127 lines
5.6 KiB
Markdown
127 lines
5.6 KiB
Markdown
# Working with diffs
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Currently we rely on different sources to present diffs, these include:
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- Gitaly service
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- Database (through `merge_request_diff_files`)
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- Redis (cached highlighted diffs)
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## Architecture overview
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### Merge request diffs
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When refreshing a Merge Request (pushing to a source branch, force-pushing to target branch, or if the target branch now contains any commits from the MR)
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we fetch the comparison information using `Gitlab::Git::Compare`, which fetches `base` and `head` data using Gitaly and diff between them through
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`Gitlab::Git::Diff.between`.
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The diffs fetching process _limits_ single file diff sizes and the overall size of the whole diff through a series of constant values. Raw diff files are
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then persisted on `merge_request_diff_files` table.
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Even though diffs larger than 10% of the value of `ApplicationSettings#diff_max_patch_bytes` are collapsed,
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we still keep them on Postgres. However, diff files larger than defined _safety limits_
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(see the [Diff limits section](#diff-limits)) are _not_ persisted in the database.
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In order to present diffs information on the Merge Request diffs page, we:
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1. Fetch all diff files from database `merge_request_diff_files`
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1. Fetch the _old_ and _new_ file blobs in batch to:
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- Highlight old and new file content
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- Know which viewer it should use for each file (text, image, deleted, etc)
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- Know if the file content changed
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- Know if it was stored externally
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- Know if it had storage errors
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1. If the diff file is cacheable (text-based), it's cached on Redis
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using `Gitlab::Diff::FileCollection::MergeRequestDiff`
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### Note diffs
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When commenting on a diff (any comparison), we persist a truncated diff version
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on `NoteDiffFile` (which is associated with the actual `DiffNote`). So instead
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of hitting the repository every time we need the diff of the file, we:
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1. Check whether we have the `NoteDiffFile#diff` persisted and use it
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1. Otherwise, if it's a current MR revision, use the persisted
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`MergeRequestDiffFile#diff`
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1. In the last scenario, go the repository and fetch the diff
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## Diff limits
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As explained above, we limit single diff files and the size of the whole diff. There are scenarios where we collapse the diff file,
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and cases where the diff file is not presented at all, and the user is guided to the Blob view. Here we'll go into details about
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these limits.
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### Diff collection limits
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Limits that act onto all diff files collection. Files number, lines number and files size are considered.
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```ruby
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Gitlab::Git::DiffCollection.collection_limits[:safe_max_files] = Gitlab::Git::DiffCollection::DEFAULT_LIMITS[:max_files] = 100
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```
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File diffs will be collapsed (but be expandable) if 100 files have already been rendered.
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```ruby
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Gitlab::Git::DiffCollection.collection_limits[:safe_max_lines] = Gitlab::Git::DiffCollection::DEFAULT_LIMITS[:max_lines] = 5000
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```
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File diffs will be collapsed (but be expandable) if 5000 lines have already been rendered.
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```ruby
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Gitlab::Git::DiffCollection.collection_limits[:safe_max_bytes] = Gitlab::Git::DiffCollection.collection_limits[:safe_max_files] * 5.kilobytes = 500.kilobytes
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```
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File diffs will be collapsed (but be expandable) if 500 kilobytes have already been rendered.
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```ruby
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Gitlab::Git::DiffCollection.collection_limits[:max_files] = Commit::DIFF_HARD_LIMIT_FILES = 1000
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```
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No more files will be rendered at all if 1000 files have already been rendered.
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```ruby
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Gitlab::Git::DiffCollection.collection_limits[:max_lines] = Commit::DIFF_HARD_LIMIT_LINES = 50000
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```
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No more files will be rendered at all if 50,000 lines have already been rendered.
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```ruby
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Gitlab::Git::DiffCollection.collection_limits[:max_bytes] = Gitlab::Git::DiffCollection.collection_limits[:max_files] * 5.kilobytes = 5000.kilobytes
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```
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No more files will be rendered at all if 5 megabytes have already been rendered.
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*Note:* All collection limit parameters are currently sent and applied on Gitaly. That is, once the limit is surpassed,
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Gitaly will only return the safe amount of data to be persisted on `merge_request_diff_files`.
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### Individual diff file limits
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Limits that act onto each diff file of a collection. Files number, lines number and files size are considered.
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#### Expandable patches (collapsed)
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Diff patches are collapsed when surpassing 10% of the value set in `ApplicationSettings#diff_max_patch_bytes`.
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That is, it's equivalent to 10kb if the maximum allowed value is 100kb.
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The diff will still be persisted and expandable if the patch size doesn't
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surpass `ApplicationSettings#diff_max_patch_bytes`.
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*Note:* Although this nomenclature (Collapsing) is also used on Gitaly, this limit is only used on GitLab (hardcoded - not sent to Gitaly).
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Gitaly will only return `Diff.Collapsed` (RPC) when surpassing collection limits.
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#### Not expandable patches (too large)
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The patch not be rendered if it's larger than `ApplicationSettings#diff_max_patch_bytes`.
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Users will see a `This source diff could not be displayed because it is too large` message.
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```ruby
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Commit::DIFF_SAFE_LINES = Gitlab::Git::DiffCollection::DEFAULT_LIMITS[:max_lines] = 5000
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```
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File diff will be suppressed (technically different from collapsed, but behaves the same, and is expandable) if it has more than 5000 lines.
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*Note:* This limit is currently hardcoded and only applied on GitLab.
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## Viewers
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Diff Viewers, which can be found on `models/diff_viewer/*` are classes used to map metadata about each type of Diff File. It has information
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whether it's a binary, which partial should be used to render it or which File extensions this class accounts for.
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`DiffViewer::Base` validates _blobs_ (old and new versions) content, extension and file type in order to check if it can be rendered.
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