HDFS-15567. [SBN Read] HDFS should expose msync() API to allow downstream applications call it explicitly. Contributed by Konstantin V Shvachko.

(cherry picked from commit b3786d6)

Author: shvachko

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/AbstractFileSystem.java

@@ -865,6 +865,19 @@ public abstract FileStatus getFileStatus(final Path f)
       throws AccessControlException, FileNotFoundException,
       UnresolvedLinkException, IOException;
 
+  /**
+   * Synchronize client metadata state.
+   * <p/>In some FileSystem implementations such as HDFS metadata
+   * synchronization is essential to guarantee consistency of read requests
+   * particularly in HA setting.
+   * @throws IOException
+   * @throws UnsupportedOperationException
+   */
+  public void msync() throws IOException, UnsupportedOperationException {
+    throw new UnsupportedOperationException(getClass().getCanonicalName() +
+        " does not support method msync");
+  }
+
   /**
    * The specification of this method matches that of
    * {@link FileContext#access(Path, FsAction)}

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/FileContext.java

@@ -1254,6 +1254,16 @@ public FileStatus next(final AbstractFileSystem fs, final Path p)
     }.resolve(this, absF);
   }
 
+  /**
+   * Synchronize client metadata state.
+   *
+   * @throws IOException
+   * @throws UnsupportedOperationException
+   */
+  public void msync() throws IOException, UnsupportedOperationException {
+    defaultFS.msync();
+  }
+
   /**
    * Checks if the user can access a path.  The mode specifies which access
    * checks to perform.  If the requested permissions are granted, then the

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/FilterFileSystem.java

@@ -456,6 +456,11 @@ public FileStatus getFileStatus(Path f) throws IOException {
     return fs.getFileStatus(f);
   }
 
+  @Override
+  public void msync() throws IOException, UnsupportedOperationException {
+    fs.msync();
+  }
+
   @Override
   public void access(Path path, FsAction mode) throws AccessControlException,
       FileNotFoundException, IOException {

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/FilterFs.java

@@ -124,6 +124,11 @@ public FileStatus getFileStatus(Path f)
     return myFs.getFileStatus(f);
   }
 
+  @Override
+  public void msync() throws IOException, UnsupportedOperationException {
+    myFs.msync();
+  }
+
   @Override
   public void access(Path path, FsAction mode) throws AccessControlException,
       FileNotFoundException, UnresolvedLinkException, IOException {