Mode select: irreversible module actions

This design document describes how the Lmod mode select feature could be implemented in Tcl modulefiles.

Goal is to describe a solution that could be the same over the different module implementations. This way the Tcl modulefiles relying on such feature will be processed the same way whatever the module implementation.

Feature description

The mode select feature allows modulefiles to specify actions that should only be executed in specific modes (load or unload). This is particularly useful for operations that are irreversible or need special handling during module load/unload cycles.

When mode select is used, the command is executed as-is if current evaluation mode corresponds to one set. Which means a setenv with a mode select for unload evaluation, is evaluated as a setenv command when unloading modulefile.

Such feature does not cope with module dependency definition.

Proposed interface for Tcl modulefile

Proposition is to add a --mode option on concerned Tcl modulefile commands. This --mode option accepts a Tcl as value, for instance:

• --mode load

• --mode {load unload}

Concerned Tcl modulefile commands are:

• setenv

• unsetenv

• prepend-path

• append-path

• remove-path

• pushenv

• module load

• module load-any

• module try-load

• module unload

Support in Modules

Mode select is not supported at the moment in Modules. Support can be added if someone shows up and expresses a need.

The implementation of this feature for the module load command may be non-trivial: currently when unloading modules, only modulefile unloads are expected. Dependency resolution mechanism is expecting to have all the properties of the system when starting by looking at the loaded environment. Adding the ability to throw modulefile loads during this unload process requires to revise the current dependency resolution mechanism.

Due to that, implementation if asked may first be done only for modulefile Tcl commands others than module.