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Design document for handling of environment variables.

The uenv CLI and Slurm plugin need to modify environment variables, to implement some key features that automatically configure the runtime environment according to flags and configuration files provided when launching a job or starting a uenv.

This document outlines a single design for how this should be achieved for all use cases and both the CLI and Slurm plugin use case.

Use Cases

Activating views

Activating a view sets environment variables like PATH, PKG_CONFIG_PATH, etc.

Configuration files

We want to support configruation files that specify a uenv, and how the environment should be configured, fo users.

This includes explicitly setting environment variables.

Constraints

The main constraint is how Slurm executes jobs. The Slurm daemon starts a single process on each node, then forks into a separate process for each MPI rank on the node. Each fork then runs the user-provided command using execve. At no point is a new shell started.

As a result, it is not possible to source a shell script in the environment, that might set environment variables. Instead, the only way to modify the environment is to use the spank_setenv function, which updates a list of environment variables to be passed to execve.

Environment modification:

  • create a new namespace and mount squashfs images.
  • set environment variables using spank_setenv, os.setenv, etc.

Design

The current design, a bash function wrapper that execs commands returned by the uenv-impl app, supports modifying the environment of the calling shell. This enables uenv view x to "load a view" in the running shell, by modifying environment variables. Unfortunately, this approach is fragile and complicated.

A more robust design would drop the ability to modify the calling shell's environment.

There would no longer be separate uenv-impl ane uenv-image commands - these would be merged into a single uenv executable that would be called directly on the CLI (instead of indirectly calling them via a bash function).

Modifications to the environment would only be performed via calls to uenv start and uenv run, each of which starts a new shell.

uenv start --view=develop icon
|
|_fork__
        |
        |
        setenv("PATH", ...)
        setenv("LD_LIBRARY_PATH", ...)
        |
        execve(squashfs-mount)
        |
        execve(bash)
        |
        bash

Features

  1. support for scalar and list variables.
  2. three different modes for setting values
    • set: scalar and list variabls
    • unset: scalar and list variabls
    • prepend: list variables
    • append: list variables

Data structure

  • var.type
    • scalar: a scalar value
    • list: a colon separated list, like PATH, LD_LIBRARY_PATH, etc.
  • var.operation is one of
    • prepend: prepend to a list
    • append: prepend to a list
    • set: set the value to equal
    • unset: each variable has three values
  • var.old
    • the value currently set in the environment, before modification
    • can be null
  • var.value
    • the value provided by uenv
  • var.new
    • the value set by uenv
    • this is apply(env.delta, env.old, env.operation)

class envVar: