rofiles: Fix --copyup when creating a new file

This tripped up the `docbook-dtds` `%post` in my experiments
with doing rpm-ostree for buildroots.

I cloned and built [xfstests](https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git)
but haven't yet investigated actually running it.

In the meantime let's do the obvious fix here; we need to distinguish
between "copyup enabled" and "actually did a copyup" in the open path
at least, since if we didn't do a copyup we don't need to re-open.

Closes: #1396
Approved by: jlebon

tests/test-rofiles-fuse.sh

@@ -121,19 +121,54 @@ echo "ok flock"
 
 # And now with --copyup enabled
 
-fusermount -u ${test_tmpdir}/mnt
-assert_not_has_file mnt/firstfile
-rofiles-fuse --copyup checkout-test2 mnt
+copyup_reset() {
+    cd ${test_tmpdir}
+    fusermount -u mnt
+    rm checkout-test2 -rf
+    $OSTREE checkout -H test2 checkout-test2
+    rofiles-fuse --copyup checkout-test2 mnt
+}
+
+assert_test_file() {
+    t=$1
+    f=$2
+    if ! test ${t} "${f}"; then
+        ls -al "${f}"
+        fatal "Failed test ${t} ${f}"
+    fi
+}
+
+copyup_reset
 assert_file_has_content mnt/firstfile first
 echo "ok copyup mount"
 
+# Test O_TRUNC directly
 firstfile_orig_inode=$(stat -c %i checkout-test2/firstfile)
-for path in firstfile{,-link}; do
-    echo truncating > mnt/${path}
-    assert_file_has_content mnt/${path} truncating
-    assert_not_file_has_content mnt/${path} first
-done
+echo -n truncating > mnt/firstfile
+assert_streq "$(cat mnt/firstfile)" truncating
 firstfile_new_inode=$(stat -c %i checkout-test2/firstfile)
 assert_not_streq "${firstfile_orig_inode}" "${firstfile_new_inode}"
+assert_test_file -f checkout-test2/firstfile
+
+copyup_reset
+firstfile_link_orig_inode=$(stat -c %i checkout-test2/firstfile-link)
+firstfile_orig_inode=$(stat -c %i checkout-test2/firstfile)
+# Now write via the symlink
+echo -n truncating > mnt/firstfile-link
+assert_streq "$(cat mnt/firstfile)" truncating
+firstfile_new_inode=$(stat -c %i checkout-test2/firstfile)
+firstfile_link_new_inode=$(stat -c %i checkout-test2/firstfile-link)
+assert_not_streq "${firstfile_orig_inode}" "${firstfile_new_inode}"
+assert_streq "${firstfile_link_orig_inode}" "${firstfile_link_new_inode}"
+assert_test_file -f checkout-test2/firstfile
+# Verify we didn't replace the link with a regfile somehow
+assert_test_file -L checkout-test2/firstfile-link
+
+# These both end up creating new files; in the sed case we'll then do a rename()
+copyup_reset
+echo "hello new file" > mnt/a-new-non-copyup-file
+assert_file_has_content_literal mnt/a-new-non-copyup-file "hello new file"
+sed -i -e s,first,second, mnt/firstfile
+assert_file_has_content_literal mnt/firstfile "second"
 
 echo "ok copyup"