From: Daira Hopwood Date: Thu, 28 May 2015 23:14:44 +0000 (+0100) Subject: More reordering. X-Git-Url: https://git.rkrishnan.org/pf/vdrive?a=commitdiff_plain;h=6be4464a4419b13e9200570d44a3952066d79719;p=tahoe-lafs%2Ftahoe-lafs.git More reordering. 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 bde8485a..e185c509 100644 --- a/docs/proposed/magic-folder/remote-to-local-sync.rst +++ b/docs/proposed/magic-folder/remote-to-local-sync.rst @@ -573,6 +573,68 @@ because another process wrote ``foo.conflicted_unique`` after we chose the filename, then we retry with a different filename. +Read/download collisions +~~~~~~~~~~~~~~~~~~~~~~~~ + +A *read/download collision* occurs when another program reads +from ``foo`` in the local filesystem, concurrently with the new +version being written by the Magic Folder client. We want to +ensure that any successful attempt to read the file by the other +program obtains a consistent view of its contents. + +On Unix, the above procedure for writing downloads is sufficient +to achieve this. There are three cases: + +A + The other process opens ``foo`` for reading before it is + renamed to ``foo.backup``. Then the file handle will continue to + refer to the old file across the rename, and the other process + will read the old contents. +B + The other process attempts to open ``foo`` after it has been + renamed to ``foo.backup``, and before it is linked in step c. + The open call fails, which is acceptable. +C + The other process opens ``foo`` after it has been linked to + the new file. Then it will read the new contents. + +On Windows, the analysis is very similar, but case A′ needs to +be split into two subcases, depending on the sharing mode the other +process uses when opening the file for reading: + +A′. + The other process opens ``foo`` before the Magic Folder + client's attempt to rename ``foo`` to ``foo.backup`` (as part + of the implementation of `ReplaceFileW`_). The subcases are: + + i. The other process uses sharing flags that deny deletion and + renames. The `ReplaceFileW`_ call fails, and the download is + reclassified as a conflict. The downloaded file ends up at + ``foo.conflicted``, which is correct. + + ii. The other process uses sharing flags that allow deletion + and renames. The `ReplaceFileW`_ call succeeds, and the + other process reads inconsistent data. This can be attributed + to a poor choice of sharing flags by the other process. +B′. + The other process attempts to open ``foo`` at the point + during the `ReplaceFileW`_ call where it does not exist. + The open call fails, which is acceptable. +C′. + The other process opens ``foo`` after it has been linked to + the new file. Then it will read the new contents. + + +For both write/download and read/download collisions, 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 less common. + + + Fire Dragons: Distinguishing conflicts from overwrites '''''''''''''''''''''''''''''''''''''''''''''''''''''' @@ -731,66 +793,6 @@ conflict loop that alice has seen it) -Read/download collisions -~~~~~~~~~~~~~~~~~~~~~~~~ - -A *read/download collision* occurs when another program reads -from ``foo`` in the local filesystem, concurrently with the new -version being written by the Magic Folder client. We want to -ensure that any successful attempt to read the file by the other -program obtains a consistent view of its contents. - -On Unix, the above procedure for writing downloads is sufficient -to achieve this. There are three cases: - -A - The other process opens ``foo`` for reading before it is - renamed to ``foo.backup``. Then the file handle will continue to - refer to the old file across the rename, and the other process - will read the old contents. -B - The other process attempts to open ``foo`` after it has been - renamed to ``foo.backup``, and before it is linked in step c. - The open call fails, which is acceptable. -C - The other process opens ``foo`` after it has been linked to - the new file. Then it will read the new contents. - -On Windows, the analysis is very similar, but case A′ needs to -be split into two subcases, depending on the sharing mode the other -process uses when opening the file for reading: - -A′. - The other process opens ``foo`` before the Magic Folder - client's attempt to rename ``foo`` to ``foo.backup`` (as part - of the implementation of `ReplaceFileW`_). The subcases are: - - i. The other process uses sharing flags that deny deletion and - renames. The `ReplaceFileW`_ call fails, and the download is - reclassified as a conflict. The downloaded file ends up at - ``foo.conflicted``, which is correct. - - ii. The other process uses sharing flags that allow deletion - and renames. The `ReplaceFileW`_ call succeeds, and the - other process reads inconsistent data. This can be attributed - to a poor choice of sharing flags by the other process. -B′. - The other process attempts to open ``foo`` at the point - during the `ReplaceFileW`_ call where it does not exist. - The open call fails, which is acceptable. -C′. - The other process opens ``foo`` after it has been linked to - the new file. Then it will read the new contents. - - -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 less common. - - Air Dragons: Collisions between local writes and uploads ''''''''''''''''''''''''''''''''''''''''''''''''''''''''