### What changes were proposed in this pull request?
Allow to run the Spark web UI behind a reverse proxy with URLs prefixed by a context root, like www.mydomain.com/spark. In particular, this allows to access multiple Spark clusters through the same virtual host, only distinguishing them by context root, like www.mydomain.com/cluster1, www.mydomain.com/cluster2, and it allows to run the Spark UI in a common cookie domain (for SSO) with other services.
### Why are the changes needed?
This PR is to take over https://github.com/apache/spark/pull/17455.
After changes, Spark allows showing customized prefix URL in all the `href` links of the HTML pages.
### Does this PR introduce _any_ user-facing change?
Yes, all the links of UI pages will be contains the value of `spark.ui.reverseProxyUrl` if it is configurated.
### How was this patch tested?
New HTML Unit tests in MasterSuite
Manual UI testing for master, worker and app UI with an nginx proxy
Spark config:
```
spark.ui.port 8080
spark.ui.reverseProxy=true
spark.ui.reverseProxyUrl=/path/to/spark/
```
nginx config:
```
server {
listen 9000;
set $SPARK_MASTER http://127.0.0.1:8080;
# split spark UI path into prefix and local path within master UI
location ~ ^(/path/to/spark/) {
# strip prefix when forwarding request
rewrite /path/to/spark(/.*) $1 break;
#rewrite /path/to/spark/ "/" ;
# forward to spark master UI
proxy_pass $SPARK_MASTER;
proxy_intercept_errors on;
error_page 301 302 307 = handle_redirects;
}
location handle_redirects {
set $saved_redirect_location '$upstream_http_location';
proxy_pass $saved_redirect_location;
}
}
```
Closes#29820 from gengliangwang/revertProxyURL.
Lead-authored-by: Gengliang Wang <gengliang.wang@databricks.com>
Co-authored-by: Oliver Köth <okoeth@de.ibm.com>
Signed-off-by: Gengliang Wang <gengliang.wang@databricks.com>
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