Fix post-simulation assertion in pkg-build (GH issue #382)

1 files changed, 70 insertions, 1 deletions

diff --git a/bpkg/pkg-build.cxx b/bpkg/pkg-build.cxx
index fac79c2..adf7938 100644
--- a/bpkg/pkg-build.cxx
+++ b/bpkg/pkg-build.cxx
@@ -6871,12 +6871,81 @@ namespace bpkg
             //
             if (!rescan)
             {
+              // Return true if the specified package is loaded as a
+              // prerequisite of some dependent package cached in the session
+              // and contained in a different database. Also unload this
+              // package from all such dependents.
+              //
+              auto unload_prereq = [&ses, &sp_session]
+                (const shared_ptr<selected_package>& sp,
+                 const odb::database* db)
+              {
+                bool r (false);
+
+                for (const auto& dps: ses.map ())
+                {
+                  // Skip dependents from the same database.
+                  //
+                  if (dps.first == db)
+                    continue;
+
+                  if (const selected_packages* sps = sp_session (dps.second))
+                  {
+                    for (const auto& p: *sps)
+                    {
+                      for (auto& pr: p.second->prerequisites)
+                      {
+                        const lazy_shared_ptr<selected_package>& lp (pr.first);
+
+                        if (lp.loaded () && lp.get_eager () == sp)
+                        {
+                          lp.unload ();
+                          r = true;
+                        }
+                      }
+                    }
+                  }
+                }
+
+                return r;
+              };
+
               for (const auto& dps: ses.map ())
               {
                 if (const selected_packages* sps = sp_session (dps.second))
                 {
                   if (old_sp.find (dps.first) == old_sp.end ())
-                    assert (sps->empty ());
+                  {
+                    // Note that the reason for these packages to still be
+                    // present in the session is that they may be referenced
+                    // as prerequisites by some dependent packages from other
+                    // databases. For example:
+                    //
+                    // new session: A (D1, 2) -> B (D1, 2) -> C (D2, 2)
+                    // old session: A
+                    //
+                    // Here C is the package in question, package A is present
+                    // in both sessions, D* are databases, the numbers are the
+                    // package reference counts, and the arrows denote the
+                    // loaded prerequisite lazy pointers.
+                    //
+                    // Let's verify that by unloading these packages from such
+                    // dependents and rescan.
+                    //
+                    if (!sps->empty ())
+                    {
+                      for (const auto& p: *sps)
+                      {
+                        if (unload_prereq (p.second, dps.first))
+                          rescan = true;
+                      }
+
+                      // If we didn't unload any of these packages, then we
+                      // consider this as a bug.
+                      //
+                      assert (rescan);
+                    }
+                  }
                 }
               }
             }