Alternate syntax for cross references (#9116)

* Add support for local vs. remote xrefs

* Add doc for references

* Docs: fix cases not currently supported

* One more doc fix

* Doc: mentions for teams and orgs

* Change !num ref concept, no change in functionality

* Fix test

* Improve table of issue reference types

* Fix paragraph mark

1 files changed, 173 insertions, 0 deletions

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+---
+date: "2019-11-21T17:00:00-03:00"
+title: "Usage: Automatically Linked References"
+slug: "automatically-linked-references"
+weight: 15
+toc: true
+draft: false
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Automatically Linked References"
+    weight: 15
+    identifier: "automatically-linked-references"
+---
+
+# Automatically Linked References in Issues, Pull Requests and Commit Messages
+
+When an issue, pull request or comment is posted, the text description is parsed
+in search for references. These references will be shown as links in the Issue View
+and, in some cases, produce certain _actions_.
+
+Likewise, commit messages are parsed when they are listed, and _actions_
+are can be triggered when they are pushed to the main branch.
+
+To prevent the creation of unintended references, there are certain rules
+for them to be recognized. For example, they should not be included inside code
+text. They should also be reasonably cleared from their surrounding text
+(for example, using spaces).
+
+## User, Team and Organization Mentions
+
+When a text in the form `@username` is found and `username` matches the name
+of an existing user, a _mention_ reference is created. This will be shown
+by changing the text into a link to said user's profile, and possibly create
+a notification for the mentioned user depending on whether they have
+the necessary permission to access the contents.
+
+Example:
+
+> [@John](#), can you give this a look?
+
+This is also valid for teams and organizations:
+
+> [@Documenters](#), we need to plan for this.
+
+> [@CoolCompanyInc](#), this issue concerns us all!
+
+Teams will receive mail notifications when appropriate, but whole organizations won't.
+
+Commit messages do not produce user notifications.
+
+## Commits
+
+Commits can be referenced using their SHA1 hash, or a portion of it of
+at least seven characters. They will be shown as a link to the corresponding
+commit.
+
+Example:
+
+> This bug was introduced in [e59ff077](#)
+
+## Issues and Pull Requests
+
+A reference to another issue or pull request can be created using the simple
+notation `#1234`, where _1234_ is the number of an issue or pull request
+in the same repository. These references will be shown as links to the
+referenced content.
+
+The effect of creating this type of reference is that a _notice_ will be
+created in the referenced document, provided the creator of the reference
+has reading permissions on it.
+
+Example:
+
+> This seems related to [#1234](#)
+
+Issues and pull requests in other repositories can be referred to as well
+using the form `owner/repository#1234`:
+
+> This seems related to [mike/compiler#1234](#)
+
+Alternatively, the `!1234` notation can be used as well. Even when in Gitea
+a pull request is a form of issue, the `#1234` form will always link to
+an issue; if the linked entry happens to be a pull request instead, Gitea
+will redirect as appropriate. With the `!1234` notation, a pull request
+link will be created, which will be redirected to an issue if required.
+However, this distinction could be important if an external tracker is
+used, where links to issues and pull requests are not interchangeable.
+
+## Actionable References in Pull Requests and Commit Messages
+
+Sometimes a commit or pull request may fix or bring back a problem documented
+in a particular issue. Gitea supports closing and reopening the referenced
+issues by preceding the reference with a particular _keyword_. Common keywords
+include "closes", "fixes", "reopens", etc. This list can be
+[customized]({{< ref "/doc/advanced/config-cheat-sheet.en-us.md" >}}) by the
+site administrator.
+
+Example:
+
+> This PR _closes_ [#1234](#)
+
+If the actionable reference is accepted, this will create a notice on the
+referenced issue announcing that it will be closed when the referencing PR
+is merged.
+
+For an actionable reference to be accepted, _at least one_ of the following
+conditions must be met:
+
