Pep8 environment to run on delta code only

Currently tox -epep8 will run flake8 on whole code.
To make this fast, flake8 support is added for only
updated(delta) code. Same can be run by "tox -efast8".

It also determines how many commits to check based on the
$FAST8_NUM_COMMITS env variable. If set to "smart",
it uses git to try to run against all unsubmitted commits.
This allows fast8 to be more useful when actively
developing a series of patches.

Closes-Bug: #1829455
Change-Id: Ic02d7a91c6f6b227abf14bc6f7cb85815591c0d3

tools/fast8.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+NUM_COMMITS=${FAST8_NUM_COMMITS:-1}
+
+if [[ $NUM_COMMITS = "smart" ]]; then
+    # Run on all commits not submitted yet
+    # (sort of -- only checks vs. "master" since this is easy)
+    NUM_COMMITS=$(git cherry master | wc -l)
+fi
+
+echo "Checking last $NUM_COMMITS commits."
+
+cd $(dirname "$0")/..
+CHANGED=$(git diff --name-only HEAD~${NUM_COMMITS} | tr '\n' ' ')
+
+# Skip files that don't exist
+# (have been git rm'd)
+CHECK=""
+for FILE in $CHANGED; do
+    if [ -f "$FILE" ]; then
+        CHECK="$CHECK $FILE"
+    fi
+done
+
+diff -u --from-file /dev/null $CHECK | flake8 --diff

tox.ini

@@ -39,6 +39,13 @@ commands =
   # Run security linter
   bandit -r keystone -x tests
 
+[testenv:fast8]
+basepython = python3
+envdir = {toxworkdir}/pep8
+commands =
+  {toxinidir}/tools/fast8.sh
+passenv = FAST8_NUM_COMMITS
+
 [testenv:bandit]
 basepython = python3
 # NOTE(browne): This is required for the integration test job of the bandit