fix coverage

Author: parthea

.github/workflows/unittest.yml

@@ -59,21 +59,35 @@ jobs:
     steps:
     - name: Checkout
       uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
-      # Use a fetch-depth of 2 to avoid error `fatal: origin/main...HEAD: no merge base`
-      # See https://github.com/googleapis/google-cloud-python/issues/12013
-      # and https://github.com/actions/checkout#checkout-head.
       with:
-        fetch-depth: 2
+        # Use fetch-depth of 0 (full history) so Git can find the merge-base with the
+        # target branch and calculate the exact list of modified packages.
+        fetch-depth: 0
         persist-credentials: false
+      # We must explicitly fetch and set the target branch reference (main, etc.)
+      # to perform a relative diff (target_branch...HEAD). Comparing against a generic "HEAD~1"
+      # is fragile and fails if developers merge main back into their PR, if the PR has
+      # multiple commits, or if running inside a Merge Queue (merge_group) environment.
+    - name: Set target branch ref
+      id: target
+      env:
+        TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref || 'main' }}
+      run: |
+        git fetch origin $TARGET_BRANCH
+        target_sha=$(git rev-parse FETCH_HEAD)
+        echo "sha=${target_sha}" >> "$GITHUB_OUTPUT"
     - name: Setup Python
       uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5
       with:
         python-version: "3.10"
     - name: Set number of files changes in packages and preview-packages directories
       id: packages
+      env:
+        # Use TARGET_SHA...HEAD instead of HEAD~1 to accurately detect all files modified across the entire PR
+        TARGET_SHA: ${{ steps.target.outputs.sha }}
       run: |
-        git diff HEAD~1 -- packages preview-packages > /dev/null
-        num_files_changed=$(git diff HEAD~1 -- packages preview-packages | wc -l | tr -d ' ')
+        git diff "$TARGET_SHA"...HEAD -- packages preview-packages > /dev/null
+        num_files_changed=$(git diff "$TARGET_SHA"...HEAD -- packages preview-packages | wc -l | tr -d ' ')
         echo "num_files_changed=${num_files_changed}" >> "$GITHUB_OUTPUT"
     - name: Install coverage
       if: ${{ steps.packages.outputs.num_files_changed > 0 }}
@@ -91,9 +105,12 @@ jobs:
         # TODO: default to 100% coverage after next gapic-generator release
         # https://github.com/googleapis/google-cloud-python/issues/17459
         DEFAULT_FAIL_UNDER: 99
+        TARGET_SHA: ${{ steps.target.outputs.sha }}
       run: |
+        # Use target_ref to find all packages modified across the entire PR branch relative to target branch
+        target_ref="$TARGET_SHA"
         # Find all modified packages
-        modified_packages=$(git diff --name-only HEAD~1 -- packages preview-packages | cut -d/ -f1,2 | sort -u)
+        modified_packages=$(git diff --name-only ${target_ref}...HEAD -- packages preview-packages | cut -d/ -f1,2 | sort -u)
 
         existing_modified_packages=()
         for pkg in ${modified_packages}; do
@@ -122,7 +139,7 @@ jobs:
           fi
 
           # Find all modified packages
-          modified_packages=$(git diff --name-only HEAD~1 -- packages preview-packages | cut -d/ -f1,2 | sort -u)
+          modified_packages=$(git diff --name-only ${target_ref}...HEAD -- packages preview-packages | cut -d/ -f1,2 | sort -u)
 
           failed_packages=()
           passed_packages=()

ci/run_conditional_tests.sh

@@ -36,14 +36,6 @@ set -eo pipefail
 export PROJECT_ROOT=$(realpath $(dirname "${BASH_SOURCE[0]}")/..)
 TARGET_BRANCH="${TARGET_BRANCH:-main}"
 
-# Redirect git clones for core dependencies to the local repository.
-# This serves two purposes:
-# 1. Performance: Avoids repeated 100MB+ downloads of the monorepo for each dependency.
-# 2. Correctness: Ensures that changes in core packages (like google-api-core) are
-#    tested against downstream packages in the same Pull Request.
-git config --global url."${PROJECT_ROOT}".insteadOf "https://github.com/googleapis/google-cloud-python"
-git config --global url."${PROJECT_ROOT}".insteadOf "https://github.com/googleapis/google-cloud-python.git"
-
 # A script file for running the test in a sub project.
 test_script="${PROJECT_ROOT}/ci/run_single_test.sh"
 
@@ -68,6 +60,18 @@ else
     GIT_DIFF_ARG=""
 fi
 
+# Redirect git clones for core dependencies to the local repository.
+# This serves two purposes:
+# 1. Performance: Avoids repeated 100MB+ downloads of the monorepo for each dependency.
+# 2. Correctness: Ensures that changes in core packages (like google-api-core) are
+#    tested against downstream packages in the same Pull Request.
+#
+# NOTE: This configuration MUST be set AFTER `git fetch origin` above, otherwise Git will
+# intercept and redirect `git fetch origin` to the local repository instead of GitHub,
+# breaking target branch fetching and package change detection.
+git config --global url."${PROJECT_ROOT}".insteadOf "https://github.com/googleapis/google-cloud-python"
+git config --global url."${PROJECT_ROOT}".insteadOf "https://github.com/googleapis/google-cloud-python.git"
+
 # Then detect changes in the test scripts.
 
 set +e