-*- Text -*- Content ======= * Context * Issue Description * Pre-Resolution State of Affairs - Single platform - Multi-platform: Windows + MacOS X * Proposed Support Library - Assumptions - Options * Proposed Normal Form * Possible Solutions - Normalization of path-input on MacOS X - Normalization of path-input everywhere - Comparison routines (client side) - Comparison routines (everywhere) * Short Term (ie before 2.0) solution * Long Term Solution (ie 2.0+) * Additional Information * References Context ======= Within Unicode, some characters - with diacritical marks - can be represented in 2 forms: Normal Form Composed (NFC) or Normal Form Decomposed (NFD). A string of unicode characters can contain any mixture of both forms. This problem explicitly does not concern itself with invisible characters, spaces or other characters unlikely to be present in filenames. Please note that this issue is explicitly excluding NFKC/NFKD (compatibility) normal forms, because they remove for example formatting (meaning they are lossy?). Because there are 2 forms for representing (some) characters in Unicode, it's possible to produce different sequences of codepoints meaning to indicate the same sequence of characters [1]. UTF-8, the internal Unicode encoding of choice for Subversion, encodes codepoints in (a series of) bytes (octets). Because the sequences of codepoints specifying a character may differ, so may the resulting UTF-8. Hence, we end up with more than one way to specify the same path. The following table specifies behaviour of OSes related to handling of Unicode filenames: OS Accepts Gives back ---------- ------- ---------- MacOS X[2] all NFD* Linux all Windows all Others ? ? *) There are some remarks to be made regarding full or partial NFD here, but the essential thing is: if you send in NFC, don't expect it back! Issue Description ================= From the above issue description, two problems follow: First, we can't generally depend on the OS to give us back the exact filename we gave it. This is mainly a client side issue, something which might be resolved in the client side libraries (client/subr/wc). Secondly, the same filename may be encoded in different codepoints. This issue is much broader than the first, especially given the fact that we already have lots of populated repositories "out there". We cannot depend on a filename coming from the operating system -- even though different from the one in the repository -- to name a different file. This has repository (ie. server-side) impact. Pre-Resolution State of Affairs =============================== This section serves to describe the problems to be expected in different combinations of client/server OSes. As indicated in the table in the context section, Linux and Windows are expected to behave equally. This section therefor leaves out the consideration of Linux as a separate system. The platforms below are strictly client side: the server side problems mentioned in the issue description section solely relates to the repository, which can be located at any server platform. Single platform --------------- This can be multiple MacOSX machines or multiple Windows machines. In this scenario, no interoperability problems are to be expected. Multi-platform: Windows + MacOSX -------------------------------- Consider a filename which contains one or more precomposed (NFC) characters being committed from Windows. When the MacOSX developer updates, a file is written in NFC form, but as stated in the context section, Mac recodes that to NFD. Now, when comparing what comes from the disk (NFD) with what's in the entries file (NFC), results in a missing file (the NFC encoded one) and an unversioned file (the NFD encoded one). Both of the filenames look exactly the same to the person reading the Subversion output on the screen. [==> confusion!] Committing a file the other way around might be less problematic, since Windows is capable of storing NFD filenames. Proposed Support Library ======================== Assumptions ----------- The main assumption is that we'll keep using APR for character set conversion, meaning that the recoding solution to choose would not need to provide any other functionality than recoding. Options ------- There are two options (that I'm aware of [dionisos]) for choosing a library which supports the required functionality: 1) International Component for Unicode (ICU)[3] -- a library with a very wide range of targeted functions, but with a memory footprint to match. In order to be able to use it, we'd need to trim this library down significantly. 2) utf8proc -- a library for processing UTF-8 encoded unicode strings. A library specifically targeted at a limited number of operations to be performed on UTF-8 encoded strings. It consists of two .c and a single .h file, with a total source size of 1MB (compiled less than 0.5MB). From these two, under the given assumption, it only makes sense to use utf8proc. Proposed Normal Form ==================== The proposed internal 'normal form' should be NFC, if only if it were because it's the most compact form of the two: when allocating memory to store a conversion result, it won't be necessary (ever) to allocate more than the size of the input buffer. This would give the maximum performance from utf8proc, which requires two recoding runs when the buffer is too small: one to retrieve the required buffer size, the second to actually store the result. Possible Solutions ================== Several options are available for resolution of this problem, each with its pros and cons, to be outlined below. 