Add conditional execution (similar to jsonpatch test)

[gberche-orange]

It is sometimes useful to conditionally execute bosh operators (in addition to the ? refer to hash keys that may or may not exist) depending on the presence of key/value in the manifest.

This issue suggests to implement a support similar to the json patch test syntax

{ "op": "test", "path": "/best_biscuit/name", "value": "Choco Leibniz" }
Tests that the specified value is set in the document. If the test fails, then the patch as a whole should not apply.

This would enable bosh operators to check their prereqs explicitly, and be ignored if preqs are not met.

/CC @voelzmo

Here is a related specific use-case (for a bosh-based on demand service broker similar to cppforlife/bosh-generic-sb-release): orange-cloudfoundry/cf-ops-automation-broker#30

[voelzmo]

@gberche-orange is this what you were asking for? https://github.com/cppforlife/go-patch/blob/master/patch/integration_test.go#L50-L74

seems like @cppforlife added this recently without telling anyone ;)

[gberche-orange]

yes this seems close to what I had in mind :-) thanks @voelzmo @cppforlife

It seems to support precondition and post conditions and fail if not met with a default error message:

Expects a specific value for a path. Fails otherwise with default error message.

- type: test
  path: /instance_groups/name=uaa/instances
  value: 0

Expects no value for a path. Fails otherwise with default error message

- type: test
  path: /instance_groups/name=uaa/instances-not
  absent: true

A possible extension would be something like the following to support skipping the current ops file (an any previously applied mutations) when a test fail, and proceed to other ops files specific to the bosh cli.

Expects a specific value for a path. Skip current ops file with a warning when unmet

- type: test
  path: /instance_groups/name=uaa/instances
  value: 0
  on_violations: skip_ops_file
  message: "my custom message: prereq X not met, skipping feature Y" 

Expects a specific value for a path. When unmet, fail the whole bosh int command and display a custom message

- type: test
  path: /instance_groups/name=uaa/instances
  value: 0
  on_violations: fail
  message: "my custom message: prereq X not met, please fix config"