Feature request: Allow makefile BuildMethod to use Amazon Linux 2 based build image about aws-sam-cli

[lucashuy]

Thanks for raising this feature request. Was there a particular way you were building the layer? Defining a layer as such:

HelloWorldLayer:
  Type: AWS::Serverless::LayerVersion
  Properties:
    ContentUri: ./layer
    CompatibleRuntimes:
      - provided.al2
    CompatibleArchitectures:
      - x86_64
  Metadata:
    BuildMethod: makefile

With a random requirements.txt and a Makefile in the ./layer folder:

# ./layer/Makefile
build-HelloWorldLayer:
	# my system does not have yum, AL2 provided does
	yum --version

	mkdir -p "$(ARTIFACTS_DIR)/python"
	python3 -m pip install -r requirements.txt -t "$(ARTIFACTS_DIR)/python"

Running sam build -u HelloWorldLayer seems to pull the provided AL2 build image and installed requirements successfully.

[rikbruil]

@lucashuy Thanks for the info, I compared your snippet to our templates and everything was identical except for the CompatibleRuntime setting which was set to python3.8 in our case.

I ran the SAM build command just for the layer resource as you did, and now it turns out the correct image gets used when building (whether I specify provided.al2 or python3.8, in both cases I get an AL2 based image). I took another look at our template and noticed there was also one lambda function in there with Runtime: provided.

It seems that SAM CLI prefers the provided runtime over our explicitly specified python3.8 compatible runtime for the layer when we build the entire stack via sam build. I'm not sure if that's correct behaviour, because the provided runtime appears to be the only one using a different AmazonLinux version compared to the rest of the runtimes. To me it seems to be the 'least' compatible, while SAM CLI output says:

For container layer build, first compatible runtime is chosen as build target for container.

Do you recommend I open a bug around this behaviour, or does first compatible runtime mean any existing runtime image in this context?