ci: 安装步骤加重试,一次 xlings update 可能静默返回陈旧索引

索引以 artifact 形式发布、由一个指针指向,而版本 bump 合入后指针是**异步**
传播的。发布当天实测:合入四分钟后跑的 update 打印了 `index updated`,随后
安装失败于

    package 'mcpp@<ver>' not found in the synced index
    (xim@artifact:<更旧的 sha>, ...), synced 0 seconds ago

什么都没出错。那次 update 取到的是**上一个** artifact,而
「synced 0 seconds ago」说的是它什么时候被取的、不是它里面有什么——这正是
这条消息读起来像「很新」的原因。

所以这不是绕过抖动的重试,而是补上单次 update 并不执行的那段等待。钉了
一个从未发布过的版本仍然会失败,在最后一次尝试之后,并说明是两种情形中的
哪一种。

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -26,8 +26,35 @@ jobs:
 
       - name: Install mcpp
         run: |
-          xlings update
-          xlings install "mcpp@$MCPP_VERSION" -y -g
+          # ⚠️ A LOOP, BECAUSE ONE `xlings update` CAN RETURN A STALE INDEX
+          # WITHOUT SAYING SO.
+          #
+          # The index is published as an artifact behind a pointer, and that
+          # pointer propagates asynchronously after a version bump is merged.
+          # Measured on release day: an update run four minutes after the merge
+          # printed `index updated`, and the install then failed with
+          #
+          #     package 'mcpp@<ver>' not found in the synced index
+          #     (xim@artifact:<an older sha>, ...), synced 0 seconds ago
+          #
+          # Nothing had gone wrong. The update fetched the PREVIOUS artifact,
+          # and "synced 0 seconds ago" describes when it was fetched rather than
+          # what it contains — which is why the message reads as freshness.
+          #
+          # So this is not a retry around flakiness; it is the wait that a
+          # single update does not perform. A pin naming a version that was
+          # never published still fails, after the last attempt, and says which
+          # of the two situations it is.
+          for attempt in 1 2 3 4 5 6; do
+            xlings update > /dev/null 2>&1 || true
+            if xlings install "mcpp@$MCPP_VERSION" -y -g; then break; fi
+            if [ "$attempt" = 6 ]; then
+              echo "::error::mcpp@$MCPP_VERSION never appeared in the index (6 attempts over 5 minutes). If it was just released, the pointer has not propagated; if the pin names a version that was never published, it never will."
+              exit 1
+            fi
+            echo "the index has not caught up yet (attempt $attempt of 6); waiting 60s"
+            sleep 60
+          done
           mcpp --version
           mcpp self config --mirror GLOBAL
 
@@ -111,8 +138,35 @@ jobs:
 
       - name: Install mcpp
         run: |
-          xlings update
-          xlings install "mcpp@$MCPP_VERSION" -y -g
+          # ⚠️ A LOOP, BECAUSE ONE `xlings update` CAN RETURN A STALE INDEX
+          # WITHOUT SAYING SO.
+          #
+          # The index is published as an artifact behind a pointer, and that
+          # pointer propagates asynchronously after a version bump is merged.
+          # Measured on release day: an update run four minutes after the merge
+          # printed `index updated`, and the install then failed with
+          #
+          #     package 'mcpp@<ver>' not found in the synced index
+          #     (xim@artifact:<an older sha>, ...), synced 0 seconds ago
+          #
+          # Nothing had gone wrong. The update fetched the PREVIOUS artifact,
+          # and "synced 0 seconds ago" describes when it was fetched rather than
+          # what it contains — which is why the message reads as freshness.
+          #
+          # So this is not a retry around flakiness; it is the wait that a
+          # single update does not perform. A pin naming a version that was
+          # never published still fails, after the last attempt, and says which
+          # of the two situations it is.
+          for attempt in 1 2 3 4 5 6; do
+            xlings update > /dev/null 2>&1 || true
+            if xlings install "mcpp@$MCPP_VERSION" -y -g; then break; fi
+            if [ "$attempt" = 6 ]; then
+              echo "::error::mcpp@$MCPP_VERSION never appeared in the index (6 attempts over 5 minutes). If it was just released, the pointer has not propagated; if the pin names a version that was never published, it never will."
+              exit 1
+            fi
+            echo "the index has not caught up yet (attempt $attempt of 6); waiting 60s"
+            sleep 60
+          done
           mcpp --version
           mcpp self config --mirror GLOBAL