CI: mock the pipenv binary

The mock gets used by the flake8 pre-commit hook, which invokes flake8
via `pipenv run flake8`. It's normally useful to use pipenv here cause
it ensures flake8 is invoked within the context of the venv. However, in
CI, there is no venv - dependencies are installed directly to the
system site-packages. `pipenv run` does not work in such case because it
tries to create a new venv if one doesn't exist (it doesn't consider the
system interpreter to be a venv).

This workaround (okay, it's a hack) creates an executable shell script
which replaces the original pipenv binary. The shell script simply
ignores the first argument (i.e. ignores `run` in `pipenv run`) and
executes the rest of the arguments as a command. It essentially makes
`pipenv run flake8` equivalent to just having ran `flake8`. When
pre-commit executes pipenv, the aforementioned script is what will run.

1 files changed, 10 insertions, 0 deletions

diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index fa85e6045..280f11a36 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -29,6 +29,16 @@ jobs:
       - script: pipenv install --dev --deploy --system
         displayName: 'Install project using pipenv'
 
+      # Create an executable shell script which replaces the original pipenv binary.
+      # The shell script ignores the first argument and executes the rest of the args as a command.
+      # It makes the `pipenv run flake8` command in the pre-commit hook work by circumventing
+      # pipenv entirely, which is too dumb to know it should use the system interpreter rather than
+      # creating a new venv.
+      - script: |
+          printf '%s\n%s' '#!/bin/bash' '"${@:2}"' > $(PythonVersion.pythonLocation)/bin/pipenv \
+          && chmod +x $(PythonVersion.pythonLocation)/bin/pipenv
+        displayName: 'Mock pipenv binary'
+
       - task: Cache@2
         displayName: 'Restore pre-commit environment'
         inputs: