From: Daira Hopwood Date: Fri, 22 May 2015 23:26:53 +0000 (+0100) Subject: Work in progress: use hard link technique to prevent Earth Dragon data loss on Unix... X-Git-Url: https://git.rkrishnan.org/specifications/components/cyclelanguage?a=commitdiff_plain;h=53c648758e2e53071e3396224abacfd5f7d42b6b;p=tahoe-lafs%2Ftahoe-lafs.git Work in progress: use hard link technique to prevent Earth Dragon data loss on Unix; ReplaceFileW on Windows. Signed-off-by: Daira Hopwood --- diff --git a/docs/proposed/magic-folder/remote-to-local-sync.rst b/docs/proposed/magic-folder/remote-to-local-sync.rst index eeabc15c..29848021 100644 --- a/docs/proposed/magic-folder/remote-to-local-sync.rst +++ b/docs/proposed/magic-folder/remote-to-local-sync.rst @@ -252,6 +252,7 @@ we classified various problems as "dragons", which as a convenient mnemonic we have named after the five classical Greek elements (Earth, Air, Water, Fire and Aether). + *Glossary* Write: a modification to a local filesystem object by a client @@ -261,50 +262,173 @@ Download: a download from the Tahoe-LAFS file store to a local object Pending notification: a local filesystem change that has been detected but not yet processed. -*Earth Dragons: write/download and read/download collisions* -Suppose that Alice changes the file ``foo`` locally, concurrently -with Alice's Magic Folder client writing a version of ``foo`` that -it has downloaded in response to a remote change. +*Earth Dragons: Write/download and read/download collisions* + +Suppose that Alice's Magic Folder client is about to write a +version of ``foo`` that it has downloaded in response to a remote +change. + +The criteria for distinguishing overwrites from conflicts are +described later in the `Fire Dragons`_ section. For now, suppose +that the remote change has been tentatively classified as an +overwrite. (As we will see below, it may be reclassified in some +circumstances.) + +.. _`Fire Dragons`: `Fire Dragons: Distinguishing conflicts from overwrites`_ + +An "write/download" conflict occurs when another program writes +to ``foo`` in the local filesystem, concurrently with the new +version being written by the Magic Folder client. We need to +ensure that this does not cause data loss, as far as possible. An important constraint on the design is that on Windows, it is not possible to rename a file to the same name as an existing -file in that directory. - -Without this constraint, it would be possible to use the following -common technique for implementing "atomic" writes on Unix: - -* Alice's Magic Folder client writes a temporary file ``foo.tmp`` -* if 'foo' is clean, i.e. there are no pending notifications, it moves -foo.tmp over foo [FIXME: if we want to preserve old versions then it -should rename the old version first; see below] -there is a race condition where the local write notification occurs -concurrently with the move, in which case we may clobber the local write. -it is impossible to detect this (even after the fact) because we can't -distinguish whether the notification was for the move or for the local -write. -(assertion: the type of event doesn't help, because the local write may -also be a move --in fact should be for a maximally well-behaved app-- -and a move event doesn't include the from filename. also Windows which -doesn't support atomic move-onto.) -this race has a small window (milliseconds or less) - -OR: alice's gateway -* writes a temporary file foo.new -* if 'foo' is clean, i.e. there are no pending notifications, it moves -foo to foo.old and then foo.new to foo -(this would work on Windows; note that the rename to foo.old will fail if -the file is locked for writing. We should probably handle that case as a -conflict.) - -TODO: on Unix, what happens with reference to inotify events if we rename a file while -it is open? Does the filename for the CLOSE_WRITE event reflect the new -name? - -did the notification event for the local change precede the write? - - -air dragons: write/upload collisions +file in that directory. Also, on Windows it may not be possible to +delete or rename a file that has been opened by another program +(depending on the sharing flags specified by that program). +Therefore we need to consider carefully how to handle failure +conditions. + +Our proposed design is as follows: + +1. Alice's Magic Folder client writes a temporary file, say + ``.foo.tmp``. +2. If there are pending notifications of changes to ``foo``, + reclassify as a conflict and stop. +3. Perform a ''file replacement'' operation (see below) + with backup filename ``foo.old``, replaced file ``foo``, + and replacement file ``.foo.tmp``. If any step of this + operation fails, reclassify as a conflict and stop. + +The implementation of file replacement differs between +Windows and Unix. On Windows, it can be implemented as a +single call to the `ReplaceFileW`_ API (with the +``REPLACEFILE_IGNORE_MERGE_ERRORS`` flag). + +Note that ReplaceFileW is not atomic. The effect of this call +is to first move ``foo`` to ``foo.old``, then move ``.foo.tmp`` +to ``foo``. It is possible for there to be a failure between +these two moves, in which case the call will fail with return +code ``ERROR_UNABLE_TO_MOVE_REPLACEMENT_2``. However, it is +still preferable to use this API over two `MoveFileExW`_ calls, +because it retains the attributes and ACLs of ``foo`` where +possible. + +.. _`ReplaceFileW`: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365512%28v=vs.85%29.aspx +.. _`MoveFileExW`: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365240%28v=vs.85%29.aspx + +On Unix, file replacement can be implemented as follows: + +a. Set the permissions of the replacement file to be the + same as the replaced file, bitwise-or'd with octal 600 + (``rw-------``), and set its ``mtime`` to be *T* seconds + before the current time (see below for further explanation). +b. Attempt to move the replaced file (``foo``) to the + backup filename (``foo.old``). +c. Attempt to create a hard link at the replaced filename + (``foo``) pointing to the replacement file (``.foo.tmp``). +d. Attempt to unlink the replacement file (``.foo.tmp``), + suppressing errors. + +To reclassify as a conflict, attempt to rename ``.foo.tmp`` to +``foo.conflicted``, suppressing errors. + +Note that, if there is no conflict, the entry for ``foo`` +recorded in the `magic