From 139e3bca2f279a2e5c4a13d2fe73ca057a0490e2 Mon Sep 17 00:00:00 2001
From: Marc Herbert <marc.herbert@intel.com>
Date: Fri, 30 Oct 2020 04:33:12 +0000
Subject: [PATCH] travis: skip check patch when shallow cloning makes the PR
 "infinite"

When "back-merging" upstream Linux into our shallow target branch,
"git rev-list" can fail to reconnect the bottom of our shallow clone
with upstream Linux. In such a case the shallowness makes git rev-list
wrongly believe that the list of new commits submitted in the pull
request is the entire git history since the dawn of time. Try to detect
such backmerges and skip checkpatch when we find one.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>
---
 .travis.yml | 43 +++++++++++++++++++++++++++++++++++++------
 1 file changed, 37 insertions(+), 6 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index c4ee4ee27ac144..723a86df6f2b9f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -17,9 +17,8 @@ dist: focal
 #
 # - When it's too shallow it will save zero time because fetching pull
 #   requests with a base older than the shallow history will take as
-#   much time as cloning the entire repo... without even providing the
-#   entire repo information as checkpatch seems to be producing very
-#   confusing garbage in that case. See example and discussion in
+#   much time as cloning the entire repo.
+#   See example and discussion in
 #   https://github.com/thesofproject/linux/pull/2341
 #   (The workaround is of course to rebase the pull request)
 git:
@@ -51,7 +50,8 @@ jobs:
       # These two are combined because they both need a merge base and
       # locally unshallowing .git/ seems time consuming for some reason.
       # Without the merge base, the --xxxstats will either fail or be
-      # wrong and checkpatch prints confusing garbage.
+      # wrong and checkpatch will check every commit since the dawn of
+      # time.
       name: checkpatch and age of git base
       script:
         # Start with some visual and plain English context. A picture is
@@ -81,6 +81,37 @@ jobs:
         # Note $behind is NOT comparable to clone depth in repos with merges.
         - '[ "$behind" -lt 1000 ] # is the PR base too far behind?'
 
+        # The previous code looks at the target branch to show how far
+        # ahead it is from the pull request and how much the pull
+        # request misses from it. This may NOT be enough to address
+        # checkpatch issues with shallow cloning. Checkpatch tries to
+        # look at the _other_ side = the content of the PR itself; more
+        # specifically at:
+        #     git log ^"${TRAVIS_BRANCH}" "${TRAVIS_PULL_REQUEST_SHA}".
+        #
+        # From time to time "${TRAVIS_PULL_REQUEST_SHA}" is a merge
+        # commit, more specifically a "back-merge" of upstream code back
+        # into our branch. Such a back-merge "defeats" the previous git
+        # merge-base check above which leaves our ${TRAVIS_BRANCH}
+        # target branch shallow. Because of this shallowness, git log
+        # ^"${TRAVIS_BRANCH}" "${TRAVIS_PULL_REQUEST_SHA}" can generally
+        # _not_ connect the bottom of the shallow target branch to the
+        # pull request, so git tells checkpatch to check every single
+        # commit since the dawn of time! Let's try to detect such
+        # backmerges by looking at the all _parents_ of
+        # "${TRAVIS_PULL_REQUEST_SHA}": see if we fail to find a
+        # merge-base for some (upstream) parent because of our
+        # shallowness.
+        - skip_checkpatch=false;
+          (set -e; set -x;
+            for PR_parent in
+              $(git log -n 1 --pretty=%P ${TRAVIS_PULL_REQUEST_SHA}); do
+                 git merge-base $PR_parent ${TRAVIS_BRANCH};
+            done
+          ) || skip_checkpatch=true
+        # This check is not bullet-proof (imagine some commit on top of
+        # the backmerge) but it catches all common backmerges.
+
         # New Ubuntu 20.04 location
         - sudo mkdir -p /usr/share/codespell && sudo ln -s
            /usr/lib/python3/dist-packages/codespell_lib/data/dictionary.txt
@@ -89,8 +120,8 @@ jobs:
         # Note TRAVIS_COMMIT_RANGE has triple dots for "git diff" and
         # needs conversion to double dots for git log's inconsistent
         # user interface.
-        - ( set -x; scripts/checkpatch.pl --strict --codespell
-                     -g "${TRAVIS_COMMIT_RANGE/.../..}" )
+        - $skip_checkpatch || ( set -x; scripts/checkpatch.pl
+                --strict --codespell -g "${TRAVIS_COMMIT_RANGE/.../..}" )
 
     - name: Sparse check
       script: