Fix race in library metadata protocol

Specifically, we need to check whether the prerequisite_member is ad hoc
before checking whether it is NULL because ad hoc ones are blanked out (set to
NULL) during execute.

1 files changed, 3 insertions, 2 deletions

diff --git a/libbuild2/cc/common.cxx b/libbuild2/cc/common.cxx
index 8150cfa..ce80e3f 100644
--- a/libbuild2/cc/common.cxx
+++ b/libbuild2/cc/common.cxx
@@ -240,9 +240,10 @@ namespace build2
         for (const prerequisite_target& pt: l.prerequisite_targets[a])
         {
           // Note: adhoc prerequisites are not part of the library meta-
-          // information protocol.
+          // information protocol (and we should check for adhoc first to
+          // avoid races).
           //
-          if (pt == nullptr || pt.adhoc)
+          if (pt.adhoc || pt == nullptr)
             continue;
 
           bool la;