avoid clearing the dependency cache unnecessarily based on wrap-mode

We actually do not and should not care about wrap-mode at all for this.
We want to cache dependency lookups whenever humanly possible, but only
use them in cases where we would anyways be using them -- which in
particular means if we said to force a subproject fallback for this dep,
we want to bypass the cache.

Currently, we handle this by always looking up the cache for all
dependencies, but clearing the cache at startup if a reconfigure means
we are changing our resolution strategy. This is bad -- we might have
many dependencies that are worth caching, and only one dependency that
should stop being cached and use a subproject instead.

The simple solution is to handle the forcefallback case when doing a
cache lookup, and not do a cache lookup at all. Now we don't have to
nuke the entire cache. In fact, if a future reconfigure changes the
forcefallback state back to not being forced, we can reuse the original
cached dependency, which is still there.

Closes #11828

mesonbuild/coredata.py

@@ -704,12 +704,6 @@ class CoreData:
 
         if key.name == 'buildtype':
             dirty |= self._set_others_from_buildtype(value)
-        elif key.name in {'wrap_mode', 'force_fallback_for'}:
-            # We could have the system dependency cached for a dependency that
-            # is now forced to use subproject fallback. We probably could have
-            # more fine-grained cache invalidation, but better be safe.
-            self.clear_deps_cache()
-            dirty = True
 
         return dirty
 

mesonbuild/interpreter/dependencyfallbacks.py

@@ -220,6 +220,8 @@ class DependencyFallbacksHolder(MesonInterpreterObject):
                 mlog.log('Dependency', mlog.bold(self._display_name),
                          'found:', mlog.red('NO'), *info)
                 return cached_dep
+        elif self.forcefallback and self.subproject_name:
+            cached_dep = None
         else:
             info = [mlog.blue('(cached)')]
             cached_dep = self.coredata.deps[for_machine].get(identifier)