From 7c28d5797387fdf68379ec66f0de3c62e6a079ca Mon Sep 17 00:00:00 2001
From: David Smiley <dsmiley@apache.org>
Date: Thu, 21 Jun 2018 14:00:12 -0400
Subject: [PATCH] SOLR-12413: Ensure pre-loaded aliases in ZK are considered.
 (fix use of zk version in Aliases.EMPTY)

---
 solr/CHANGES.txt                                  |  3 +++
 .../org/apache/solr/common/cloud/Aliases.java     |  6 +++---
 .../apache/solr/common/cloud/ZkStateReader.java   | 15 +++++++++++++++
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/solr/CHANGES.txt b/solr/CHANGES.txt
index 118267297d7..960baf238d8 100644
--- a/solr/CHANGES.txt
+++ b/solr/CHANGES.txt
@@ -102,6 +102,9 @@ Bug Fixes
 
 * SOLR-11807: Restoring collection now treats maxShardsPerNode=-1 as unlimited (Varun Thacker)
 
+* SOLR-12413: If Zookeeper was pre-loaded with data before first-use, then the aliases information would be ignored.
+  (David Smiley, Gaël Jourdan, Gus Heck)
+
 Optimizations
 ----------------------
 
diff --git a/solr/solrj/src/java/org/apache/solr/common/cloud/Aliases.java b/solr/solrj/src/java/org/apache/solr/common/cloud/Aliases.java
index 671c27b0534..439b9363eeb 100644
--- a/solr/solrj/src/java/org/apache/solr/common/cloud/Aliases.java
+++ b/solr/solrj/src/java/org/apache/solr/common/cloud/Aliases.java
@@ -39,10 +39,10 @@ public class Aliases {
   /**
    * An empty, minimal Aliases primarily used to support the non-cloud solr use cases. Not normally useful
    * in cloud situations where the version of the node needs to be tracked even if all aliases are removed.
-   * A version of 0 is provided rather than -1 to minimize the possibility that if this is used in a cloud
-   * instance data is written without version checking.
+   * The -1 version makes it subordinate to any real version, and furthermore we never "set" this EMPTY instance
+   * into ZK.
    */
-  public static final Aliases EMPTY = new Aliases(Collections.emptyMap(), Collections.emptyMap(), 0);
+  public static final Aliases EMPTY = new Aliases(Collections.emptyMap(), Collections.emptyMap(), -1);
 
   // These two constants correspond to the top level elements in aliases.json. The first one denotes
   // a section containing a list of aliases and their attendant collections, the second contains a list of
diff --git a/solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java b/solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java
index a86c5e28448..2c1209e2ff6 100644
--- a/solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java
+++ b/solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java
@@ -1698,6 +1698,19 @@ public class ZkStateReader implements Closeable {
      * The caller should understand it's possible the aliases has further changed if it examines it.
      */
     public void applyModificationAndExportToZk(UnaryOperator<Aliases> op) {
+      // The current aliases hasn't been update()'ed yet -- which is impossible?  Any way just update it first.
+      if (aliases.getZNodeVersion() == -1) {
+        try {
+          boolean updated = update();
+          assert updated;
+        } catch (InterruptedException e) {
+          Thread.currentThread().interrupt();
+          throw new ZooKeeperException(ErrorCode.SERVER_ERROR, e.toString(), e);
+        } catch (KeeperException e) {
+          throw new ZooKeeperException(ErrorCode.SERVER_ERROR, e.toString(), e);
+        }
+      }
+
       final long deadlineNanos = System.nanoTime() + TimeUnit.SECONDS.toNanos(30);
       // note: triesLeft tuning is based on ConcurrentCreateRoutedAliasTest
       for (int triesLeft = 30; triesLeft > 0; triesLeft--) {
@@ -1769,6 +1782,7 @@ public class ZkStateReader implements Closeable {
         // note: it'd be nice to avoid possibly needlessly parsing if we don't update aliases but not a big deal
         setIfNewer(Aliases.fromJSON(data, stat.getVersion()));
       } catch (KeeperException.ConnectionLossException | KeeperException.SessionExpiredException e) {
+        // note: aliases.json is required to be present
         LOG.warn("ZooKeeper watch triggered, but Solr cannot talk to ZK: [{}]", e.getMessage());
       } catch (KeeperException e) {
         LOG.error("A ZK error has occurred", e);
@@ -1786,6 +1800,7 @@ public class ZkStateReader implements Closeable {
      * @param newAliases the potentially newer version of Aliases
      */
     private boolean setIfNewer(Aliases newAliases) {
+      assert newAliases.getZNodeVersion() >= 0;
       synchronized (this) {
         int cmp = Integer.compare(aliases.getZNodeVersion(), newAliases.getZNodeVersion());
         if (cmp < 0) {