Add support for Heroku-20

This adds support for generating binaries for the upcoming Heroku-20
stack, and testing them in CI.

The Heroku-20 Dockerfile is based on that for Heroku-18, but with:
* the unused `python-{pip-whl,setuptools,wheel}` dependencies removed
* `python-pip` replaced with `python3-pip` since the former does not
  exist on Ubuntu 20.04.

The switch to Python 3 pip in the builder image only affects bob-builder,
since everything else (such as pip-diff) runs against the Python binaries
downloaded as part of the compile.

bob-builder has been updated to v0.0.18 to pick up the Python 3 fixes in:
heroku-python/bob-builder#48

Before this is merged, the binaries will need to be built/uploaded using
the steps here:
https://github.com/heroku/heroku-buildpack-python/blob/master/builds/README.md

A decision will need to be made about whether to backfill older Python
versions, however for now we should at least provide the latest point
release of each supported Python branch:
https://devguide.python.org/#status-of-python-branches

Which are:
* `3.5.9`
* `3.6.10`
* `3.7.7`
* `3.8.2`

We'll also want to upload at least one version of PyPy, however the
current script is for the out of date `pypy3.6-7.2.0`, so we'll want to
generate newer binaries for all stacks, which is out of scope for now.

Refs W-7485877.

.travis.yml

@@ -17,6 +17,8 @@ script:
   - docker run --rm -e "STACK=heroku-16" travis-build-heroku-16 bash $TESTFOLDER
   - docker build --pull --tag travis-build-heroku-18 --file $(pwd)/builds/heroku-18.Dockerfile .
   - docker run --rm -e "STACK=heroku-18" travis-build-heroku-18 bash $TESTFOLDER
+  - docker build --pull --tag travis-build-heroku-20 --file $(pwd)/builds/heroku-20.Dockerfile .
+  - docker run --rm -e "STACK=heroku-20" travis-build-heroku-20 bash $TESTFOLDER
 
 jobs:
   include:

Makefile

@@ -1,7 +1,7 @@
 # These targets are not files
 .PHONY: tests
 
-test: test-heroku-18 test-heroku-16 test-cedar-14
+test: test-heroku-20 test-heroku-18 test-heroku-16 test-cedar-14
 
 check:
 	@shellcheck -x bin/compile bin/detect bin/release bin/test-compile bin/utils bin/warnings bin/default_pythons
@@ -23,6 +23,11 @@ test-heroku-18:
 	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=heroku-18" heroku/heroku:18-build bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run-deps; test/run-features; test/run-versions;'
 	@echo ""
 
+test-heroku-20:
+	@echo "Running tests in docker (heroku-20)..."
+	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=heroku-20" heroku/heroku:20-build bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run-deps; test/run-features; test/run-versions;'
+	@echo ""
+
 buildenv-heroku-16:
 	@echo "Creating build environment (heroku-16)..."
 	@echo
@@ -49,6 +54,19 @@ buildenv-heroku-18:
 	@echo
 	@docker run -it --rm python-buildenv-heroku-18
 
+buildenv-heroku-20:
+	@echo "Creating build environment (heroku-20)..."
+	@echo
+	@docker build --pull -f $(shell pwd)/builds/heroku-20.Dockerfile -t python-buildenv-heroku-20 .
+	@echo
+	@echo "Usage..."
+	@echo
+	@echo "  $$ export AWS_ACCESS_KEY_ID=foo AWS_SECRET_ACCESS_KEY=bar  # Optional unless deploying"
+	@echo "  $$ bob build runtimes/python-2.7.13"
+	@echo "  $$ bob deploy runtimes/python-2.7.13"
+	@echo
+	@docker run -it --rm python-buildenv-heroku-20
+
 tools:
 	git clone https://github.com/kennethreitz/pip-pop.git
 	mv pip-pop/bin/* vendor/pip-pop/

README.md

@@ -81,6 +81,7 @@ make test
 Or to test in a particular stack:
 
 ```
+make test-heroku-20
 make test-heroku-18
 make test-heroku-16
 ```

builds/README.md

@@ -7,6 +7,7 @@
     $ docker build --pull --tag heroku-python-build-cedar-14 --file $(pwd)/builds/cedar-14.Dockerfile .
     $ docker build --pull --tag heroku-python-build-heroku-16 --file $(pwd)/builds/heroku-16.Dockerfile .
     $ docker build --pull --tag heroku-python-build-heroku-18 --file $(pwd)/builds/heroku-18.Dockerfile .
+    $ docker build --pull --tag heroku-python-build-heroku-20 --file $(pwd)/builds/heroku-20.Dockerfile .
 
 ## Using the Image
 
@@ -15,6 +16,7 @@ You can e.g. `bash` into each of the images you built using their tag:
     docker run --rm -ti heroku-python-build-cedar-14 bash
     docker run --rm -ti heroku-python-build-heroku-16 bash
     docker run --rm -ti heroku-python-build-heroku-18 bash
+    docker run --rm -ti heroku-python-build-heroku-20 bash
 
 You then have a shell where you can run `bob build`, `bob deploy`, and so forth. You can of course also invoke these programs directly with `docker run`:
 

builds/heroku-20.Dockerfile

@@ -0,0 +1,19 @@
+FROM heroku/heroku:20-build
+
+WORKDIR /app
+ENV WORKSPACE_DIR="/app/builds" \
+    S3_BUCKET="lang-python" \
+    S3_PREFIX="heroku-20/" \
+    DEBIAN_FRONTEND=noninteractive \
+    STACK="heroku-20"
+
+RUN apt-get update \
+    && apt-get install --no-install-recommends -y \
+        libsqlite3-dev \
+        python3-pip \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt /app/
+RUN pip3 install --disable-pip-version-check --no-cache-dir -r /app/requirements.txt
+
+COPY . /app

requirements.txt

@@ -1,3 +1,3 @@
 docopt==0.6.2
-bob-builder==0.0.17
-boto==2.48.0
+bob-builder==0.0.18
+boto==2.49.0

tests.sh

@@ -1,21 +1,8 @@
 #!/usr/bin/env bash
 
-if [[ ! "$STACK" ]]; then
-    echo '$STACK must be set! (heroku-16 | cedar-14)'
+if [[ -z "$STACK" ]]; then
+    echo '$STACK must be set! (eg: `STACK=heroku-18 ./tests.sh`)'
     exit 1
 fi
 
-if [[ "$STACK" == "cedar-14" ]]; then
-    make test-cedar-14
-    exit $?
-fi
-
-if [[ "$STACK" == "heroku-16" ]]; then
-    make test-heroku-16
-    exit $?
-fi
-
-if [[ "$STACK" == "heroku-18" ]]; then
-    make test-heroku-18
-    exit $?
-fi
+make "test-${STACK}"