1) Normalization of (path) input on MacOSX. Since the Mac seems to be the only platform which mutilates its pathname input to be NFD, this seems like a logical (low impact) solution. 2) Normalization of (path) input on all platforms. Since paths can't differ only in encoding if we standardize on encoding, this seems like a logical (relatively low) impact solution. 3) Normalization of path input in the client and server. On the server side, non-normalized paths may have become part of the repository. We can achieve full in-memory standardization by converting any path coming from the repository as well as the client. 4) Client and server-side path comparison routines. Because paths read from the repository may be used to access said repository, possibly by calculating hash values, paths from can't be munged (repository-side). To eliminate the effect, we acknowledge we're not going to be 'clean': we'll always need path comparison routines. Solution (1) has a very strong CON: it will break all pre-existing MacOSX-only workshops. Consider a client which starts sending NFC encoded paths in an environment where all paths have been NFD encoded until that time - without proper support in the server. This would result in commits with NFC encoded paths to files for which the path in the repository is NFD encoded: breakage. Solution (2) has the same problem as solution (1) on MacOSX, but on the upside it prevents new NFD paths from entering into the repository (for sufficiently broad definitions of 'client' [think mod_dav_svn]). As already stated, solution (3) may prevent paths from being found, if the retrieval mechanism is hash-based. Meaning this could break any repository backend using hashing to store information about paths. (Don't we store locks in FSFS based on hashing?) Solution (4) defines no internal standard representation, assuming it's not possible to maintain a clean in-memory state, given all problems found in the earlier solutions. Instead, it requires all path comparisons to be performed using special NFC/NFD encoding aware functions. Short Term Solution =================== Because of our interoperability guarantees, the client and server should be considered separate universes, each of which can use its own (internal) solution. However, the client should at all times use the exact path the server sent it. The same applies the other way around. Given the above, the short term (before 2.0) solution should be to use path comparison routines as stated in solution (4). Long Term Solution ================== The long term (2.0+) solution would be to use option (2), which ensures recoding of all input paths into the 'normal' normal form (NFC). In that case, it'll no longer require the use of specialised path comparison routines (although that might still be desired for other design considerations). Short Term Solution Implementation Consequences =============================================== As stated before, since we don't know whether the other side of the equation might be a pre-normalization-aware client or server until we break backward compat in 2.0, the client and server should be able to talk backward compatibly with a pre-NF-aware 'other side'. Hence, solving this problem means considering the client and the server separate universes, each of which can employ its own internal solution. Implementing option (4) means: A. Comparing file names with entry paths using NFC/NFD aware comparison functions. Then, when there's a match, *use the pathname from the entries file* to communicate with the server; after all, the path might have been added with a different encoding than we got back from the disk. B. Match working copy paths with entries-file paths using NFC/NFD aware comparison functions. On a match, use the entries-file path to communicate with the server. The above means the client has to be very careful to preserve the encoding from the server and use that when talking to the server otherwise the server may not recognize the path as a versioned entity. Locally however, we can't be sure the filesystem enforces the encoding the server sent to the client, meaning there are (contrived) cases where a file exists in a different encoding locally than in the repository. Which means we have to be very careful about how we find our files and to use the encoding we got from the local filesystem. Implementation details: * The hash keys in svn_wc_adm_access_t's are hashed on the normalized path encoding, not the repository path, in order to be able to calculate the hash key from both the wc path as well as the repo path. * The same line of reasoning applies to the hash keys in the entries hash. New conventions: * Variables containing a path as encoded in the local filesystem should contain the (sub)string 'wc_path'. * Variables containing a path as encoded in the repository should contain the (sub)string 'repo_path'. Additional Information ====================== * "UTF-8 NFC/NFD paths issue" dev@ mailing list thread: http://svn.haxx.se/dev/archive-2010-09/0319.shtml References ========== 1) UAX #15: Unicode normalization forms http://unicode.org/reports/tr15/ 2) Apple Technical Q&A: Path encodings in VFS http://developer.apple.com/qa/qa2001/qa1173.html 3) ICU - International Component for Unicode http://www-306.ibm.com/software/globalization/icu/index.jsp 4) utf8proc - a library targeted at processing UTF-8 encoded unicode strings http://www.flexiguided.de/publications.utf8proc.en.html