Allow running terraform plan without persisting refreshed state

[toofishes]

In the case of running with a remote state file that one only has read-only
permissions on, terraform plan currently fails as it can't rewrite the remote
state file. Running with -refresh=false isn't a great option either, as then
the state we are planning from is not guaranteed to be current.

Add a new -persist flag that defaults to true to match existing behavior. If
this is set to false, we will continue to refresh remote state, but won't try
to write back our changes, allowing a read-only state file.

This addresses #2548.

[toofishes]

This is my first shot at a contribution to terraform. Please let me know if there are docs I missed, tests I should be writing, etc.!

Looking now, I'm guessing that to get this merged, I should update website/source/docs/commands/plan.html.markdown with the new flag, and possibly add some sort of test case to command/plan_test.go?

[apparentlymart]

Thanks for writing this, @toofishes. It is an interesting idea.

One concern I have with this change is that I think it is in conflict with the change I'm trying to make in #4616, where a generated plan refers to a particular version of the state and Terraform will refuse to apply a plan that refers to an older state. If we allowed a plan to apply to a non-persisted refresh result, then on the subsequent apply the serial recorded in the current state wouldn't match.

Personally I think the mechanism in #4616 is important to avoid bad consequences of a real-world mistake I've made and seen people make several times. I wonder if we can adjust your proposed feature somehow so that it can coexist with the stale plan protection mechanism... but nothing came to my mind after some initial pondering. 😖

[radeksimko]

Not persisting state on terraform plan is now the default behaviour.

See #6811

Is there still any value in turning off this behaviour - i.e. making plan behave exactly as it behaves in <= 0.6.16? 💭

[jen20]

This feature was merged as #6811 after some reworking - thanks for opening a PR @toofishes! This behaviour is now standard in 0.7rc1

[toofishes]

Thanks!

[ghost]

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