fixing paper build

paper/.latexmkrc

@@ -4,5 +4,23 @@ $bibtex_use = 2;                       # run biber when biblatex .bcf changes
 # biber cwd is paper/build; scripts/nx_paper.sh symlinks ../build/bib -> ../src/bib so
 # datasources log as bib/references.bib and latexmk's -e check works from paper/src
 $biber    = 'biber %O %S';
+
+# Stale latexmk db: biblatex uses biber + .bcf, but the fdb can keep a "bibtex" rule after a bad
+# run. Then biber never runs and citations stay undefined. Read whole fdb (small) so the rule
+# line is never missed after a long dependency list.
+for my $job (qw(main main-genpop summary)) {
+    my $bcf = "../build/$job.bcf";
+    my $bbl = "../build/$job.bbl";
+    my $fdb = "../build/$job.fdb_latexmk";
+    next unless -e $fdb && -e $bcf;
+    my $drop = !-e $bbl;
+    if ( !$drop && open my $fh, '<', $fdb ) {
+        local $/;
+        my $body = <$fh>;
+        close $fh;
+        $drop = 1 if defined $body && $body =~ /\["bibtex $job"\]/;
+    }
+    unlink $fdb if $drop;
+}
 $pdf_previewer = 'zathura %O %S';
 $clean_ext = 'synctex.gz bbl bcf run.xml fls fdb_latexmk glg glo gls ist blg lof lot out toc';