Fix get_changed_files in infra/ci. (#5055)

Fixes https://github.com/google/oss-fuzz/issues/5022

1 files changed, 10 insertions, 5 deletions

diff --git a/infra/ci/build.py b/infra/ci/build.py
index 30ff4ea4..9a1ed623 100755
--- a/infra/ci/build.py
+++ b/infra/ci/build.py
@@ -35,16 +35,21 @@ DEFAULT_SANITIZERS = ['address', 'undefined']
 LANGUAGES_WITH_COVERAGE_SUPPORT = ['c', 'c++', 'go']
 
 
-def get_changed_files():
+def get_changed_files_output():
   """Returns the output of a git command that discovers changed files."""
-  return subprocess.check_output(['git', 'diff', '--name-only',
-                                  'FETCH_HEAD']).decode()
+  main_branch = subprocess.check_output(
+      ['git', 'rev-parse', '--abbrev-ref', 'origin/HEAD']).strip().decode()
+  branch_commit_hash = subprocess.check_output(
+      ['git', 'merge-base', 'FETCH_HEAD', main_branch]).strip().decode()
+
+  return subprocess.check_output(
+      ['git', 'diff', '--name-only', branch_commit_hash + '..']).decode()
 
 
 def get_modified_buildable_projects():
   """Returns a list of all the projects modified in this commit that have a
   build.sh file."""
-  git_output = get_changed_files()
+  git_output = get_changed_files_output()
   projects_regex = '.*projects/(?P<name>.*)/.*\n'
   modified_projects = set(re.findall(projects_regex, git_output))
   projects_dir = os.path.join(get_oss_fuzz_root(), 'projects')
@@ -196,7 +201,7 @@ def build_modified_projects():
 
 def is_infra_changed():
   """Returns True if the infra directory was changed."""
-  git_output = get_changed_files()
+  git_output = get_changed_files_output()
   infra_code_regex = '.*infra/.*\n'
   return re.search(infra_code_regex, git_output) is not None