Allow shell expanded .conf values

[lrascao]

Allow for shell commmand substitution of .conf values,
only the `ShellCommand` form is allowed.

[lrascao]

This one should be merged after #13 and conflicts have been resolved

[lrascao]

64c7aca has the actual change, 3c8439a is needed to bring up the coverage percentage so coveralls green lights this

[lrascao]

@lukebakken this is all i have in the pipe for 2.5.0, proposed merge order is #12 , #13 , pause for rebase and then #14

[lukebakken]

My feeling is that executing arbitrary shell commands will not be appreciated by end-users (cc @michaelklishin)

@lrascao can you give a concrete example of the kinds of problems this change addresses?

[lukebakken]

I can hear the screams of the security review now.

[lrascao]

My feeling is that executing arbitrary shell commands will not be appreciated by end-users (cc @michaelklishin)

I hadn't really considered the security aspect of it, i guess mostly due to the fact that a malicious actor that is able to add arbitrary shell commands to the .conf file should also be able to do a lot of other things (eg. find the cookie of the node and connect remotely and os:cmd becomes available as well, alter the extended start script which is also just shell script), if this is an issue we can probably come up with some alternative to mitigate it

@lrascao can you give a concrete example of the kinds of problems this change addresses?

Sure, a specific one i'm currently working on is running the Erlang VM on Kubernetes. Without getting into too much detail, k8s offers two features (configMap and downwardAPI) that allow us to expose the resources that are being requested as files in the Erlang VM container. A typical scenario is tweaking the number of VM schedulers available based on the resource limits imposed (the adoptingerlang.org site section on k8s has more detail on this). So here's my vision on this:

• Using the configMap/downwardAPI features, you expose a file under etc/conf.d/erlang-vm-total-schedulers containing a single number which is the desired total number of schedulers. K8s doesn't allow any formatting of this data so the file really only contains a number
• A erlang-vm-k8s.conf file is generated in etc/conf.d that specifies the erlang.schedulers.total parameter as:

erlang.schedulers.total = `cat erlang-vm-total-schedulers`

• erlang-vm-k8s.conf is automatically picked up and used to generate the apropriate vm.args that will instruct the VM to only run as many schedulers

If the security restriction is a concern i suppose a viable alternative would be to make use the $() substitution feature to also support files, it's not as flexible as running arbitrary shell script though, thoughts?

[lukebakken]

If the security restriction is a concern i suppose a viable alternative would be to make use the $() substitution feature to also support files

Yes, let's do that. Please use the same syntax as what bash supports which is $(< path/to/file):

$ echo 'FILE CONTENTS' > testfile.txt

$ echo $(<testfile.txt)
FILE CONTENTS

[lrascao]

@lukebakken reworked as agreed

[lukebakken]

@lrascao if you have time to review the commits I added to handle some edge-cases, I would appreciate it!

[lrascao]

👍 changes look good, let's hope coveralls doesn't come back with more coverage demands

[lrascao]

@lukebakken that's the end of my pipeline, if there's nothing else that you intend do add could you please cut a 2.5.0 and push it to hex? thanks!

[lukebakken]

@lrascao I am blocked by this issue - erlef/rebar3_hex#194

[lukebakken]

@lrascao https://hex.pm/packages/cuttlefish/2.5.0