On a large debuginfod installation, where a full traversal might take hours but things hardly ever change, it would help to focus debuginfod's traversal to directories known to have changed. For example, an external monitor of a build system could notify us that a new build XYZ exists, and map that to a likely subdirectory.
More specifically, let debuginfod be run with a --traversal-trigger=FILENAME option. In the traversal thread that normally sleeps quite a while, let it check for the existence & nonemptiness of this file. If it exists & has content, the traversal thread should immediately begin a traversal/scan process for the named directories, as though they were given on command line, probably into some sibling vector to source_paths. Then it should truncate the file (if is still unmodified from the beginning of the read operation - so as to avoid losing data on a race).
If a traversal is already in progress, so the traversal thread is sitting inside scan_source_paths()'s fts loop, there probably isn't a super simple way of suspending that search, focusing on the new directories, then resuming ... so probably not worth accelerating this case. We could let the fts finish, and then let the thread process the trigger file back out there.
An external program could "echo /path/ >> FILENAME" whenever, and debuginfod should have that content scanned rapidly. With that in place, such sites might choose a very long -tNNN time for thorough full traversals.