fix(local invoke): ensure later Lambda layers overwrite earlier layers in Docker image#8688
Merged
roger-zhangg merged 3 commits intodevelopfrom Feb 24, 2026
Merged
fix(local invoke): ensure later Lambda layers overwrite earlier layers in Docker image#8688roger-zhangg merged 3 commits intodevelopfrom
roger-zhangg merged 3 commits intodevelopfrom
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Docker's ADD/COPY with directories may merge rather than overwrite files in BuildKit, breaking AWS Lambda's layer precedence where later layers should override earlier ones. Fix by staging each layer in a temp directory and using 'cp -rf' to explicitly overwrite files in /opt, matching real Lambda behavior. Also add cleanup_samcli_images() call to tearDownClass for proper cleanup between parameterized test classes.
valerena
previously approved these changes
Feb 23, 2026
| docker_patch.from_env.return_value = docker_client_mock | ||
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| expected_docker_file = "FROM python\nADD aws-lambda-rie-arm64 /var/rapid/\nRUN mv /var/rapid/aws-lambda-rie-arm64 /var/rapid/aws-lambda-rie && chmod +x /var/rapid/aws-lambda-rie\nADD layer1 /opt\n" | ||
| expected_docker_file = "FROM python\nADD aws-lambda-rie-arm64 /var/rapid/\nRUN mv /var/rapid/aws-lambda-rie-arm64 /var/rapid/aws-lambda-rie && chmod +x /var/rapid/aws-lambda-rie\nADD layer1 /tmp/layer0\nRUN cp -rf /tmp/layer0/. /opt/ && rm -rf /tmp/layer0\n" |
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Do we have something in the integration tests that checks that 2 layers are built correctly? (is it the test that you mention in the description?).
It's interesting that we don't have a unit test that checks the behavior with 2 or more layers, but if we have an integration test that checks that, then it should be good enough.
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This is a good point, it seems we were checking the layer override (this test). But not really 2 layer working together. I can take a look
reedham-aws
previously approved these changes
Feb 23, 2026
Docker's ADD with directories may not overwrite existing files in all build backends (e.g. BuildKit). This breaks AWS Lambda's layer precedence where later layers should override earlier ones. Fix: the first layer still uses ADD directly to /opt. Subsequent layers are staged in a temp directory and copied with 'cp -rf' to guarantee overwrite semantics. Also adds test_two_independent_layers integration test that verifies two layers providing different modules both work in a single function, and adds cleanup_samcli_images() to tearDownClass.
6dd935a
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Added an addtional test to import 2 layers and test function from both Also for the first layer, still use Add directly for better performance. |
Member
Author
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Seems new integ test is failing, fixing |
The nested template parents were not forwarding the new layer parameters, causing SAM CLI to use the default fake ARNs and fail with lambda:GetLayerVersion permission errors.
valerena
approved these changes
Feb 24, 2026
reedham-aws
approved these changes
Feb 24, 2026
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Problem
When a function has multiple layers with overlapping files (e.g. both layer1 and layer2 provide `simple_python_module/simple_python.py`), the second layer should win per AWS Lambda semantics. However, Docker's `ADD layer2 /opt` after `ADD layer1 /opt` does not reliably overwrite files in all build backends (BuildKit vs legacy builder).
This caused `test_download_two_layers` to fail with `"Layer1" != "Layer2"`.
Fix
Stage each layer in a unique temp directory (`/tmp/layer0`, `/tmp/layer1`, etc.) and use `RUN cp -rf /tmp/layerN/. /opt/ && rm -rf /tmp/layerN` to explicitly overwrite files, matching real Lambda behavior.
Also adds `cleanup_samcli_images()` call to `tearDownClass` for proper image cleanup between parameterized test classes.
Changes
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.