Skip vendoring libffi/libmemcached on Heroku-16 (#465)

* Add a test for the cryptography (cffi) compile step

Since it's currently untested.

* Skip vendoring libffi/libmemcached on Heroku-16

Unlike for Cedar-14, Heroku-16 comes with these packages pre-installed:
https://github.com/heroku/stack-images/blob/5a341970cfc1f201014262ad64c3b3e47514f663/heroku-16/installed-packages.txt#L111
https://github.com/heroku/stack-images/blob/5a341970cfc1f201014262ad64c3b3e47514f663/heroku-16/installed-packages.txt#L172

As such the build scripts had already been made a no-op on Heroku-16:
https://github.com/heroku/heroku-buildpack-python/blob/fedae5ceda5a42f594012f911c3808dd5dc6fd9e/builds/libraries/vendor/libffi#L6-L9
https://github.com/heroku/heroku-buildpack-python/blob/fedae5ceda5a42f594012f911c3808dd5dc6fd9e/builds/libraries/vendor/libmemcache#L6-L9

...meaning the Heroku-16 archives for them on S3 contain zero files.

However until now, the buildpack was still unnecessarily downloading
and extracting these empty archives - and not just on the first compile
(like on cedar-14), but every compile since the directory check will
never succeed.

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Python Buildpack Changelog
 
+# Unreleased
+
+The libffi/libmemcached vendoring step is now skipped on Heroku-16 (since they are installed in the base image).
+
 # 115
 
 Revert a pull request.

bin/steps/cryptography

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-# This script serves as the Cryptography build step of the
+# This script serves as the cffi build step of the
 # [**Python Buildpack**](https://github.com/heroku/heroku-buildpack-python)
 # compiler.
 #
@@ -9,6 +9,11 @@
 #
 # This script is invoked by [`bin/compile`](/).
 
+if [[ "$STACK" == "heroku-16" ]]; then
+    # libffi is pre-installed in the stack image so there is no need to vendor it.
+    return 0
+fi
+
 # The location of the pre-compiled libffi binary.
 VENDORED_LIBFFI="https://lang-python.s3.amazonaws.com/$STACK/libraries/vendor/libffi.tar.gz"
 

bin/steps/pylibmc

@@ -9,6 +9,11 @@
 #
 # This script is invoked by [`bin/compile`](/).
 
+if [[ "$STACK" == "heroku-16" ]]; then
+    # libmemcached is pre-installed in the stack image so there is no need to vendor it.
+    return 0
+fi
+
 # The location of the pre-compiled libmemcached binary.
 VENDORED_MEMCACHED="https://lang-python.s3.amazonaws.com/$STACK/libraries/vendor/libmemcache.tar.gz"
 
@@ -16,7 +21,6 @@ VENDORED_MEMCACHED="https://lang-python.s3.amazonaws.com/$STACK/libraries/vendor
 # shellcheck source=bin/utils
 source "$BIN_DIR/utils"
 
-
 # If pylibmc exists within requirements, use vendored libmemcached.
 if (pip-grep -s requirements.txt pylibmc &> /dev/null) then
 

test/fixtures/cffi/requirements.txt

@@ -0,0 +1 @@
+cffi

test/run

@@ -47,6 +47,12 @@ testPsycopg2() {
   assertCapturedSuccess
 }
 
+testCffi() {
+  compile "cffi"
+  assertCaptured "cffi"
+  assertCapturedSuccess
+}
+
 testPylibmc() {
   compile "pylibmc"
   assertCaptured "pylibmc"