Answer by Jack for Use FallbackResource even if directory exists

I'm answering this myself as well, because I'm pretty sure that the issue is related to how mod_dir.c works internally and I think it's a bug.

If a resource can't be mapped to the local file system, the function fixup_dflt() will run, using the FallbackResource to determine which document should be loaded instead.

However, when a resource can be mapped to the local file system and it's a directory, it will attempt to resolve the document by running fixup_dir(); this function iterates over the list of DirectoryIndex values until it finds the first suitable document.

In my case, the configuration has an empty list of DirectoryIndex values, so fixup_dir() will fail and a 404 is returned.

The following patch works for me (PR):

static int dir_fixups(request_rec *r){    if (r->finfo.filetype == APR_DIR) {-        return fixup_dir(r);+        if (fixup_dir(r) != OK) {+           /* use fallback */+           return fixup_dflt(r);+        }++        return OK;    }    else if ((r->finfo.filetype == APR_NOFILE) && (r->handler == NULL)) {        /* No handler and nothing in the filesystem - use fallback */        return fixup_dflt(r);    }    return DECLINED;}

It basically tries fixup_dflt() after fixup_dir() failed.

Update 2015-04-21

A fix has been submitted to the project, scheduled for 2.5; it may be ported to 2.4 as well.

Update 2015-05-18

The fix has been reverted because:

[...] it [at least] causes FallBackResource to kick in before mod_autoindex might have kicked in.

I'm still trying to figure out how to avoid this kind of situation.