copy: rework copy_file_atomic() to copy the specified file via O_TMPFILE if possible

src/test/test-copy.c

@@ -240,7 +240,27 @@ static void test_copy_bytes_regular_file(const char *src, bool try_reflink, uint
         unlink(fn3);
 }
 
+static void test_copy_atomic(void) {
+        _cleanup_(rm_rf_physical_and_freep) char *p = NULL;
+        const char *q;
+        int r;
+
+        assert_se(mkdtemp_malloc(NULL, &p) >= 0);
+
+        q = strjoina(p, "/fstab");
+
+        r = copy_file_atomic("/etc/fstab", q, 0644, 0, COPY_REFLINK);
+        if (r == -ENOENT)
+                return;
+
+        assert_se(copy_file_atomic("/etc/fstab", q, 0644, 0, COPY_REFLINK) == -EEXIST);
+
+        assert_se(copy_file_atomic("/etc/fstab", q, 0644, 0, COPY_REPLACE) >= 0);
+}
+
 int main(int argc, char *argv[]) {
+        log_set_max_level(LOG_DEBUG);
+
         test_copy_file();
         test_copy_file_fd();
         test_copy_tree();
@@ -251,6 +271,7 @@ int main(int argc, char *argv[]) {
         test_copy_bytes_regular_file(argv[0], true, 1000);
         test_copy_bytes_regular_file(argv[0], false, 32000); /* larger than copy buffer size */
         test_copy_bytes_regular_file(argv[0], true, 32000);
+        test_copy_atomic();
 
         return 0;
 }