HADOOP-18526. s3a instrumentation

* use the weak ref when registering
* log state of cache.disabled in trace-level constructor

The reason for the logging is that running out of memory
due to instrumentation refs is probably a sign of s3a fs instances
being created and not closed.
HADOOP-18476 should partially address this through FileContext;
the trace is there to identify other possible causes.

Change-Id: If78d16e8bfc63eea93ec328543ab110f82c93d90

Author: steveloughran

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AFileSystem.java

@@ -461,8 +461,9 @@ public void initialize(URI name, Configuration originalConf)
       if (LOG.isTraceEnabled()) {
         // log a full trace for deep diagnostics of where an object is created,
         // for tracking down memory leak issues.
-        LOG.trace("Filesystem for {} creation stack",
-            name, new RuntimeException(super.toString()));
+        LOG.trace("Filesystem for {} created; fs.s3a.impl.disable.cache = {}",
+            name, originalConf.getBoolean("fs.s3a.impl.disable.cache", false),
+            new RuntimeException(super.toString()));
       }
       // clone the configuration into one with propagated bucket options
       Configuration conf = propagateBucketOptions(originalConf, bucket);

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AInstrumentation.java

@@ -263,7 +263,7 @@ static boolean hasMetricSystem() {
   }
 
   /**
-   * Register this instance as a metrics source.
+   * Register this instance as a metrics source via a weak reference.
    * @param name s3a:// URI for the associated FileSystem instance
    */
   private void registerAsMetricsSource(URI name) {
@@ -277,7 +277,7 @@ private void registerAsMetricsSource(URI name) {
     String msName = METRICS_SOURCE_BASENAME + number;
     String metricsSourceName = msName + "-" + name.getHost();
     metricsSourceReference = new WeakRefMetricsSource(metricsSourceName, this);
-    metricsSystem.register(metricsSourceName, "", this);
+    metricsSystem.register(metricsSourceName, "", metricsSourceReference);
   }
 
   /**