+* The commenter has permissions to close or reopen the issue at the moment
+of creating the reference.
+* The reference is inside a commit message.
+* The reference is posted as part of the pull request description.
+
+In the last case, the issue will be closed or reopened only if the merger
+of the pull request has permissions to do so.
+
+Additionally, only pull requests and commit messages can create an action,
+and only issues can be closed or reopened this way.
+
+The default _keywords_ are:
+
+* **Closing**: close, closes, closed, fix, fixes, fixed, resolve, resolves, resolved
+* **Reopening**: reopen, reopens, reopened
+
+## External Trackers
+
+Gitea supports the use of external issue trackers, and references to issues
+hosted externally can be created in pull requests. However, if the external
+tracker uses numbers to identify issues, they will be indistinguishable from
+the pull requests hosted in Gitea. To address this, Gitea allows the use of
+the `!` marker to identify pull requests. For example:
+
+> This is issue [#1234](#), and links to the external tracker.
+
+> This is pull request [!1234](#), and links to a pull request in Gitea.
+
+The `!` and `#` can be used interchangeably for issues and pull request _except_
+for this case, where a distinction is required.
+
+## Issues and Pull Requests References Summary
+
+This table illustrates the different kinds of cross-reference for issues and pull requests.
+In the examples, `User1/Repo1` refers to the repository where the reference is used, while
+`UserZ/RepoZ` indicates a different repository.
+
+| Reference in User1/Repo1  | Repo1 issues are external | RepoZ issues are external | Should render                    |
+|---------------------------|:-------------------------:|:-------------------------:|----------------------------------|
+| `#1234`                   |     no                    |    N/A                    | A link to issue/pull 1234 in `User1/Repo1` |
+| `!1234`                   |     no                    |    N/A                    | A link to issue/pull 1234 in `User1/Repo1` |
+| `#1234`                   |    yes                    |    N/A                    | A link to _external issue_ 1234 for `User1/Repo1` |
+| `!1234`                   |    yes                    |    N/A                    | A link to _PR_ 1234 for `User1/Repo1` |
+| `User1/Repo1#1234`        |     no                    |    N/A                    | A link to issue/pull 1234 in `User1/Repo1` |
+| `User1/Repo1!1234`        |     no                    |    N/A                    | A link to issue/pull 1234 in `User1/Repo1` |
+| `User1/Repo1#1234`        |    yes                    |    N/A                    | A link to _external issue_ 1234 for `User1/Repo1` |
+| `User1/Repo1!1234`        |    yes                    |    N/A                    | A link to _PR_ 1234 for `User1/Repo1` |
+| `UserZ/RepoZ#1234`        |    N/A                    |     no                    | A link to issue/pull 1234 in `UserZ/RepoZ` |
+| `UserZ/RepoZ!1234`        |    N/A                    |     no                    | A link to issue/pull 1234 in `UserZ/RepoZ` |
+| _Not supported_           |    N/A                    |    yes                    | A link to _external issue_ 1234 for `UserZ/RepoZ` |
+| `UserZ/RepoZ!1234`        |    N/A                    |    yes                    | A link to _PR_ 1234 for `UserZ/RepoZ` |
+| **Alphanumeric issue IDs:** | -                       | -                         | - |
+| `AAA-1234`                |    yes                    |    N/A                    | A link to _external issue_ `AAA-1234` for `User1/Repo1` |
+| `!1234`                   |    yes                    |    N/A                    | A link to _PR_ 1234 for `User1/Repo1` |
+| `User1/Repo1!1234`        |    yes                    |    N/A                    | A link to _PR_ 1234 for `User1/Repo1` |
+| _Not supported_           |    N/A                    |    yes                    | A link to _external issue_ `AAA-1234` for `UserZ/RepoZ` |
+| `UserZ/RepoZ!1234`        |    N/A                    |    yes                    | A link to _PR_ 1234 in `UserZ/RepoZ` |
+
+_The last section is for repositories with external issue trackers that use alphanumeric format._
+
+_**N/A**: not applicable._
+
+Note: automatic references between repositories with different types of issues (external vs. internal) are not fully supported
+and may render invalid links.