Have check.sh fallback to checking everything (#4614)

Sometimes PRs have been cut from so long ago that there's no merge base
with master in the shallow clone that Travis makes. In this case, fall
back on just checking everything.

Author: wilhuff

scripts/check.sh

@@ -84,6 +84,7 @@ cd "${top_dir}"
 
 ALLOW_DIRTY=false
 COMMIT_METHOD="none"
+CHECK_DIFF=false
 START_REVISION="origin/master"
 TEST_ONLY=false
 VERBOSE=false
@@ -98,12 +99,14 @@ fi
 # is not available and we need to compute the START_REVISION from the common
 # ancestor of $TRAVIS_COMMIT and origin/master.
 if [[ -n "${TRAVIS_COMMIT_RANGE:-}" ]] ; then
+  CHECK_DIFF=true
   START_REVISION="$TRAVIS_COMMIT_RANGE"
 elif [[ -n "${TRAVIS_COMMIT:-}" ]] ; then
   if ! git rev-parse origin/master >& /dev/null; then
     git remote set-branches --add origin master
     git fetch origin
   fi
+  CHECK_DIFF=true
   START_REVISION=$(git merge-base origin/master "${TRAVIS_COMMIT}")
 fi
 
@@ -146,6 +149,7 @@ while [[ $# -gt 0 ]]; do
       ;;
 
     *)
+      CHECK_DIFF=true
       START_REVISION="$1"
       shift
       break
@@ -187,9 +191,24 @@ if [[ "${START_REVISION}" == *..* ]]; then
     git fetch origin
   fi
 
-  NEW_RANGE_START=$(git merge-base origin/master "${RANGE_END}")
-  START_REVISION="${START_REVISION/$RANGE_START/$NEW_RANGE_START}"
-  START_SHA="${START_REVISION}"
+  # Try to come up with a more accurate representation of the merge, so that
+  # checks will operate on just the differences the PR would merge into master.
+  # The start of the revision range that Travis supplies can sometimes be a
+  # seemingly random value.
+  NEW_RANGE_START=$(git merge-base origin/master "${RANGE_END}" || echo "")
+  if [[ -n "$NEW_RANGE_START" ]]; then
+    START_REVISION="${NEW_RANGE_START}..${RANGE_END}"
+    START_SHA="${START_REVISION}"
+  else
+    # In the shallow clone that Travis has created there's no merge base
+    # between the PR and master. In this case just fall back on checking
+    # everything.
+    echo "Unable to detect base commit for change detection."
+    echo "Failling back on just checking everything."
+    CHECK_DIFF=false
+    START_REVISION="origin/master"
+    START_SHA="origin/master"
+  fi
 
 else
   START_SHA=$(git rev-parse "${START_REVISION}")
@@ -236,7 +255,9 @@ style_cmd=("${top_dir}/scripts/style.sh")
 if [[ "${TEST_ONLY}" == true ]]; then
   style_cmd+=(test-only)
 fi
-style_cmd+=("${START_SHA}")
+if [[ "$CHECK_DIFF" == true ]]; then
+  style_cmd+=("${START_SHA}")
+fi
 
 # Restyle and commit any changes
 "${style_cmd[@]}"
@@ -247,7 +268,9 @@ fi
 # If there are changes to the Firestore project, ensure they're ordered
 # correctly to minimize conflicts.
 if [ -z "${GITHUB_WORKFLOW-}" ]; then
-  if ! git diff --quiet "${START_SHA}" -- Firestore; then
+  if [[ "$CHECK_DIFF" == "false" ]] || \
+      ! git diff --quiet "${START_SHA}" -- Firestore; then
+
     sync_project_cmd=("${top_dir}/scripts/sync_project.rb")
     if [[ "${TEST_ONLY}" == true ]]; then
       sync_project_cmd+=(--test-only)
@@ -268,4 +291,8 @@ fi
 "${top_dir}/scripts/check_cmake_files.py"
 
 # Google C++ style
-"${top_dir}/scripts/check_lint.py" "${START_SHA}"
+lint_cmd=("${top_dir}/scripts/check_lint.py")
+if [[ "$CHECK_DIFF" == true ]]; then
+  lint_cmd+=("${START_SHA}")
+fi
+"${lint_cmd[@]}"