windows: fix fs_read with nbufs > 1 and offset

ReadFile() does not seem to update the offset at all.

src/win/fs.c

@@ -557,11 +557,6 @@ void fs__read(uv_fs_t* req) {
 
   if (offset != -1) {
     memset(&overlapped, 0, sizeof overlapped);
-
-    offset_.QuadPart = offset;
-    overlapped.Offset = offset_.LowPart;
-    overlapped.OffsetHigh = offset_.HighPart;
-
     overlapped_ptr = &overlapped;
   } else {
     overlapped_ptr = NULL;
@@ -571,6 +566,13 @@ void fs__read(uv_fs_t* req) {
   bytes = 0;
   do {
     DWORD incremental_bytes;
+
+    if (offset != -1) {
+      offset_.QuadPart = offset + bytes;
+      overlapped.Offset = offset_.LowPart;
+      overlapped.OffsetHigh = offset_.HighPart;
+    }
+
     result = ReadFile(handle,
                       req->bufs[index].base,
                       req->bufs[index].len,
@@ -623,7 +625,6 @@ void fs__write(uv_fs_t* req) {
   do {
     DWORD incremental_bytes;
 
-    /* WriteFile() does not advance overlapped as ReadFile() does. */
     if (offset != -1) {
       offset_.QuadPart = offset + bytes;
       overlapped.Offset = offset_.LowPart;

test/test-fs.c

@@ -109,6 +109,7 @@ static uv_fs_t utime_req;
 static uv_fs_t futime_req;
 
 static char buf[32];
+static char buf2[32];
 static char test_buf[] = "test-buffer\n";
 static char test_buf2[] = "second-buffer\n";
 static uv_buf_t iov;
@@ -2200,12 +2201,15 @@ TEST_IMPL(fs_write_multiple_bufs) {
   uv_fs_req_cleanup(&open_req1);
 
   memset(buf, 0, sizeof(buf));
-  iov = uv_buf_init(buf, sizeof(buf));
-  r = uv_fs_read(loop, &read_req, open_req1.result, &iov, 1, -1, NULL);
+  memset(buf2, 0, sizeof(buf2));
+  /* Read the strings back to separate buffers. */
+  iovs[0] = uv_buf_init(buf, sizeof(test_buf));
+  iovs[1] = uv_buf_init(buf2, sizeof(test_buf2));
+  r = uv_fs_read(loop, &read_req, open_req1.result, iovs, 2, 0, NULL);
   ASSERT(r >= 0);
   ASSERT(read_req.result >= 0);
-  ASSERT(memcmp(buf, test_buf, sizeof(test_buf)) == 0);
-  ASSERT(strcmp(buf + sizeof(test_buf), test_buf2) == 0);
+  ASSERT(strcmp(buf, test_buf) == 0);
+  ASSERT(strcmp(buf2, test_buf2) == 0);
   uv_fs_req_cleanup(&read_req);
 
   iov = uv_buf_init(buf, sizeof(buf));