.

wcfs/wcfs.go

@@ -1062,7 +1062,8 @@ func (f *BigFile) Read(_ nodefs.File, dest []byte, off int64, fctx *fuse.Context
 		end = f.size
 	}
 	if end <= off {
-		// XXX off >= size -> EINVAL? (but when size=0 kernel issues e.g. [0 +4K) read)
+		// the kernel issues e.g. [0 +4K) read for f.size=0 and expects to get (0, ok)
+		// POSIX also says to return 0 if off >= f.size
 		return fuse.ReadResultData(nil), fuse.OK
 	}
 

wcfs/wcfs_test.py

@@ -44,7 +44,7 @@ from zodbtools.util import ashex as h, fromhex
 import pytest; xfail = pytest.mark.xfail
 from pytest import raises, fail
 from six import reraise
-from .internal import mm
+from .internal import io, mm
 from .internal.wcfs_test import read_nogil, install_sigbus_trap, fadvise_dontneed
 
 # setup:
@@ -1248,6 +1248,10 @@ def test_wcfs_basic():
     f.assertCache([0,0,0])  # initially not cached
     f.assertData (['','','c1'], mtime=t.head)
 
+    # read after file size returns (0, ok)
+    # (the same behaviour as on e.g. ext4 and as requested by posix)
+    assert 0 == io.readat(f.f.fileno(), 100*blksize, bytearray(10))
+
     # >>> (@at2) commit again -> we can see both latest and snapshotted states
     # NOTE blocks d(4) and f(5) will be accessed only in the end
     at2 = t.commit(zf, {2:'c2', 3:'d2', 5:'f2'})