HADOOP-8406. CompressionCodecFactory.CODEC_PROVIDERS iteration is thread-unsafe. Contributed by Todd Lipcon.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1339475 13f79535-47bb-0310-9956-ffa450edef68

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -35,6 +35,9 @@ Release 2.0.1-alpha - UNRELEASED
 
     HADOOP-8400. All commands warn "Kerberos krb5 configuration not found" when security is not enabled. (tucu)
 
+    HADOOP-8406. CompressionCodecFactory.CODEC_PROVIDERS iteration is
+    thread-unsafe (todd)
+
 Release 2.0.0-alpha - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/io/compress/CompressionCodecFactory.java

@@ -109,8 +109,12 @@ public class CompressionCodecFactory {
     List<Class<? extends CompressionCodec>> result
       = new ArrayList<Class<? extends CompressionCodec>>();
     // Add codec classes discovered via service loading
-    for (CompressionCodec codec : CODEC_PROVIDERS) {
-      result.add(codec.getClass());
+    synchronized (CODEC_PROVIDERS) {
+      // CODEC_PROVIDERS is a lazy collection. Synchronize so it is
+      // thread-safe. See HADOOP-8406.
+      for (CompressionCodec codec : CODEC_PROVIDERS) {
+        result.add(codec.getClass());
+      }
     }
     // Add codec classes from configuration
     String codecsString = conf.get("io.compression.codecs");