fix(FileSystemApi): read file with offset and length

Author: richardschneider

src/CoreApi/FileSystemApi.cs

@@ -182,8 +182,17 @@ internal FileSystemApi(IpfsClient ipfs)
 
         public Task<Stream> ReadFileAsync(string path, long offset, long length = 0, CancellationToken cancel = default(CancellationToken))
         {
-            // TODO: length is not yet supported by daemons
-            return ipfs.DownloadAsync("cat", cancel, path, $"offset={offset}");
+            // https://github.com/ipfs/go-ipfs/issues/5380
+            if (offset > int.MaxValue)
+                throw new NotSupportedException("Only int offsets are currently supported.");
+            if (length > int.MaxValue)
+                throw new NotSupportedException("Only int lengths are currently supported.");
+
+            if (length == 0)
+                length = int.MaxValue; // go-ipfs only accepts int lengths
+            return ipfs.DownloadAsync("cat", cancel, path, 
+                $"offset={offset}",
+                $"length={length}");
         }
 
         /// <summary>

test/CoreApi/FileSystemApiTest.cs

@@ -58,6 +58,35 @@ public void Read_With_Offset()
             using (var outdata = ipfs.FileSystem.ReadFileAsync(node.Id, offset: 1).Result)
             {
                 Assert.AreEqual(20, outdata.ReadByte());
+                Assert.AreEqual(30, outdata.ReadByte());
+                Assert.AreEqual(-1, outdata.ReadByte());
+            }
+        }
+
+        [TestMethod]
+        public void Read_With_Offset_Length_1()
+        {
+            var ipfs = TestFixture.Ipfs;
+            var indata = new MemoryStream(new byte[] { 10, 20, 30 });
+            var node = ipfs.FileSystem.AddAsync(indata).Result;
+            using (var outdata = ipfs.FileSystem.ReadFileAsync(node.Id, offset: 1, count: 1).Result)
+            {
+                Assert.AreEqual(20, outdata.ReadByte());
+                Assert.AreEqual(-1, outdata.ReadByte());
+            }
+        }
+
+        [TestMethod]
+        public void Read_With_Offset_Length_2()
+        {
+            var ipfs = TestFixture.Ipfs;
+            var indata = new MemoryStream(new byte[] { 10, 20, 30 });
+            var node = ipfs.FileSystem.AddAsync(indata).Result;
+            using (var outdata = ipfs.FileSystem.ReadFileAsync(node.Id, offset: 1, count: 2).Result)
+            {
+                Assert.AreEqual(20, outdata.ReadByte());
+                Assert.AreEqual(30, outdata.ReadByte());
+                Assert.AreEqual(-1, outdata.ReadByte());
             }
         }