[Bug debuginfod/27918] New: add control file for triggered traversal

["fche at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27918

            Bug ID: 27918
           Summary: add control file for triggered traversal
           Product: elfutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: debuginfod
          Assignee: unassigned at sourceware dot org
          Reporter: fche at redhat dot com
                CC: elfutils-devel at sourceware dot org
  Target Milestone: ---

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.

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