Also reuse subproject-based fallback dependencies

This allows a project to use the same fallbacks dependency from the same
subproject multiple times in the same way that external dependencies can be.

Also change the format of the dependency identifier to ensure that fallback
checks with different dirname/varname aren't mistakenly reused. We now use
a tuple for this because the format is simpler to construct and it gives us the
same immutability guarantees as a string which is needed for using it as
a dictionary key.

mesonbuild/dependencies.py

@@ -1148,7 +1148,15 @@ def get_dep_identifier(name, kwargs):
         modlist = [modlist]
     for module in modlist:
         elements.append(module)
-    return '/'.join(elements) + '/main' + str(kwargs.get('main', False)) + '/static' + str(kwargs.get('static', False))
+    # We use a tuple because we need a non-mutable structure to use as the key
+    # of a dictionary and a string has potential for name collisions
+    identifier = tuple(elements)
+    identifier += ('main', kwargs.get('main', False))
+    identifier += ('static', kwargs.get('static', False))
+    if 'fallback' in kwargs:
+        f = kwargs.get('fallback')
+        identifier += ('fallback', f[0], f[1])
+    return identifier
 
 def find_external_dependency(name, environment, kwargs):
     required = kwargs.get('required', True)

mesonbuild/interpreter.py

@@ -1626,7 +1626,9 @@ class Interpreter():
                 dep = dependencies.find_external_dependency(name, self.environment, kwargs)
             except dependencies.DependencyException:
                 if 'fallback' in kwargs:
-                    return self.dependency_fallback(kwargs)
+                    dep = self.dependency_fallback(kwargs)
+                    self.coredata.deps[identifier] = dep.held_object
+                    return dep
                 raise
         self.coredata.deps[identifier] = dep
         return DependencyHolder(dep)