This commit forward ports the changes made for 0.6.17, in order to store
the type and sensitive flag against outputs.
It also refactors the logic of the import for V0 to V1 state, and
fixes up the call sites of the new format for outputs in V2 state.
Finally we fix up tests which did not previously set a state version
where one is required.
The retryablehttp package implements basic retries w/ exponential
backoff, which helps the remote state push recover in cases of
connectivity blips or transient errors.
Atlas returns an HTTP 409 - Conflict if the pushed state reports the same
Serial number but the checksum of the raw content differs. This can
sometimes happen when Terraform changes state representation internally
between versions in a way that's semantically neutral but affects the JSON
output and therefore the checksum.
Here we detect and handle this situation by ticking the serial and retrying
iff for the previous state and the proposed state:
* the serials match
* the parsed states are Equal (semantically equivalent)
In other words, in this situation Terraform can override Atlas's detected
conflict by asserting that the state it is pushing is indeed correct.
