Report io.EOF from UnionFile.ReadAt

[youdie006]

UnionFile.ReadAt returns a nil error on a short read, so callers that rely on io.EOF to stop
never do.

io.ReaderAt, verbatim from $GOROOT/src/io/io.go:210:

When ReadAt returns n < len(p), it returns a non-nil error explaining why more bytes were not
returned. In this respect, ReadAt is stricter than Read.

Same calls through three implementations, 10-byte file:

                  ReadAt(buf[8], 6)      ReadAt(buf[8], 10)
os.File           n=4 err=EOF            n=0 err=EOF
afero mem.File    n=4 err=EOF            n=0 err=EOF
afero UnionFile   n=4 err=<nil>          n=0 err=<nil>

The (0, nil) case is what bites in practice. io.NewSectionReader — the natural thing to write
when the exact size is not known — cannot terminate:

os.File          terminated after 3 calls
afero mem.File   terminated after 3 calls
afero UnionFile  still going at 200 calls

Both of the other columns are afero's own code, so this is not an os-vs-mem difference.

Cause

unionFile.go:70:

n, err := f.Layer.ReadAt(s, o)
if (err == nil || err == io.EOF) && f.Base != nil {
    _, err = f.Base.Seek(o+int64(n), io.SeekStart)
}
return n, err

The assignment replaces the layer's io.EOF with the seek's nil. Read, sixteen lines above, gets
this right and carries the comment saying why:

if _, seekErr := f.Base.Seek(int64(n), io.SeekCurrent); seekErr != nil {
    // only overwrite err in case the seek fails: we need to
    // report an eventual io.EOF to the caller
    err = seekErr
}

Change

Five lines, mirroring Read. A test is added to composite_test.go covering the short read and the
read at EOF. Red with only unionFile.go reverted:

composite_test.go:537: ReadAt short read = (4, <nil>), want (4, EOF)
composite_test.go:540: ReadAt at EOF = (0, <nil>), want (0, EOF)

Green with the change; go test ./... passes across all packages and gofmt -l lists neither file.

Reachable through the public API via NewCacheOnReadFs(base, layer, 0).OpenFile(name, os.O_RDWR, perm), which returns a *UnionFile (cacheOnReadFs.go:233).

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Disclosure: prepared with AI assistance; I verified the three-way comparison, the section-reader
behaviour and the red/green runs myself.

[CLAassistant]

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