Implement "embedded" command to extract static resources (#9982)

* draft

* Implement extract command

* Fix nits and force args on extract

* Add !bindata stub, support Windows, fmt

* fix vendored flag

* Remove leading slash for matching

* Add docs

* Fix typos

* Add embedded view command

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

2 files changed, 128 insertions, 6 deletions

diff --git a/docs/content/doc/advanced/cmd-embedded.en-us.md b/docs/content/doc/advanced/cmd-embedded.en-us.md
new file mode 100644
index 0000000000..e036d17498
--- /dev/null
+++ b/docs/content/doc/advanced/cmd-embedded.en-us.md
@@ -0,0 +1,115 @@
+---
+date: "2020-01-25T21:00:00-03:00"
+title: "Embedded data extraction tool"
+slug: "cmd-embedded"
+weight: 40
+toc: true
+draft: false
+menu:
+  sidebar:
+    parent: "advanced"
+    name: "Embedded data extraction tool"
+    weight: 40
+    identifier: "cmd-embedded"
+---
+
+# Embedded data extraction tool
+
+Gitea's executable contains all the resources required to run: templates, images, style-sheets
+and translations. Any of them can be overridden by placing a replacement in a matching path
+inside the `custom` directory (see [Customizing Gitea]({{< relref "doc/advanced/customizing-gitea.en-us.md" >}})).
+
+To obtain a copy of the embedded resources ready for editing, the `embedded` command from the CLI
+can be used from the OS shell interface.
+
+## Listing resources
+
+To list resources embedded in Gitea's executable, use the following syntax:
+
+```
+gitea embedded list [--include-vendored] [patterns...]
+```
+
+The `--include-vendored` flag makes the command include vendored files, which are
+normally excluded; that is, files from external libraries that are required for Gitea
+(e.g. [font-awesome](https://fontawesome.com/), [octicons](https://octicons.github.com/), etc).
+
+A list of file search patterns can be provided. Gitea uses [gobwas/glob](https://github.com/gobwas/glob)
+for its glob syntax. Here are some examples:
+
+- List all template files, in any virtual directory: `**.tmpl`
+- List all mail template files: `templates/mail/**.tmpl`
+- List all files inside `public/img`: `public/img/**`
+
+Don't forget to use quotes for the patterns, as spaces, `*` and other characters might have
+a special meaning for your command shell.
+
+If no pattern is provided, all files are listed.
+
+#### Example
+
+Listing all embedded files with `openid` in their path:
+
+```
+$ gitea embedded list '**openid**'
+public/img/auth/openid_connect.png
+public/img/openid-16x16.png
+templates/user/auth/finalize_openid.tmpl
+templates/user/auth/signin_openid.tmpl
+templates/user/auth/signup_openid_connect.tmpl
+templates/user/auth/signup_openid_navbar.tmpl
+templates/user/auth/signup_openid_register.tmpl
+templates/user/settings/security_openid.tmpl
+```
+
+## Extracting resources
+
+To extract resources embedded in Gitea's executable, use the following syntax:
+
+```
+gitea [--config {file}] embedded extract [--destination {dir}|--custom] [--overwrite|--rename] [--include-vendored] {patterns...}
+```
+
+The `--config` option tells gitea the location of the `app.ini` configuration file if
+it's not in its default location. This option is only used with the `--custom` flag.
+
+The `--destination` option tells gitea the directory where the files must be extracted to.
+The default is the current directory.
+
+The `--custom` flag tells gitea to extract the files directly into the `custom` directory.
+For this to work, the command needs to know the location of the `app.ini` configuration
+file (`--config`) and, depending of the configuration, be ran from the directory where
+gitea normally starts. See [Customizing Gitea]({{< relref "doc/advanced/customizing-gitea.en-us.md" >}}) for details.
+
+The `--overwrite` flag allows any existing files in the destination directory to be overwritten.
+
+The `--rename` flag tells gitea to rename any existing files in the destination directory
+as `filename.bak`. Previous `.bak` files are overwritten.
+
+At least one file search pattern must be provided; see `list` subcomand above for pattern
+syntax and examples.
+
+#### Important notice
+
+Make sure to **only extract those files that require customization**. Files that
+are present in the `custom` directory are not upgraded by Gitea's upgrade process.
+When Gitea is upgraded to a new version (by replacing the executable), many of the
+embedded files will suffer changes. Gitea will honor and use any files found
+in the `custom` directory, even if they are old and incompatible.
+
+#### Example
+
+Extracting mail templates to a temporary directory:
+
+```
+$ mkdir tempdir
+$ gitea embedded extract --destination tempdir 'templates/mail/**.tmpl'
+Extracting to tempdir:
+tempdir/templates/mail/auth/activate.tmpl
+tempdir/templates/mail/auth/activate_email.tmpl
+tempdir/templates/mail/auth/register_notify.tmpl
+tempdir/templates/mail/auth/reset_passwd.tmpl
+tempdir/templates/mail/issue/assigned.tmpl
+tempdir/templates/mail/issue/default.tmpl
+tempdir/templates/mail/notify/collaborator.tmpl
+```
diff --git a/docs/content/doc/advanced/customizing-gitea.en-us.md b/docs/content/doc/advanced/customizing-gitea.en-us.md
index f42ade7799..73127dd81a 100644
--- a/docs/content/doc/advanced/customizing-gitea.en-us.md
+++ b/docs/content/doc/advanced/customizing-gitea.en-us.md
@@ -57,14 +57,21 @@ the url `http://gitea.domain.tld/image.png`.
 
 Place the png image at the following path: `custom/public/img/avatar_default.png`
 
-## Customizing Gitea pages
+## Customizing Gitea pages and resources
 
-The `custom/templates` folder allows changing every single page of Gitea. Templates
-to override can be found in the [`templates`](https://github.com/go-gitea/gitea/tree/master/templates) directory of Gitea source (Note: the example link is from `master` branch. Make sure to copy templates from same release you are using). Override by
-making a copy of the file under `custom/templates` using a full path structure
-matching source.
+Gitea's executable contains all the resources required to run: templates, images, style-sheets
+and translations. Any of them can be overridden by placing a replacement in a matching path
+inside the `custom` directory. For example, to replace the default `.gitignore` provided
+for C++ repositories, we want to replace `options/gitignore/C++`. To do this, a replacement
+must be placed in `custom/options/gitignore/C++` (see about the location of the `custom`
+directory at the top of this document).
 
-Any statement contained inside `{{` and `}}` are Gitea's template syntax and
+Every single page of Gitea can be changed. Dynamic content is generated using [go templates](https://golang.org/pkg/html/template/),
+which can be modified by placing replacements below the `custom/templates` directory.
+
+To obtain any embedded file (including templates), the [`gitea embedded` tool]({{< relref "doc/advanced/cmd-embedded.en-us.md" >}}) can be used. Alternatively, they can be found in the [`templates`](https://github.com/go-gitea/gitea/tree/master/templates) directory of Gitea source (Note: the example link is from the `master` branch. Make sure to use templates compatible with the release you are using).
+
+Be aware that any statement contained inside `{{` and `}}` are Gitea's template syntax and
 shouldn't be touched without fully understanding these components.
 
 ### Customizing startpage / homepage