ResourceServlet commits response preventing filters from modifying response headers after `filterChain.doFilter` - regression from Jetty 11

[Spikhalskiy]

Jetty version(s)
Jetty 12.0.16

Jetty Environment
EE10

Description

I'm migrating from Jetty 11 to Jetty 12 and noticed a regression. Headers set by our Filters after a call to filterChain.doFilter are missing or incorrect on certain URLs without any debug logging.
I was able to trace it to the fact that ResourceServlet is committing the response here:

	at org.eclipse.jetty.server.ResourceService.writeHttpContent(ResourceService.java:718)
	at org.eclipse.jetty.server.ResourceService.sendData(ResourceService.java:670) //response.write(true, buffer, callback);
	at org.eclipse.jetty.server.ResourceService.doGet(ResourceService.java:206)
	at org.eclipse.jetty.ee10.servlet.ResourceServlet.doGet(ResourceServlet.java:530)
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:527)
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:614)
	at org.eclipse.jetty.ee10.servlet.ServletHolder$NotAsync.service(ServletHolder.java:1379)
	at org.eclipse.jetty.ee10.servlet.ServletHolder.handle(ServletHolder.java:736)
	at org.eclipse.jetty.ee10.servlet.ServletHandler$ChainEnd.doFilter(ServletHandler.java:1619)

After which modification of response headers in the filters after filterChain.doFilter() call are silently ignored in ServletContextResponse. For example:

@Override
public boolean onRemoveField(HttpField field) {
            if (isCommitted())
                return false;

This looks like a regression from Jetty 11, as the filters were able to correctly modify the headers of the response with Jetty 11.
Could ResourceServlet / ResourceService not commit the response to let the filters do their job?

For additional context, this issue was hard to track and untangle because of the silent discarding of header modification. It would be nice to have at least a debug log if the header or status modification was rejected inside Jetty code because the response is already committed.

Test

I modified ResourceServletTest#testOutputStreamAndCharsetFiltered to reproduce the issue:
Spikhalskiy@8054950

[joakime]

Agreed, a flush/commit from a servlet should have no impact on if a filter is executed or not.

[Spikhalskiy]

I added a reproduction within Jetty tests to make it easier to diagnose:
Spikhalskiy@8054950

[gregw]

Headers cannot be changed after a servlet is committed. We are required by the servlet spec to close (commit) the response once content-length bytes are written. Because the resource is a static one from a file system, then the size is known and writing the data will always cause the response to be committed and completed.

Headers cannot be altered after doFilter unless the servlet did not commit the response