Packer plugin protocol version negotiation and automatic upgrade behavior

Answer in a nutshell

In a CI pipeline you should pin exact plugin versions for production or release‑grade builds and use floating constraints only in lightweight validation jobs that run frequently. To detect drift, periodically query the plugin’s source (GitHub releases or the HashiCorp registry) and compare the latest tag against the version recorded in .packer.d/plugins.json. Automate packer init -upgrade in a dedicated CI job that runs a packer validate and a minimal build; only merge the result if the negotiated protocol remains compatible with the pinned core version.

Confirmed facts

• Packer 1.7+ uses a gRPC protocol; if a plugin advertises a different major protocol version, the core rejects it with a clear error.
• The required_plugins block accepts ~> 1.2.3 (pessimistic) and ^1.2.3 (caret) constraints. At packer init the newest matching release is fetched and recorded in .packer.d/plugins.json.
• There is no automatic re‑resolution on core upgrade; you must run packer init -upgrade or delete the lock file.
• The packer plugins installed command lists the installed binaries and their protocol versions but does not show newer compatible releases.
• No formal core‑to‑plugin protocol compatibility matrix exists; the safest practice is to treat core and plugin upgrades as a coupled change.

Practical steps for a CI pipeline

1. Pin core and plugin versions for release builds:
   • In required_plugins use exact version strings, e.g., "vagrant" = "2.2.19".
   • Commit the .packer.d/plugins.json lock file and the ~/.packer.d/plugins directory (or a versioned cache) to source control.
2. Allow floating constraints in quick‑validation jobs:
   • Use ~> 1.2.0 or ^1.2.0 for plugins that are known to be stable and backward‑compatible.
   • Run packer validate after every commit; if packer init fails, investigate the protocol error.
3. Detect drift automatically:
   • Schedule a nightly job that queries the plugin’s release API (GitHub releases or the HashiCorp registry) for the latest tag.
   • Parse .packer.d/plugins.json to read the currently installed version.
   • If the latest tag differs from the installed version but the major protocol version matches, emit a warning or open a pull request with the updated constraint.
4. Automate safe upgrades:
   • In a separate branch, run packer init -upgrade to fetch the newest compatible plugin.
   • Execute packer validate and a minimal packer build (e.g., using a local builder or a dummy provider) to ensure the upgraded plugin still negotiates the protocol with the current core.
   • If the job succeeds, merge the updated lock file back into the main branch; otherwise, keep the previous pin.

One diagnostic detail that changes the recommendation

Do you pull plugins from the official HashiCorp registry or a custom registry? The availability of a formal registry.hcl file and the API used to fetch releases can affect how you programmatically detect newer compatible versions. Knowing the source will let you tailor the drift‑detection script to the correct API endpoint.

Diagram

Core	Protocol Negotiation	Plugin	CI Pipeline
Core 1.7+	Major version handshake	pinned / floating	validate / upgrade job