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[vscode] Improve runInTerminal and support linux

Authored by wallace on Dec 28 2020, 12:00 PM.

Description

[vscode] Improve runInTerminal and support linux

Depends on D93874.

runInTerminal was using --wait-for, but it was some problems because it uses process polling looking for a single instance of the debuggee:

  • it gets to know of the target late, which renders breakpoints in the main function almost impossible
  • polling might fail if there are already other processes with the same name
  • polling might also fail on some linux machine, as it's implemented with the ps command, and the ps command's args and output are not standard everywhere

As a better way to implement this so that it works well on Darwin and Linux, I'm using now the following process:

  • lldb-vscode notices the runInTerminal, so it spawns lldb-vscode with a special flag --launch-target <target>. This flags tells lldb-vscode to wait to be attached and then it execs the target program. I'm using lldb-vscode itself to do this, because it makes finding the launcher program easier. Also no CMAKE INSTALL scripts are needed.
  • Besides this, the debugger creates a temporary FIFO file where the launcher program will write its pid to. That way the debugger will be sure of which program to attach.
  • Once attach happend, the debugger creates a second temporary file to notify the launcher program that it has been attached, so that it can then exec. I'm using this instead of using a signal or a similar mechanism because I don't want the launcher program to wait indefinitely to be attached in case the debugger crashed. That would pollute the process list with a lot of hanging processes. Instead, I'm setting a 20 seconds timeout (that's an overkill) and the launcher program seeks in intervals the second tepmorary file.

Some notes:

  • I preferred not to use sockets because it requires a lot of code and I only need a pid. It would also require a lot of code when windows support is implemented.
  • I didn't add Windows support, as I don't have a windows machine, but adding support for it should be easy, as the FIFO file can be implemented with a named pipe, which is standard on Windows and works pretty much the same way.

The existing test which didn't pass on Linux, now passes.

Differential Revision: https://reviews.llvm.org/D93951

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