HDFS-13249. Document webhdfs support for getting snapshottable directory list. Contributed by Lokesh Jain.

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

@@ -51,6 +51,7 @@ The HTTP REST API supports the complete [FileSystem](../../api/org/apache/hadoop
     * [`GETALLSTORAGEPOLICY`](#Get_all_Storage_Policies) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).getAllStoragePolicies)
     * [`GETSTORAGEPOLICY`](#Get_Storage_Policy) (see [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).getStoragePolicy)
     * [`GETSNAPSHOTDIFF`](#Get_Snapshot_Diff)
+    * [`GETSNAPSHOTTABLEDIRECTORYLIST`](#Get_Snapshottable_Directory_List)
 *   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)
@@ -1282,6 +1283,45 @@ See also: [FileSystem](../../api/org/apache/hadoop/fs/FileSystem.html).renameSna
 
         {"SnapshotDiffReport":{"diffList":[],"fromSnapshot":"s3","snapshotRoot":"/foo","toSnapshot":"s4"}}
 
+### Get Snapshottable Directory List
+
+* Submit a HTTP GET request.
+
+        curl -i GET "http://<HOST>:<PORT>/webhdfs/v1/?user.name=<USER>&op=GETSNAPSHOTTABLEDIRECTORYLIST"
+
+    If the USER is not the hdfs super user, the call lists only the snapshottable directories owned by the user. If the USER is the hdfs super user, the call lists all the snapshottable directories. The client receives a response with a [`SnapshottableDirectoryList` JSON object](#SnapshottableDirectoryList_JSON_Schema):
+
+        HTTP/1.1 200 OK
+        Content-Type: application/json
+        Transfer-Encoding: chunked
+
+        {
+            "SnapshottableDirectoryList":
+            [
+                {
+                  "dirStatus":
+                    {
+                        "accessTime":0,
+                        "blockSize":0,
+                        "childrenNum":0,
+                        "fileId":16386,
+                        "group":"hadoop",
+                        "length":0,
+                        "modificationTime":1520761889225,
+                        "owner":"random",
+                        "pathSuffix":"bar",
+                        "permission":"755",
+                        "replication":0,
+                        "storagePolicy":0,
+                        "type":"DIRECTORY"
+                    },
+                  "parentFullPath":"/",
+                  "snapshotNumber":0,
+                  "snapshotQuota":65536
+                }
+            ]
+        }
+
 Delegation Token Operations
 ---------------------------
 
@@ -2135,6 +2175,58 @@ var diffReportEntries =
 }
 ```
 
+### SnapshottableDirectoryList JSON Schema
+
+```json
+{
+  "name": "SnapshottableDirectoryList",
+  "type": "object",
+  "properties":
+  {
+    "SnapshottableDirectoryList":
+    {
+      "description": "An array of SnapshottableDirectoryStatus",
+      "type"        : "array",
+      "items"       : snapshottableDirectoryStatus,
+      "required"    : true
+    }
+  }
+}
+```
+
+#### SnapshottableDirectoryStatus
+
+JavaScript syntax is used to define `snapshottableDirectoryStatus` so that it can be referred in `SnapshottableDirectoryList` JSON schema.
+
+```javascript
+var snapshottableDirectoryStatus =
+{
+  "type": "object",
+  "properties":
+  {
+    "dirStatus": fileStatusProperties,
+    "parentFullPath":
+    {
+      "description" : "Full path of the parent of snapshottable directory",
+      "type"        : "string",
+      "required"    : true
+    },
+    "snapshotNumber":
+    {
+      "description" : "Number of snapshots created on the snapshottable directory",
+      "type"        : "integer",
+      "required"    : true
+    },
+    "snapshotQuota":
+    {
+      "description" : "Total number of snapshots allowed on the snapshottable directory",
+      "type"        : "integer",
+      "required"    : true
+    }
+  }
+}
+```
+
 HTTP Query Parameter Dictionary
 -------------------------------