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。
原创文章，作者：奋斗，如若转载，请注明出处：https://blog.ytso.com/48470.html