "__RESULT\0/home/lujun9972/bin:/ho"..., 15933) = 298
read(8, 0x7ffd1d39ce9d, 15635)          = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=10949, si_uid=1000, si_status=0, si_utime=10, si_stime=7} ---
rt_sigreturn({mask=[]})                 = -1 EINTR (被中断的系统调用)
read(8, "", 15635)                      = 0
wait4(10949, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 10949
close(8)                                = 0
getpid()                                = 10818
faccessat(AT_FDCWD, "/home/lujun9972/bin/printf", X_OK) = -1 ENOENT (没有那个文件或目录)
faccessat(AT_FDCWD, "/usr/local/sbin/printf", X_OK) = -1 ENOENT (没有那个文件或目录)
faccessat(AT_FDCWD, "/usr/local/bin/printf", X_OK) = -1 ENOENT (没有那个文件或目录)
faccessat(AT_FDCWD, "/usr/bin/printf", X_OK) = 0
stat("/usr/bin/printf", {st_mode=S_IFREG|0755, st_size=51176, ...}) = 0
openat(AT_FDCWD, "/dev/null", O_RDONLY|O_CLOEXEC) = 7
faccessat(AT_FDCWD, "/proc/5070/fd/.", F_OK) = 0
faccessat(AT_FDCWD, "/proc/5070/fd/.", F_OK) = 0
faccessat(AT_FDCWD, "/bin/bash", X_OK)  = 0
stat("/bin/bash", {st_mode=S_IFREG|0755, st_size=903440, ...}) = 0
pipe2([8, 9], O_CLOEXEC)                = 0
rt_sigprocmask(SIG_BLOCK, [INT CHLD], [], 8) = 0
vfork()                                 = 11679
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
close(9)                                = 0
close(7)                                = 0
read(8, "setterm: \347\273\210\347\253\257 xterm-256color \344"..., 16384) = 51
read(8, "Couldn't get a file descriptor r"..., 16333) = 56
read(8, "/home/lujun9972/.bash_profile: \347"..., 16277) = 72
read(8, "Your display number is 0\nTest wh"..., 16205) = 78
read(8, "Fcitx is running correctly.\n\n==="..., 16127) = 104
read(8, "stdin isn't a tty!\n", 16023)  = 19
read(8, "__RESULT\0b269cd09e7ec4e8a115188c"..., 16004) = 298
read(8, 0x7ffd1d39cba6, 15706)          = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=11679, si_uid=1000, si_status=0, si_utime=1, si_stime=1} ---
rt_sigreturn({mask=[]})                 = -1 EINTR (被中断的系统调用)
read(8, 

很容易就可以看出，当 Emacs 卡顿时，它在尝试从 8 号文件句柄中读取内容。

那么 8 号文件句柄在哪里定义的呢？往前看可以看到：

pipe2([8, 9], O_CLOEXEC)                = 0
rt_sigprocmask(SIG_BLOCK, [INT CHLD], [], 8) = 0
vfork()                                 = 11679
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
close(9)                                = 0

可以推测出，Emacs 主进程 fork 出一个子进程（进程号为 11679），并通过管道读取子进程的内容。

然而，从

--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=11679, si_uid=1000, si_status=0, si_utime=1, si_stime=1} ---
rt_sigreturn({mask=[]})                 = -1 EINTR (被中断的系统调用)
read(8, 

可以看出，实际上子进程已经退出了（父进程收到 SIGCHLD 信号），父进程确依然在尝试从管道中读取内容，导致的阻塞。

而且从

read(8, "setterm: \347\273\210\347\253\257 xterm-256color \344"..., 16384) = 51
read(8, "Couldn't get a file descriptor r"..., 16333) = 56
read(8, "/home/lujun9972/.bash_profile: \347"..., 16277) = 72
read(8, "Your display number is 0\nTest wh"..., 16205) = 78
read(8, "Fcitx is running correctly.\n\n==="..., 16127) = 104
read(8, "stdin isn't a tty!\n", 16023)  = 19
read(8, "__RESULT\0b269cd09e7ec4e8a115188c"..., 16004) = 298
read(8, 0x7ffd1d39cba6, 15706)          = ? ERESTARTSYS (To be restarted if SA_RESTART is set)

看到，子进程的输出似乎是我的交互式登录 bash 启动时的输出（加载了 .bash_profile）

在往前翻发现这么一段信息：

readlinkat(AT_FDCWD, "/home", 0x7ffd1d3abb50, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972", 0x7ffd1d3abf00, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972/.emacs.d", 0x7ffd1d3ac2b0, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972/.emacs.d/elpa", 0x7ffd1d3ac660, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972/.emacs.d/elpa/exec-path-from-shell-20180323.1904", 0x7ffd1d3aca10, 1024) = -1 EINVAL (无效的参数)
readlinkat(AT_FDCWD, "/home/lujun9972/.emacs.d/elpa/exec-path-from-shell-20180323.1904/exec-path-from-shell.elc", 0x7ffd1d3acdc0, 1024) = -1 EINVAL (无效的参数)
lseek(7, -2655, SEEK_CUR)               = 1441
read(7, "\n(defvar exec-path-from-shell-de"..., 4096) = 4096

这很明显是跟 exec-path-from-shell 有关啊。

通过查看 exec-path-from-shell 的实现，发现 exec-path-from-shell 的实现原理是通过实际调启一个 shell，然后输出 PATH 和 MANPATH 的值的。 而且对于 bash 来说，默认的启动参数为 -i -l（可以通过exec-path-from-shell-arguments来设置）。也就是说 bash 会作为交互式的登录shell来启动的，因此会加载 .bash_profile 和 .bashrc。

既然发现跟 exec-path-from-shell 这个包有关，而且据说这个包对 Linux 其实意义不大，那不如直接禁用掉好了。

dotspacemacs-excluded-packages '(exec-path-from-shell)

再次重启Emacs，发现这次启动速度明显快了许多了。