#21005 Use of FaaS `runtime=...` when there's no built-in complete platform silently falls back to `--platform=...` and thus often has confusing broken builds Issue created by huonw Describe the bug The FaaS targets like

python_aws_lambda_function

,

python_aws_lambda_layer

have some built-in complete platform JSON files, computed from the Docker images AWS publishes. These are used based on

runtime="python3.X"

(or the inferred runtime from interpreter constraints) automatically, e.g. if someone writes

runtime="python3.10"

or has

interpreter_constraints = ["==3.10.*"]

, we'll pass

--complete-platforms ...

with this file https://github.com/pantsbuild/pants/blob/cc5577ed867400cb003da75aa1a9d44812ecf940/src/python/pants/backend/awslambda/python/complete_platform_3.10-x86_64.json to the PEX build, to do a more accurate cross-build. However, someone is using an (implied) runtime that Pants doesn't have support for (e.g.

runtime="python3.12"

currently, before #21004 lands), then we silently fall back to computing an equivalent

--platform

argument. This leads to spurious errors, because

--platform

is an coarse summary: pants/src/python/pants/backend/python/util_rules/faas.py Lines 400 to 403 in </pantsbuild/pants/commit/2e5c50511f809aa3c007bb54924277c3b0348f64|2e5c505> This is conceptually the same problem as #19978, but requires a separate solution. We should deprecate and remove this behaviour, as it's too error prone. Steps to make this nicer to for users: 1. add more complete platforms for GCF (currently has none #18195 ) 2. expand the

runtime

fields's documentation to reference the supported ones 3. make sure the error message is specific about what to do (either use

complete_platforms

or run within a docker image) Pants version 2.21.0 OS macOS Additional info Example that's likely to be caused by this: https://chat.pantsbuild.org/t/18879314/i-upgraded-the-python-interpreter-version-from-3-9-to-3-12-f#4340b966-a9dc-4202-9d16-bb30085d4cd6 pantsbuild/pants