Pavel resurrecting terminal emulation


Guille is working on the Iceberg improvements and he wanted to be able to open a terminal window on top of the repository and interact with it via the command line. So we looked at this issue because I already in December made some experiments with the terminal emulation in Pharo.

In past, the Squeak had a working terminal emulation that used PseudoTTYPlugin. The VM is not built with this code for a long time but I tried to replace it with a small C library and then wrote an FFI interface to it. Together with that, I ported most of the old code Squeak code to Pharo.

With Guille we tried to avoid usage of such external library and wrote an FFI interface to all the required LibC functions. We were successful but we realized that there are several issues that are limiting us.

When you want to execute a separate process for the program that you want to open in terminal (typically the Bash), you need to redirect the standard IO files, create a fork of your process, do some additional initialization in it and call ‘exec’ on it. In the parent process, you change redirected IO files back to the original values.

But the problem is that between the FFI calls from Smalltalk the VM can do a lot of things including garbage collection etc. On OS X the fork() function has the following limitation described in man:

“There are limits to what you can do in the child process. To be totally safe you should restrict your yourself to only executing async-signal safe operations until such time as one of the exec functions is called.  All APIs, including global data symbols, in any framework or library should be assumed to be unsafe after a fork() unless explicitly documented to be safe or async-signal safe.  If you need to use these frameworks in the child process, you must exec.  In this situation it is reasonable to exec your-self. yourself.”

As the result in most cases (but not all) the fork() and exec() pair from the Smalltalk side fails on OS X. Linux does not have this limitation however even there we found an issue. It is bound to the fact that fork() makes a fork of all the parent process that uses the same resources. As soon as Pharo is opened in a window and X11 is involved (the window wants to be repainted), it can lead to the VM crash.

So we learned that unfortunately we currently cannot use image-only FFI code for this task. We need a C library or VM plugin.

The repository of the terminal emulator is here:

and can be loaded using the following code:

Metacello new
baseline: ‘TerminalEmulator’;
repository: ‘github://pavel-krivanek/terminal/src’;

#TerminalEmulator asClass compileLibrary.

It compiles and links the small library with only one function using the GCC so the machine needs to have a proper development environment.
The terminal emulator is in very early stage and has a lot of issues like processes cleanup, drawing, keyboard input etc. etc. If you are interested in it, feel free to contribute.

— Pavel

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