vfs: Don't error a r/w file open without cache; delay error until Read called

If we open a file for r/w without the cache we now always return a
handle and return an error if the file is ever read from.  This fixes
incompatibility with cmount under windows.

vfs/file.go

@@ -382,11 +382,11 @@ func (f *File) Open(flags int) (fd Handle, err error) {
 	if read && write {
 		if CacheMode >= CacheModeMinimal {
 			fd, err = f.OpenRW(flags)
-		} else if flags&os.O_TRUNC != 0 {
-			fd, err = f.OpenWrite()
 		} else {
-			fs.Errorf(f, "Can't open for read and write without cache")
-			return nil, EPERM
+			// Open write only and hope the user doesn't
+			// want to read.  If they do they will get an
+			// EPERM plus an Error log.
+			fd, err = f.OpenWrite()
 		}
 	} else if write {
 		if CacheMode >= CacheModeWrites {

vfs/file_test.go

@@ -172,7 +172,9 @@ func TestFileOpen(t *testing.T) {
 	require.NoError(t, fd.Close())
 
 	fd, err = file.Open(os.O_RDWR)
-	assert.Equal(t, EPERM, err)
+	assert.NoError(t, err)
+	_, ok = fd.(*WriteFileHandle)
+	assert.True(t, ok)
 
 	fd, err = file.Open(3)
 	assert.Equal(t, EPERM, err)

vfs/write.go

@@ -248,3 +248,17 @@ func (fh *WriteFileHandle) Truncate(size int64) (err error) {
 	// File is correct size
 	return nil
 }
+
+// Read reads up to len(p) bytes into p.
+func (fh *WriteFileHandle) Read(p []byte) (n int, err error) {
+	fs.Errorf(fh.remote, "Read: Can't read and write to file without cache")
+	return 0, EPERM
+}
+
+// ReadAt reads len(p) bytes into p starting at offset off in the
+// underlying input source. It returns the number of bytes read (0 <=
+// n <= len(p)) and any error encountered.
+func (fh *WriteFileHandle) ReadAt(p []byte, off int64) (n int, err error) {
+	fs.Errorf(fh.remote, "ReadAt: Can't read and write to file without cache")
+	return 0, EPERM
+}

vfs/write_test.go

@@ -56,6 +56,15 @@ func TestWriteFileHandleMethods(t *testing.T) {
 	assert.Equal(t, int64(5), fi.Size())
 	assert.Equal(t, "file1", fi.Name())
 
+	// Read
+	var buf = make([]byte, 16)
+	_, err = fh.Read(buf)
+	assert.Equal(t, EPERM, err)
+
+	// ReadAt
+	_, err = fh.ReadAt(buf, 0)
+	assert.Equal(t, EPERM, err)
+
 	// Close
 	assert.NoError(t, fh.Close())