ci: retry the scalafix lint when Maven cannot resolve the plugin

.github/workflows/pr_build_linux.yml

@@ -202,9 +202,45 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-java-maven-
 
+      # Split a Maven Central hiccup from a real lint violation. When the
+      # scalafix plugin fails to download (Central has answered 403, 429 and
+      # 404), Maven stops with "No plugin found for prefix 'scalafix'" and the
+      # red lint looks exactly like a violation. A scala-compiler jar that did
+      # not download shows up as "ClassNotFoundException: scala.tools.nsc.Main".
+      # The scalafix-syntactic job above lints offline after a retried
+      # download. Maven has no equally safe offline mode here, because the
+      # build resolves plugin and compiler artifacts as it goes, so this step
+      # retries the whole build instead, and only when Maven's error names a
+      # download. A lint violation or a compile error still fails on the first
+      # attempt. Maven caches a 404 in the local repository and will not ask
+      # again on its own, so the retries pass -U. Backoff matches
+      # .github/actions/maven-bootstrap: 10s, 20s, 40s plus 0-4s of jitter.
       - name: Run scalafix check
+        # Bash rather than the container default `sh`: the retry below needs
+        # pipefail, $RANDOM, and shell arithmetic.
+        shell: bash
         run: |
-          ./mvnw -B package -DskipTests scalafix:scalafix -Dscalafix.mode=CHECK -Psemanticdb ${{ matrix.profile.maven_opts }}
+          set -o pipefail
+          log=$(mktemp)
+          update=
+          for attempt in 1 2 3 4; do
+            if ./mvnw -B $update package -DskipTests scalafix:scalafix -Dscalafix.mode=CHECK \
+                 -Psemanticdb ${{ matrix.profile.maven_opts }} 2>&1 | tee "$log"; then
+              exit 0
+            fi
+            if ! grep -qE '^\[ERROR\] .*(No plugin found for prefix|could not be resolved|Could not transfer|ClassNotFoundException: scala\.tools\.nsc\.Main)' "$log"; then
+              echo "::error::Maven failed for a reason other than a download, such as a lint violation or a compile error; not retrying."
+              exit 1
+            fi
+            if [ "$attempt" -eq 4 ]; then
+              echo "::error::Maven could not download a plugin or dependency after $attempt attempts; this is not a lint violation."
+              exit 1
+            fi
+            delay=$((10 * (1 << (attempt - 1)) + RANDOM % 5))
+            echo "::warning::Maven failed to download a plugin or dependency (attempt $attempt of 4); retrying in ${delay}s with -U."
+            sleep "$delay"
+            update=-U
+          done
 
       - name: Setup Node.js
         uses: actions/setup-node@v7