125 lines
5.6 KiB
Markdown
125 lines
5.6 KiB
Markdown
When `git annex export treeish --to remote` is used to export to a remote,
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and the remote allows files to somehow be edited on it, then there ought
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to be a way to import the changes back from the remote into the git repository.
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The command could be `git annex import --from remote`
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See [[design/importing_trees_from_special_remotes]] for current design for
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this.
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## implementation notes
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* Database.ContentIdentifier needs a way to update the database with
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information coming from the git-annex branch. This will allow multiple
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clones to import from the same remote, and share content identifier
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information amoung them.
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It will only need to be updated when listContents returns a
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ContentIdentifier that is not already known in the database.
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## race conditions
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(Some thoughts about races that the design should cover now, but kept here
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for reference.)
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A file could be modified on the remote while
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it's being exported, and if the remote then uses the mtime of the modified
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file in the content identifier, the modification would never be noticed by
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imports.
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To fix this race, we need an atomic move operation on the remote. Upload
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the file to a temp file, then get its content identifier, and then move it
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from the temp file to its final location. Alternatively, upload a file and
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get the content identifier atomically, which eg S3 with versioning enabled
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provides. It would make sense to have the storeExport operation always return
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a content identifier and document that it needs to get it atomically by
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either using a temp file or something specific to the remote.
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----
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There's also a race where a file gets changed on the remote after an
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import tree, and an export then overwrites it with something else.
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One solution would be to only allow one of importtree or exporttree
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to a given remote. This reduces the use cases a lot though, and perhaps
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so far that the import tree feature is not worth building. The adb
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special remote needs both. Also, such a limitation seems like one that
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users might try to work around by initializing two remotes using the same
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data and trying to use one for import and the other for export.
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Really fixing this race needs locking or an atomic operation. Locking seems
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unlikely to be a portable enough solution.
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An atomic rename operation could at least narrow the race significantly, eg:
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1. get content identifier of $file, check if it's what was expected else
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abort (optional but would catch most problems)
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2. upload new version of $file to $tmp1
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3. rename current $file to $tmp2
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4. Get content identifier of $tmp2, check if it's what was expected to
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be. If not, $file was modified after the last import tree, and that
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conflict has to be resolved. Otherwise, delete $tmp2
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5. rename $tmp1 to $file
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That leaves a race if the file gets overwritten after it's moved out
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of the way. If the rename refuses to overwrite existing files, that race
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would be detected by it failing. renameat(2) with `RENAME_NOREPLACE` can do that,
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but probably many special remote interfaces don't provide a way to do that.
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S3 lacks a rename operation, can only copy and then delete. Which is not
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good enough; it risks the file being replaced with new content before
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the delete and the new content being deleted.
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Is this race really a significant problem? One way to look at it is
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analagous to a git merge overwriting a locally modified file.
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Git can certianly use similar techniques to entirely detect and recover
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from such races (but not the similar race described in the next section).
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But, git does not actually do that! I modified git's
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merge.c to sleep for 10 seconds after `refresh_index()`, and verified
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that changes made to the work tree in that window were silently overwritten
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by git merge. In git's case, the race window is normally quite narrow
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and this is very unlikely to happen (the similar race described in the next
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section is more likely).
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If git-annex could get the race window similarly small out would perhaps be
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ok. Eg:
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1. upload new version of $file to $tmp
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2. get content identifier of $file, check if it's what was expected else
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abort
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3. rename (or copy and delete) $tmp to $file
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The race window between #2 and #3 could be quite narrow for some remotes.
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But S3, lacking a rename, does a copy that can be very slow for large files.
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S3, with versioning, could detect the race after the fact, by listing
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the versions of the file, and checking if any of the versions is one
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that git-annex did not know the file already had.
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[Using this api](https://docs.aws.amazon.com/AmazonS3/latest/API/RESTBucketGETVersion.html),
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with version-id-marker set to the previous version of the file,
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should list only the previous and current versions; if there's an
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intermediate version then the race occurred and it could roll the change
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back, or otherwise recover the overwritten version. This could be done at
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import time, to detect a previous race, and recover from it; importing
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a tree with the file(s) that were overwritten due to the race, leading to a
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tree import conflict that the user can resolve. This likely generalizes
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to importing a sequence of trees, so each version written to S3 gets
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imported.
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----
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A remaining race is that, if the file is open for write at the same
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time it's renamed, the write might happen after the content identifer
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is checked, and then whatever is written to it will be lost.
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But: Git worktree update has the same race condition. Verified with
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this perl oneliner, run in a worktree and a second later
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followed by a git pull. The lines that it appended to the
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file got lost:
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perl -e 'open (OUT, ">>foo") || die "$!"; sleep(10); while (<>) { print OUT $_ }'
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Since this is acceptable in git, I suppose we can accept it here too..
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----
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See also, [[adb_special_remote]]
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