ci: cap ecosystem format parallelism to fix transient walk error

the formatter ecosystem check intermittently failed with a single project
error — `ruff format --preview` emitting many pathless `No such file or
directory (os error 2)` from the parallel file walk on one large project (a
different one each run: ibis, then airflow).

root cause is load: ruff-ecosystem defaults to 50 concurrent projects, each
running a 12-thread `buff format` walk plus git clone/reset. on the fork's
small `ubuntu-latest` runner this massively oversubscribes cpu and fs and the
parallel walk transiently races; astral's 32-core depot runner absorbs it, so
upstream stays green. the walker code itself is byte-identical to upstream.

add a `--max-parallelism` flag to the vendored ruff-ecosystem cli (default
unchanged at 50) and pass `--max-parallelism 2` for the two `format` runs.
the read-only `check` runs never errored, so they keep the default.

not reproducible locally on macos (linux/load-specific); validated the flag
plumbing end-to-end. the 360-minute job budget absorbs the slower run.

Author: KotlinIsland

.github/workflows/ci.yaml

@@ -709,13 +709,21 @@ jobs:
           cat ecosystem-result-check-preview >> ecosystem-result
           echo "" >> ecosystem-result
 
+      # `--max-parallelism 2`: the default (50) oversubscribes the fork's small
+      # `ubuntu-latest` runner — dozens of concurrent multi-threaded `buff format`
+      # walks plus git clone/reset churn make the parallel file walk intermittently
+      # surface a transient `No such file or directory` on one large project (a
+      # different one each run), which the "Fail on ecosystem errors" step turns
+      # into a hard failure. astral's 32-core depot runner absorbs the load; ours
+      # needs the concurrency capped. (format only — the read-only check runs are
+      # unaffected.)
       - name: Run `ruff format` stable ecosystem check
         if: ${{ needs.determine_changes.outputs.formatter == 'true' }}
         run: |
           # Set pipefail to avoid hiding errors with tee
           set -eo pipefail
 
-          ruff-ecosystem format ./target/debug/buff-baseline ./target/debug/buff --cache ./checkouts --output-format markdown | tee ecosystem-result-format-stable
+          ruff-ecosystem format ./target/debug/buff-baseline ./target/debug/buff --cache ./checkouts --output-format markdown --max-parallelism 2 | tee ecosystem-result-format-stable
 
           cat ecosystem-result-format-stable > "$GITHUB_STEP_SUMMARY"
           echo "### Formatter (stable)" >> ecosystem-result
@@ -728,7 +736,7 @@ jobs:
           # Set pipefail to avoid hiding errors with tee
           set -eo pipefail
 
-          ruff-ecosystem format ./target/debug/buff-baseline ./target/debug/buff --cache ./checkouts --output-format markdown --force-preview | tee ecosystem-result-format-preview
+          ruff-ecosystem format ./target/debug/buff-baseline ./target/debug/buff --cache ./checkouts --output-format markdown --force-preview --max-parallelism 2 | tee ecosystem-result-format-preview
 
           cat ecosystem-result-format-preview > "$GITHUB_STEP_SUMMARY"
           echo "### Formatter (preview)" >> ecosystem-result

python/ruff-ecosystem/ruff_ecosystem/cli.py

@@ -99,6 +99,7 @@ def entrypoint():
                 project_dir=Path(cache),
                 raise_on_failure=args.pdb,
                 format_comparison=format_comparison,
+                max_parallelism=args.max_parallelism,
             )
         )
         # https://stackoverflow.com/a/58840987/3549270
@@ -151,6 +152,16 @@ def parse_args() -> argparse.Namespace:
         action="store_true",
         help="Force preview mode to be enabled for all projects",
     )
+    parser.add_argument(
+        "--max-parallelism",
+        type=int,
+        default=50,
+        help=(
+            "Maximum number of projects to clone/format concurrently. Lower this on "
+            "small CI runners: high concurrency thrashes the filesystem and can make "
+            "the parallel file walk race with concurrent git clone/reset operations."
+        ),
+    )
     parser.add_argument(
         "--format-comparison",
         choices=[option.value for option in FormatComparison],