CSHARP-1993: Fix invalid seek in GridFS, more tests added

Author: Anatoliy Koperin

src/MongoDB.Driver.GridFS/GridFSSeekableDownloadStream.cs

@@ -125,13 +125,17 @@ public override long Seek(long offset, SeekOrigin origin)
             {
                 case SeekOrigin.Begin: newPosition = offset; break;
                 case SeekOrigin.Current: newPosition = _position + offset; break;
-                case SeekOrigin.End: newPosition = _position + offset; break;
+                case SeekOrigin.End: newPosition = FileInfo.Length + offset; break;
                 default: throw new ArgumentException("Invalid origin.", "origin");
             }
             if (newPosition < 0)
             {
                 throw new IOException("Position must be greater than or equal to zero.");
             }
+            if (FileInfo.Length <= newPosition)
+            {
+                throw new IOException("Position must be less than to length of stream.");
+            }
             Position = newPosition;
             return newPosition;
         }

tests/MongoDB.Driver.GridFS.Tests/GridFSSeekableDownloadStreamTests.cs

@@ -191,28 +191,47 @@ public void Read_should_throw_when_offset_is_invalid(int bufferLength, int offse
 
         [Theory]
         [InlineData(0, 1, SeekOrigin.Begin, 1)]
-        [InlineData(1, 1, SeekOrigin.Current, 2)]
-        [InlineData(2, -1, SeekOrigin.End, 1)]
+        [InlineData(5, 1, SeekOrigin.Begin, 1)]
+        [InlineData(9, 1, SeekOrigin.Begin, 1)]
+        [InlineData(0, 1, SeekOrigin.Current, 1)]
+        [InlineData(5, 1, SeekOrigin.Current, 6)]
+        [InlineData(8, 1, SeekOrigin.Current, 9)]
+        [InlineData(0, -1, SeekOrigin.End, 9)]
+        [InlineData(5, -1, SeekOrigin.End, 9)]
+        [InlineData(9, -1, SeekOrigin.End, 9)]
         public void Seek_should_return_expected_result(
             long position,
             long offset,
             SeekOrigin origin,
             long expectedResult)
         {
-            var subject = CreateSubject();
+            var subject = CreateSubject(10);
             subject.Position = position;
 
             var result = subject.Seek(offset, origin);
 
             result.Should().Be(expectedResult);
         }
 
-        [Fact]
-        public void Seek_should_throw_when_new_position_is_negative()
+        [Theory]
+        [InlineData(0, -1, SeekOrigin.Begin)]
+        [InlineData(5, -1, SeekOrigin.Begin)]
+        [InlineData(9, 10, SeekOrigin.Begin)]
+        [InlineData(0, -1, SeekOrigin.Current)]
+        [InlineData(5, -6, SeekOrigin.Current)]
+        [InlineData(8, 3, SeekOrigin.Current)]
+        [InlineData(0, 0, SeekOrigin.End)]
+        [InlineData(5, 1, SeekOrigin.End)]
+        [InlineData(9, -11, SeekOrigin.End)]
+        public void Seek_should_throw_when_new_position_is_out_of_range(
+            long position,
+            long offset,
+            SeekOrigin origin)
         {
-            var subject = CreateSubject();
+            var subject = CreateSubject(10);
+            subject.Position = position;
 
-            Action action = () => subject.Seek(-1, SeekOrigin.Begin);
+            Action action = () => subject.Seek(offset, origin);
 
             action.ShouldThrow<IOException>();
         }