fix base branch detection in fuzzer jobs

Author: strtgbb

docker/test/fuzzer/run-fuzzer.sh

@@ -50,7 +50,21 @@ function clone
                 git checkout "$SHA_TO_TEST"
                 echo "Checked out nominal SHA $SHA_TO_TEST for PR $PR_TO_TEST"
             fi
-            git diff --name-only master HEAD | tee ci-changed-files.txt
+            # Use BASE_BRANCH from environment variable if available, otherwise fall back to master
+            if [ -n "$BASE_BRANCH" ]; then
+                echo "Using base branch from environment: $BASE_BRANCH"
+                MERGE_BASE=$(git merge-base HEAD "origin/$BASE_BRANCH" 2>/dev/null || echo "")
+                if [ -z "$MERGE_BASE" ]; then
+                    echo "ERROR: No merge base found with base branch: $BASE_BRANCH"
+                    echo "Available remote branches:"
+                    git branch -r
+                    exit 1
+                fi
+                git diff --name-only "$MERGE_BASE" HEAD | tee ci-changed-files.txt
+            else
+                echo "No BASE_BRANCH environment variable, using master"
+                git diff --name-only master HEAD | tee ci-changed-files.txt
+            fi
         else
             if [ -v SHA_TO_TEST ]; then
                 git fetch --depth 2 origin "$SHA_TO_TEST"

tests/ci/ci_fuzzer_check.py

@@ -34,6 +34,7 @@ def get_run_command(
         f"-e SHA_TO_TEST={pr_info.sha}",
         f"-e BINARY_URL_TO_DOWNLOAD='{build_url}'",
         f"-e FUZZER_TO_RUN='{fuzzer_name}'",
+        f"-e BASE_BRANCH='{pr_info.base_ref}'",
     ]
 
     env_str = " ".join(envs)