util-lib: optionally, when writing a string to a file, verify string on failure

With this change, the idiom:

    r = write_string_file(p, buf, 0);
    if (r < 0) {
           if (verify_one_line_file(p, buf) > 0)
                   r = 0;
    }

gets reduced to:

    r = write_string_file(p, buf, WRITE_STRING_FILE_VERIFY_ON_FAILURE);

i.e. when writing the string fails and the new flag
WRITE_STRING_FILE_VERIFY_ON_FAILURE is specified we'll not return a
failure immediately, but check the contents of the file. If it matches
what we wanted to write we suppress the error and exit cleanly.

src/test/test-fileio.c

@@ -363,6 +363,26 @@ static void test_write_string_file_no_create(void) {
         unlink(fn);
 }
 
+static void test_write_string_file_verify(void) {
+        _cleanup_free_ char *buf = NULL, *buf2 = NULL;
+        int r;
+
+        assert_se(read_one_line_file("/proc/cmdline", &buf) >= 0);
+        assert_se((buf2 = strjoin(buf, "\n", NULL)));
+
+        r = write_string_file("/proc/cmdline", buf, 0);
+        assert_se(r == -EACCES || r == -EIO);
+        r = write_string_file("/proc/cmdline", buf2, 0);
+        assert_se(r == -EACCES || r == -EIO);
+
+        assert_se(write_string_file("/proc/cmdline", buf, WRITE_STRING_FILE_VERIFY_ON_FAILURE) == 0);
+        assert_se(write_string_file("/proc/cmdline", buf2, WRITE_STRING_FILE_VERIFY_ON_FAILURE) == 0);
+
+        r = write_string_file("/proc/cmdline", buf, WRITE_STRING_FILE_VERIFY_ON_FAILURE|WRITE_STRING_FILE_AVOID_NEWLINE);
+        assert_se(r == -EACCES || r == -EIO);
+        assert_se(write_string_file("/proc/cmdline", buf2, WRITE_STRING_FILE_VERIFY_ON_FAILURE|WRITE_STRING_FILE_AVOID_NEWLINE) == 0);
+}
+
 static void test_load_env_file_pairs(void) {
         char fn[] = "/tmp/test-load_env_file_pairs-XXXXXX";
         int fd;
@@ -419,6 +439,7 @@ int main(int argc, char *argv[]) {
         test_write_string_stream();
         test_write_string_file();
         test_write_string_file_no_create();
+        test_write_string_file_verify();
         test_load_env_file_pairs();
 
         return 0;