Do not scan through all PIDs to find parent PID command

[Fil0sOFF]

There's one command that takes a long time after profiling bash startup.
We currently scan though all processes just to find out $PPID command.
The time to do it grows linearly with a number of processes running.
Let's fix that.

Compare:

strace -e openat ps -o comm= -p $PPID

vs

strace -e openat ps -o comm= -q $PPID

Technical checklist:

• code follows our shell standards:
  • correct use of IFS
  • careful quoting
  • cautious array access
  • portable array indexing with _LP_FIRST_INDEX
  • printing a "%" character is done with _LP_PERCENT
  • functions/variable naming conventions
  • functions have local variables
  • data functions have optimization guards (early exits)
  • subshells are avoided as much as possible
  • string substitutions may be done differently in Bash and Zsh (use _LP_SHELL_*)
• tests and checks have been added, ran, and their warnings fixed:
  • unit tests have been updated (see tests/test_*.sh files)
  • ran tests.sh
  • ran shellcheck.sh (requires shellcheck).
• documentation have been updated accordingly:
  • functions and attributes are documented in alphabetical order
  • new sections are documented in the default theme
  • tag .. versionadded:: X.Y or .. versionchanged:: Y.Z
  • functions signatures have arguments, returned code, and set value(s)
  • attributes have types and defaults
  • ran docs/docs-lint.sh (requires Python 3 and requirements.txt
    installed (cd docs/; python3 -m venv venv; . venv/bin/activate; pip install -r requirements.txt))

[Rycieos]

ps does not support the -q flag on MacOS. This can't be merged as is.

Can you post some timing info (time ps -o comm= -p "$PPID") from the situations that you think this is slow?

If this is really a problem, we could try to have two different paths for those that support the -q flag vs those that don't.

[Fil0sOFF]

Oops, -q seems to be available on only on linux, since 2014

[Rycieos]

I'm probably missing something, but why was a separate option added? If -q only is allowed when no other filters are specified, why not just use the optimized search when -p is specified but nothing else is.

[Fil0sOFF]

I'm not sure why ps works that way. I only found about -q from a manpage, which i opened after strace ps -o comm= -p $PPID showed a long list of /proc/xx files.

-p is not that slow most of the time, but it can be in some cases.

$ time ps -o comm= -p "$PPID"
wezterm-gui

real    0m0.054s
user    0m0.026s
sys     0m0.031s


$ time ps -o comm= -q "$PPID"
wezterm-gui

real    0m0.012s
user    0m0.012s
sys     0m0.002s

after running 1000 sleeps:

$ for i in `seq 1000`; do sleep 30 & done

$ time ps -o comm= -p "$PPID"
wezterm-gui

real    0m0.104s
user    0m0.058s
sys     0m0.048s

$ time ps -o comm= -q "$PPID"
wezterm-gui

real    0m0.012s
user    0m0.011s
sys     0m0.003s

If different code paths for different systems are possible, then var=$(<file) might be the fastest option for linux. No forking, no external command execution. Bash- and zsh-compatible

[Rycieos]

If different code paths for different systems are possible, then var=$(<file) might be the fastest option for linux. No forking, no external command execution. Bash- and zsh-compatible

What file are you proposing to read? /proc/$PPID/comm? That should work, and I agree, it would be faster.

If you want to try to implement it, here are a few tips:

• Read with a command like IFS="" read -r sess_parent <"$_LP_LINUX_COMM_FILE". Make sure to set local sess_parent first.
• Use the var for the file path to allow for mocking it in tests. See

  liquidprompt/liquidprompt

  Lines 608 to 609 in 046d830

  	# For mocking tests.
  	_LP_LINUX_POWERSUPPLY_PATH="/sys/class/power_supply"

  for where those are defined.
• Likely split line 1644 into a function, like __lp_sess_parent() { sess_parent="$(ps -o comm= -p "$PPID" 2> /dev/null)"; }. Then you can put the above in a different function.
• Don't define both functions, define one based on the OS at load time. See

  liquidprompt/liquidprompt

  Lines 3589 to 3591 in 046d830

  	case "$LP_OS" in
  	Linux)
  	__lp_wifi_signal_strength_raw() {

  for an example. Set the local before you call that function.
• Make sure to add reading from this file to the external tool tester, even though it likely won't be very useful.