[Solved] Using TProcess to execute and interact with pacman package
manager
lazarus / 32-Linux

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SuperSathanas · 2024-05-20 15:38

I think that I have a couple different problems/things I lack
understanding in that are causing me some issues here, so forgive me
if I'm not being very clear or leaving out relevant information. I
don't know what I don't know at this point. I'll be happy to provide
more information if needed.

Some background on what I'm doing: I have a bash script (appropriately
named backupdate) that queries pacman for updateable packages, and if
there are any, updates and ranks pacman's mirrors, creates a system
snapshot using timeshift, performs pacman updates, then executes a
separate program I wrote in FPC that gets rid of excessive timeshift
snapshots, before optionally rebooting. This works fine, but I wanted
to write the entire thing in FPC.

I'm using TProcess to query pacman, run reflector to update and rank
mirrors and call my other program, capture and act on their outputs,
and that's all working fine. Where I'm running into an issue is with
actually executing pacman and doing the updates. I need to call Code:
Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i pacman -Syu
and then automatically have it respond to prompts and continue to
capture the output to display to the user (me).

I've managed to have it automatically respond to the prompt asking the
user to proceed (:: Proceed with installation? [Y/n]), by scanning
each line of the process's output, looking for a "y/n" or a variant
thereof and writing a "y" to the input stream. However, after
responding to the prompt and receiving the next line of output from
pacman, ":: Retrieving packages...", nothing seems to happen. The
TProcess instance keeps running, but I no longer receive any output
from pacman, pacman seems to not move on (if I interrupt the process,
no updates have been installed including no partial updates), and the
whole program is just stuck waiting on that TProcess.

However, if I don't write to the input stream, and just manually enter
"y" when prompted, everything works and pacman does it's thing.

Essentially what I'm trying to do is the equivalent of Code: Pascal
 [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i yes |
pacman -Syu in the terminal, just "yessing" through prompts.

Currently, this is what the procedure that handles updates looks like.
It's not optimal, but I'm not worried about cleaning it up and doing
things more efficiently until I get my current problem solved.

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
DoUpdates();varproc: TProcess;OList: TStringList;CheckString:
String;I: Integer;  begin    OList := TStringList.Create();     proc
:= TProcess.Create(nil);    proc.Options := proc.Options +
[poUsePipes, poWaitOnExit];    proc.CommandLine := 'pacman -Syu';   
proc.Execute();     while proc.Running do begin     
OList.LoadFromStream(proc.OutPut);       if OList.Count > 0 then
begin        for I = 0 to OList.Count - 1 do begin         
WriteLn(OList[I]);          CheckString := LowerCase(OList[I]);      
   if Pos('y/n', CheckString) <> 0 then begin            CheckString
:= 'y' + #10;            proc.Input.Write(CheckString[0],
Length(CheckString));            proc.CloseInput();          end;    
   end;      end;     end;     proc.Free();    OList.Free();   end; 
Not super sophisticated, but we're not too concerned with speed at
this point.

Any idea what it is about writing that "y" + #10 to the input stream
is causing the whole thing to hang? It's weird to me that I'm able to
respond to the prompt, get the next line of output from TProcess, but
then it stalls, whereas if I do not handle the prompt
programmatically, it all works.

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Roland57 · 2024-05-20 17:59

Hello!

I suggest you to take a look at this demo:
https://sourceforge.net/p/lazarus-ccr/svn/HEAD/tree/examples/process/

Unless I am wrong, it does what you are trying to do.

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SuperSathanas · 2024-05-20 20:00

That example does show a better way of waiting for and receiving
output than what I did (waiting until TProcess.NumBytesAvailable > 0
instead of me just trying to read the output into a string list and
then checking the length of list). My sloppiness there may be causing
the issue, so I'll try doing it the way shown in the example and see
if it still hangs.

I see that they do not call TProcess.CloseInput(), whereas I currently
am. I ended up adding that in to see if that would stop it from
hanging, basically just throwing darts at the board when I was out of
ideas, but I guess I'll go ahead and remove that as well and see how
it goes with handling the output differently.

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SuperSathanas · 2024-05-21 00:41

Well, it turns out it was as simple as checking for
TProcess.Output.NumBytesAvailable <> 0 instead of just trying to read
the output into the string list each iteration. I guess maybe next
time I won't be so sloppy, intending to fix things up later.