HDFS-16952. Support getLinkTarget API in WebHDFS (#5517)

Co-authored-by: Zhtttylz <hualong.z@hotmail.com>
Reviewed-by: Shilun Fan <slfan1989@apache.org>
Signed-off-by: Shilun Fan <slfan1989@apache.org>

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/web/WebHdfsFileSystem.java

@@ -2147,6 +2147,19 @@ FsServerDefaults decodeResponse(Map<?, ?> json) throws IOException {
     }.run();
   }
 
+  @Override
+  public Path getLinkTarget(Path f) throws IOException {
+    statistics.incrementReadOps(1);
+    storageStatistics.incrementOpCounter(OpType.GET_LINK_TARGET);
+    final HttpOpParam.Op op = GetOpParam.Op.GETLINKTARGET;
+    return new FsPathResponseRunner<Path>(op, f) {
+      @Override
+      Path decodeResponse(Map<?, ?> json) {
+        return new Path((String) json.get(Path.class.getSimpleName()));
+      }
+    }.run();
+  }
+
   @VisibleForTesting
   InetSocketAddress[] getResolvedNNAddr() {
     return nnAddrs;

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/web/resources/GetOpParam.java

@@ -64,6 +64,7 @@ public enum Op implements HttpOpParam.Op {
     GETSNAPSHOTDIFF(false, HttpURLConnection.HTTP_OK),
     GETSNAPSHOTDIFFLISTING(false, HttpURLConnection.HTTP_OK),
     GETSNAPSHOTTABLEDIRECTORYLIST(false, HttpURLConnection.HTTP_OK),
+    GETLINKTARGET(false, HttpURLConnection.HTTP_OK),
     GETSNAPSHOTLIST(false, HttpURLConnection.HTTP_OK);
 
     final boolean redirect;

hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/router/RouterWebHdfsMethods.java

@@ -385,6 +385,7 @@ protected Response get(
       case GETXATTRS:
       case LISTXATTRS:
       case CHECKACCESS:
+      case GETLINKTARGET:
       {
         return super.get(ugi, delegation, username, doAsUser, fullpath, op,
             offset, length, renewer, bufferSize, xattrNames, xattrEncoding,

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/web/resources/NamenodeWebHdfsMethods.java

@@ -1383,6 +1383,11 @@ protected Response get(
       final String js = JsonUtil.toJsonString(snapshotList);
       return Response.ok(js).type(MediaType.APPLICATION_JSON).build();
     }
+    case GETLINKTARGET: {
+      String target = cp.getLinkTarget(fullpath);
+      final String js = JsonUtil.toJsonString("Path", target);
+      return Response.ok(js).type(MediaType.APPLICATION_JSON).build();
+    }
     default:
       throw new UnsupportedOperationException(op + " is not supported");
     }

hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/WebHDFS.md

@@ -58,6 +58,7 @@ The HTTP REST API supports the complete [FileSystem](../../api/org/apache/hadoop
     * [`GETFILEBLOCKLOCATIONS`](#Get_File_Block_Locations) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).getFileBlockLocations)
     * [`GETECPOLICY`](#Get_EC_Policy) (see [HDFSErasureCoding](./HDFSErasureCoding.html#Administrative_commands).getErasureCodingPolicy)
     * [`GETSERVERDEFAULTS`](#Get_Server_Defaults) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).getServerDefaults)
+    * [`GETLINKTARGET`](#Get_Link_Target) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).getLinkTarget)
 *   HTTP PUT
     * [`CREATE`](#Create_and_Write_to_a_File) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).create)
     * [`MKDIRS`](#Make_a_Directory) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).mkdirs)
@@ -1139,6 +1140,22 @@ See also: [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).access
 
 See also: [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).getServerDefaults
 
+### Get Link Target
+
+* Submit a HTTP GET request.
+
+        curl -i "http://<HOST>:<PORT>/webhdfs/v1/<PATH>?op=GETLINKTARGET"
+
+    The client receives a response with a [`Path` JSON object](#Path_JSON_Schema):
+
+        HTTP/1.1 200 OK
+        Content-Type: application/json
+        Transfer-Encoding: chunked
+
+        {"Path": "/user/username/targetFile"}
+
+See also: [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).getLinkTarget
+
 Storage Policy Operations
 -------------------------
 

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/web/TestWebHDFS.java

@@ -2202,6 +2202,34 @@ public void testStatistics() throws Exception {
       cluster.shutdown();
     }
   }
+
+  @Test
+  public void testLinkTarget() throws Exception {
+    final Configuration conf = WebHdfsTestUtil.createConf();
+    try {
+      cluster = new MiniDFSCluster.Builder(conf)
+          .numDataNodes(3)
+          .build();
+      cluster.waitActive();
+
+      final WebHdfsFileSystem webHdfs =
+          WebHdfsTestUtil.getWebHdfsFileSystem(conf,
+              WebHdfsConstants.WEBHDFS_SCHEME);
+
+      // Symbolic link
+      Path root = new Path("/webHdfsTest/");
+      Path targetFile = new Path(root, "debug.log");
+      FileSystemTestHelper.createFile(webHdfs, targetFile);
+
+      Path symLink = new Path(root, "debug.link");
+
+      webHdfs.createSymlink(targetFile, symLink, false);
+      assertEquals(webHdfs.getLinkTarget(symLink), targetFile);
+    } finally {
+      cluster.shutdown();
+    }
+  }
+
   /**
    * Get FileStatus JSONObject from ListStatus response.
    */