Emit metrics for how the Python version was chosen

Currently an app's Python version can be set via a few different means:
- explicitly by the user (via `runtime.txt` or `Pipfile.lock`)
- implicitly via the sticky versions feature (for existing apps)
- implicitly via default version for new apps / those with empty cache

In order to determine the priority of features like automatic Python
patch version upgrades for sticky-versioned apps, it's useful to have
metrics for these.

There were previously no tests for either the sticky versions feature,
or changing the Python version by updating the `runtime.txt` file, so
I've added some now (given that I updated the conditional to add the
metrics, so useful to have coverage).

I've also removed the confusing overwrite of `DEFAULT_PYTHON_VERSION`
with the cached version, and kept them as two separate variables.

Closes @W-8099632@.
Closes @W-8099645@.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Emit metrics for how the Python version was chosen for an app (#1069).
 - Emit Python version metric events for all builds, not just clean installs (#1066).
 
 ## v178 (2020-09-07)

bin/compile

@@ -189,7 +189,7 @@ source "$BIN_DIR/steps/hooks/pre_compile"
 # continue to use that version of Python in perpituity (warnings will be raised if
 # they are out–of–date).
 if [ -f "$CACHE_DIR/.heroku/python-version" ]; then
-  DEFAULT_PYTHON_VERSION=$(cat "$CACHE_DIR/.heroku/python-version")
+  CACHED_PYTHON_VERSION=$(cat "$CACHE_DIR/.heroku/python-version")
 fi
 
 # We didn't always record the stack version. This code is in place because of that.
@@ -206,9 +206,14 @@ fi
 # shellcheck source=bin/steps/pipenv-python-version
 source "$BIN_DIR/steps/pipenv-python-version"
 
-# If no runtime was provided by the user, assume the default Python runtime version.
-if [ ! -f runtime.txt ]; then
-  echo "$DEFAULT_PYTHON_VERSION" > runtime.txt
+if [[ -f runtime.txt ]]; then
+  mcount "version.reason.python.specified"
+elif [[ -n "${CACHED_PYTHON_VERSION:-}" ]]; then
+  mcount "version.reason.python.cached"
+  echo "${CACHED_PYTHON_VERSION}" > runtime.txt
+else
+  mcount "version.reason.python.default"
+  echo "${DEFAULT_PYTHON_VERSION}" > runtime.txt
 fi
 
 # Create the directory for .profile.d, if it doesn't exist.

test/run-versions

@@ -214,6 +214,30 @@ testPypy2_7_warn() {
   fi
 }
 
+testStickyPythonVersion() {
+  local cache_dir="$(mktmpdir)"
+  compile "python3_6_warn" "$cache_dir"
+  assertCaptured "Installing python-3.6.7"
+  assertCapturedSuccess
+  compile "no-runtime-txt" "$cache_dir"
+  assertCaptured "Installing python-3.6.7"
+  assertCapturedSuccess
+  # Whilst this file seems like an implementation detail (so something that should
+  # not be tested), we must guarantee the filename remains consistent for backwards
+  # compatibility across buildpack versions for already-built apps.
+  assertFile "python-3.6.7" ".heroku/python-version"
+}
+
+testPythonVersionChange() {
+  local cache_dir="$(mktmpdir)"
+  compile "python3_6_warn" "$cache_dir"
+  assertCaptured "Installing python-3.6.7"
+  assertCapturedSuccess
+  compile "python3_6" "$cache_dir"
+  assertCaptured "Found python-3.6.7, removing"
+  assertCapturedSuccess
+}
+
 pushd $(dirname 0) >/dev/null
 popd >/dev/null