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0081-process-specific-working-directory.md

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  • Name: Process Specific Working Directory
  • Start Date: 2021-03-09
  • Author(s): ForestEckhardt, ryanmoran, fg-j
  • Status: Implemented
  • RFC Pull Request: rfcs#144
  • CNB Pull Request: (leave blank)
  • CNB Issue: buildpacks/spec#212, buildpacks/spec#216
  • Supersedes: N/A

Summary

For any given process, the buildpack should be able to specify a working directory that is not CNB_APP_DIR.

Motivation

Allowing a buildpack to specify a custom working directory would allow buildpacks to write start commands that need to be invoked from a specfic directory without needing to invoke a shell.

What it is

It is a common pattern in buildpacks to execute the process from a directory that is not CNB_APP_DIR. In these cases, buildpack authors are required to write a process command that includes a change into the directory that the process should run from. This often looks like cd $CNB_APP_DIR/sub-directory && ./execute-process. Unfortunately, this means that the command can only be run on a stack that includes a shell. It also means that including additional flags or arguments to the command at runtime is cumbersome.

Here are some examples of where buildpacks have had to work around this issue:

As an alternative, the buildpack lifecycle could allow buildpacks to specify the working directory of the process when it is invoked.

How it Works

A new field working-directory would be added the the processes in launch.toml.

[[processes]]
type = "<process type>"
command = "<command>"
args = ["<arguments>"]
direct = false
working-directory = "<working directory>"

If working-directory is not set the process will be invoked in CNB_APP_DIR as normal. However, if given a relative path such as src/app the process should be invoked from $CNB_APP_DIR/src/app. If given an absolute path such as /mounted/bin, the process should invoke from /mounted/bin.

The working-directory specified for the process should not affect the working directory for either profile.d or exec.d scripts.

The process specific working-directory should not affect the OCI image metadata WorkingDir field.

Prior Art

  • Go's exec.Cmd accepts a directory which that command will be executed in.

Spec. Changes (OPTIONAL)

Add a new field working-directory field to processes inlaunch.toml