add securityContext.enabled deprecation doc to the design folder

Signed-off-by: Nicolas Fischer <nicolas@emberspark.io>

design/20191218-security-context-deprecation.md

@@ -0,0 +1,178 @@
+# Logic for deprecating securityContext.enabled
+
+The idea is to be able to define a `securityContext` from an arbitrary yaml block without being restricted to just the attributes currently supported.
+
+For example, by supporting a block like the following in the `values.yaml` file:
+
+```yaml
+securityContext:
+  fsGroup: 1111
+  runAsUser: 2222
+  runAsNonRoot: true
+```
+
+The logic for supporting the new `securityContext` in the `values.yaml` file should be backwards compatible with the `securityContext.enabled` parameter.
+
+## Pseudo-code logic
+
+No security context block should be defined if:
+
+```
+securityContext is empty
+or securityContext.enabled is defined and securityContext.enabled is false
+```
+
+Otherwise a securityContext block should be defined
+
+When a securityContext block has to be generated, we should fall back and support the deprecated structure if:
+
+```
+securityContext.enabled is true
+```
+
+Otherwise, we copy the `securityContext` block as it is to support the new format.
+
+## Test cases
+
+### Test case 1
+
+Defaults
+
+Input:
+
+No security context specified
+
+Output:
+
+Nothing generated
+
+### Test case 2
+
+Legacy parameters with `enabled` only and no additional parameters
+
+Input:
+
+```yaml
+securityContext:
+  enabled: true
+```
+
+Output:
+
+```yaml
+securityContext:
+  fsGroup: 1001
+  runAsUser: 1001
+```
+
+### Test case 3
+
+Legacy parameters with `enabled`, group and user
+
+Input:
+
+```yaml
+securityContext:
+  enabled: true
+  fsGroup: 1111
+  runAsUser: 2222
+```
+
+Output:
+
+```yaml
+securityContext:
+  fsGroup: 1111
+  runAsUser: 2222
+```
+
+### Test case 4:
+
+New default format
+
+Input:
+
+```yaml
+securityContext: {}
+```
+
+Output:
+
+```yaml
+```
+
+### Test case 5:
+
+New format with arbitrary block
+
+Input:
+
+```yaml
+securityContext:
+  fsGroup: 1111
+  runAsUser: 2222
+  runAsNonRoot: true
+```
+
+Output:
+
+```yaml
+securityContext:
+  fsGroup: 1111
+  runAsNonRoot: true
+  runAsUser: 2222
+```
+
+### Test case 6:
+
+Legacy parameters with `enabled` set to false and extra parameters that should be ignored
+
+Input:
+
+```yaml
+securityContext:
+  enabled: false
+  fsGroup: 1111
+  runAsUser: 2222
+```
+
+Output:
+
+```yaml
+```
+
+### Test case 7:
+
+Legacy parameters with `enabled` set to false
+
+Input:
+
+```yaml
+securityContext:
+  enabled: false
+```
+
+Output:
+
+```yaml
+```
+
+### Test case 8:
+
+Block with only fsGroup and runAsUser
+
+Input:
+
+```yaml
+securityContext:
+  fsGroup: 1111
+  runAsUser: 2222
+```
+
+Output:
+
+```yaml
+securityContext:
+  fsGroup: 1111
+  runAsUser: 2222
+```