folder db`_ will reflect the ``mtime`` +set in step a. The link in step c will cause an ``IN_CREATE`` +event for ``foo``, but this will not trigger an upload, +because the metadata recorded in the database entry will +exactly match the metadata for the file's inode on disk. +(The two hard links -- ``foo`` and, while it still exists, +``.foo.tmp`` -- share the same inode and therefore the same +metadata.) + +.. _`magic folder db`: `filesystem_integration.rst#Local scanning and database`_ + +[TODO: on Unix, what happens with reference to inotify events if we +rename a file while it is open? Does the filename for the ``CLOSE_WRITE`` +event reflect the new name?] + +To determine whether this procedure adequately protects against data +loss, we need to consider what happens if another process has ``foo`` +open for writing: + +* On Unix, open file handles refer to inodes, not paths. When the other + program closes the file, changes will have been written to the file + at the same inode, now linked at ``foo.old``. This avoids data loss. + +* On Windows, we have two subcases, depending on whether the sharing + flags specified by the other process when it opened its file handle + included ``FILE_SHARE_DELETE``. (This flag covers both deletion and + rename operations.) + i. If the sharing flags *do not* allow deletion/renaming, the + `ReplaceFileW`_ operation will fail without renaming ``foo``. + In this case we will end up with ``foo`` changed by the other + process, and the downloaded file still in ``foo.tmp``. + This avoids data loss. + ii. If the sharing flags *do* allow deletion/renaming, then + data loss or corruption may occur. This is unavoidable and + can be attributed to other process making a poor choice of + sharing flags (either explicitly if it used `CreateFile`_, or + via whichever higher-level API it used). + +.. _`CreateFile`: https://msdn.microsoft.com/en-us/library/windows/desktop/aa363858%28v=vs.85%29.aspx + +[TODO: on Windows, what is the default sharing of a file opened for +writing by _open/_wopen?] + +We also need to consider what happens if another process attempts to +update ``foo`` by renaming another file, say ``foo.other``, onto it. +Again this differs between Windows and Unix: + +On Unix, we need to consider all possible interleavings between the +operations performed by the Magic Folder client and the other process. +(Note that atomic operations on a directory are totally ordered.) + +* Interleaving 1a: the other process' rename precedes our rename in + step b, and we get an ``IN_MOVED_TO`` event for its rename before + we do ours. Then we reclassify as a conflict; its changes end up + at ``foo`` and ours end up at ``foo.conflicted``. This avoids + data loss. +* Interleaving 1b: its rename precedes ours in step b, and we do + not get an ``IN_MOVED_TO`` event for its rename before ours. Its + changes end up at ``foo.old`` and ours end up at ``foo``. This + avoids data loss. +* Interleaving 2: its rename happens between our rename in step b, + and our link operation in step c of the file replacement. The + latter fails with an ``EEXIST`` error because ``foo`` already + exists. We reclassify as a conflict; the old version ends up at + ``foo.old``, the other process' changes end up at ``foo``, and + ours at ``foo.conflicted``. This avoids data loss. +* Interleaving 3: its rename happens after our link in step c, and + causes an ``IN_MOVED_TO`` event for ``foo``. Its rename also changes + the ``mtime`` for ``foo`` so that it is different from the ``mtime`` + calculated in step a, and therefore different from the metadata + recorded for ``foo`` in the magic folder db. (Assuming no system + clock changes, its rename will set an ``mtime`` timestamp + corresponding to a time after step c, which is not equal to the + timestamp *T* seconds before step a, provided that *T* seconds + is sufficiently greater than the timestamp granularity.) + Therefore, an upload will be triggered for ``foo`` after its change, + which is correct and avoids data loss. + +Note that it is possible that another process tries to open the file +between steps b and c. In this case the open will fail because ``foo`` +does not exist. Nevertheless, no data will be lost. (Probably, the user +will be able to retry the operation.) + +Above we have considered only interleavings with a single other process, +and only the most common possibilities for the other process' interaction +with the file. If multiple other processes are involved, or if a process +performs operations other than those considered, then we cannot say much +about the outcome in general; however, we believe that such cases will be +much rarer. + +[TODO: discuss read/download collisions] + + +*Air Dragons: write/upload collisions* we can't read a file atomically. therefore, when we read a file in order to upload it, we may read an inconsistent version if it was also being @@ -332,7 +456,7 @@ we have implemented the pending delay but we will not implement the abort/re-upload for the OTF grant -fire dragons: distinguishing conflicts from overwrites +*Fire Dragons: Distinguishing conflicts from overwrites* alice sees a change by bob to 'foo' and needs to know whether that change is an overwrite or a conflict @@ -368,7 +492,7 @@ filesystem notifications for filenames that match the conflicted pattern are ignored -water dragons: resolving conflict loops +*Water Dragons: Resolving conflict loops* suppose that we've detected a remote write to file 'foo' that conflicts with a local write @@ -414,7 +538,7 @@ conflict loop that alice has seen it) -aether dragons: handling renames +*Aether Dragons: Handling renames* suppose that a subfolder of the Magic Folder is renamed on one of the Magic Folder clients. it is not clear how to handle this at all: @@ -435,11 +559,8 @@ treat the move event as a directory creation, and also pretend that there has been a modification of the directory at the old name by all other Magic Folder clients. this is the easiest option to implement. -other design issues: + +*Other design issues* + * choice of conflicted filenames (e.g. foo.by_bob_at_YYYYMMDD_HHMMSS[v].type) - -[*] the association of dragons with the classical Greek elements -admittedly owes more to modern fantasy gaming than historically or -culturally accurate dragon mythology. consider them just as codenames for -now