ref: add basic symref content check for files backend

We have code that checks regular ref contents, but we do not yet check
the contents of symbolic refs. By using "parse_loose_ref_content" for
symbolic refs, we will get the information of the "referent".

We do not need to check the "referent" by opening the file. This is
because if "referent" exists in the file system, we will eventually
check its correctness by inspecting every file in the "refs" directory.
If the "referent" does not exist in the filesystem, this is OK as it is
seen as the dangling symref.

So we just need to check the "referent" string content. A regular ref
could be accepted as a textual symref if it begins with "ref:", followed
by zero or more whitespaces, followed by the full refname, followed only
by whitespace characters. However, we always write a single SP after
"ref:" and a single LF after the refname. It may seem that we should
report a fsck error message when the "referent" does not apply above
rules and we should not be so aggressive because third-party
reimplementations of Git may have taken advantage of the looser syntax.
Put it more specific, we accept the following contents:

1. "ref: refs/heads/master   "
2. "ref: refs/heads/master   \n  \n"
3. "ref: refs/heads/master\n\n"

When introducing the regular ref content checks, we created two fsck
infos "refMissingNewline" and "trailingRefContent" which exactly
represents above situations. So we will reuse these two fsck messages to
write checks to info the user about these situations.

But we do not allow any other trailing garbage. The followings are bad
symref contents which will be reported as fsck error by "git-fsck(1)".

1. "ref: refs/heads/master garbage\n"
2. "ref: refs/heads/master \n\n\n garbage  "

And we introduce a new "badReferentName(ERROR)" fsck message to report
above errors by using "is_root_ref" and "check_refname_format" to check
the "referent". Since both "is_root_ref" and "check_refname_format"
don't work with whitespaces, we use the trimmed version of "referent"
with these functions.

In order to add checks, we will do the following things:

1. Record the untrimmed length "orig_len" and untrimmed last byte
   "orig_last_byte".
2. Use "strbuf_rtrim" to trim the whitespaces or newlines to make sure
   "is_root_ref" and "check_refname_format" won't be failed by them.
3. Use "orig_len" and "orig_last_byte" to check whether the "referent"
   misses '\n' at the end or it has trailing whitespaces or newlines.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions