Refactor output filename handling in ci-fetch-log.sh (#43901)

Signed-off-by: Michael Goin <[email protected]>

.buildkite/scripts/ci-fetch-log.sh

@@ -9,6 +9,13 @@
 # Find <build_number> and <job_uuid> via:
 #   gh pr checks <PR> --repo vllm-project/vllm
 # Each failing row's URL is .../builds/<build_number>#<job_uuid>.
+#
+# Default output path: ci-<build>-<uuid_first_13_chars>.log (e.g.
+# ci-68478-019e6b07-daae.log). Jobs in the same build share the UUID's
+# first 8 chars, so the second segment is needed for uniqueness when
+# fetching multiple jobs in parallel. The script refuses to overwrite an
+# existing output file; pass an explicit path or set CI_FETCH_LOG_FORCE=1
+# to override.
 
 set -euo pipefail
 
@@ -26,12 +33,12 @@ if [ $# -lt 1 ]; then usage; fi
 if [[ "$1" == https://* ]]; then
     BUILD=$(echo "$1" | sed -nE 's#.*/builds/([0-9]+).*#\1#p')
     JOB=$(echo "$1" | grep -oE '[0-9a-f]{8}-[0-9a-f-]+' | head -n 1)
-    OUT="${2:-ci-${BUILD}-${JOB:0:8}.log}"
+    OUT="${2:-}"
 else
     if [ $# -lt 2 ]; then usage; fi
     BUILD="$1"
     JOB="$2"
-    OUT="${3:-ci-${BUILD}-${JOB:0:8}.log}"
+    OUT="${3:-}"
 fi
 
 if [ -z "$BUILD" ] || [ -z "$JOB" ]; then
@@ -39,6 +46,18 @@ if [ -z "$BUILD" ] || [ -z "$JOB" ]; then
     usage
 fi
 
+# Jobs in the same build share the UUID's first segment, so include the
+# second segment (chars 9-13, e.g. "019e6b07-daae") to keep default filenames
+# unique when fetching multiple jobs from one build in parallel.
+if [ -z "$OUT" ]; then
+    OUT="ci-${BUILD}-${JOB:0:13}.log"
+fi
+
+if [ -e "$OUT" ] && [ -z "${CI_FETCH_LOG_FORCE:-}" ]; then
+    echo "Refusing to overwrite existing $OUT (set CI_FETCH_LOG_FORCE=1 or pass an explicit output path)." >&2
+    exit 1
+fi
+
 COOKIES=$(mktemp)
 trap 'rm -f "$COOKIES"' EXIT