sg-report-flight: Add a comment re same-flight search narrowing

In afe851ca1771e5da6395b596afa69e509dbbc278
  sg-report-flight: When justifying, disregard out-of-flight build jobs
we narrowed sg-report-flight's search algorith.

An extensive justification is in the commit message.  I think much of
this information belongs in-tree, so c&p it (with slight edits) here.

No code change.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

diff --git a/sg-report-flight b/sg-report-flight
index 49f7ba6a95664c2c7acfa32d6cd1f406bfca8955..1c3721291bef2fe7fd7a6b8d302913ef704ff46f 100755 (executable)
--- a/sg-report-flight
+++ b/sg-report-flight
@@ -242,9 +242,27 @@ END
        # jobs.  We start with all jobs in $tflight, and for each job
        # we also process any other jobs it refers to in *buildjob runvars.
        #
+       # The real thing we want to check that the build jobs *in the
+       # same flight as the justifying job* used the right revisions.
+       # Build jobs from other flights were either (i) build jobs for
+       # components not being targed for testing by this branch, but
+       # which were necessary for the justifying job and for which we
+       # decided to reuse another build job (in which case we don't
+       # really care what versions they used, even if underlying it
+       # all there might be a different version of a tree we are
+       # actually interested in (ii) the kind of continuous update
+       # thing seen with freebsdbuildjob.
+       #
+       # (This is rather different to cs-bisection-step, which is
+       # less focused on changes in a particular set of trees.)
+       #
+       # So we limit the scope of our recursive descent into build
+       # jobs, to jobs in the same flight.
+       #
        # We don't actually use a recursive algorithm because that
        # would involve recursive use of the same sql query object;
        # hence the @binfos_todo queue.
+
        my @binfos_todo;
        my $binfos_queue = sub {
            my ($inflight,$q,$why) = @_;