Temporarily add role membership when managing permissions

[zytek]

Add GRANT $owner TO $connected_user when managing permissions.

Notice: we should probably remove this grant afterward to not leak permissions BUT only if it wasn't granted before we did it/checked for it. So, as far as I understand, provider should check if this membership is added, add it if not and later remove it.

Mentioned in #53
Related hashicorp/terraform#11452

when using this on RDS:

resource "postgresql_default_privileges" "priv-sequence-for-user" {
  database    = "${var.db}"
  owner       = "${var.owner}"
  role        = "${var.user}"
  schema      = "public"
  object_type = "sequence"
  privileges  = ["ALL"]
  depends_on  = ["postgresql_database.db"]
}

we need to first run `GRANT ${var.owner} to ${provider.username}"

[zytek]

If anyone has this in progress please mention me, if not I will try to submit PR this week

[cyrilgdn]

@zytek Thanks, for opening the issue.

Notice: we should probably remove this grant afterward to not leak permissions BUT only if it wasn't granted before we did it/checked for it. So, as far as I understand, provider should check if this membership is added, add it if not and later remove it.

True ! And if you give it a try, note that I already had to do something similar here: https://github.com/terraform-providers/terraform-provider-postgresql/blob/master/postgresql/resource_postgresql_database.go#L132-L142
So helpers already exist for that. (Unfortunately I added them after creating the PR for grant & default privileges :) )