Can specify a subproject fallback for dependencies.

interpreter.py

@@ -1262,6 +1262,9 @@ class Interpreter():
         if len(args) != 1:
             raise InterpreterException('Subproject takes exactly one argument')
         dirname = args[0]
+        return self.do_subproject(dirname, kwargs)
+
+    def do_subproject(self, dirname, kwargs):
         if self.subdir != '':
             segs = os.path.split(self.subdir)
             if len(segs) != 2 or segs[0] != self.subproject_dir:
@@ -1537,10 +1540,24 @@ class Interpreter():
         else:
             dep = dependencies.Dependency() # Returns always false for dep.found()
         if not dep.found():
-            dep = dependencies.find_external_dependency(name, self.environment, kwargs)
+            try:
+                dep = dependencies.find_external_dependency(name, self.environment, kwargs)
+            except dependencies.DependencyException:
+                if 'fallback' in kwargs:
+                    return self.dependency_fallback(kwargs)
+                raise
         self.coredata.deps[identifier] = dep
         return DependencyHolder(dep)
 
+    def dependency_fallback(self, kwargs):
+        fbinfo = kwargs['fallback']
+        check_stringlist(fbinfo)
+        if len(fbinfo) != 2:
+            raise InterpreterException('Fallback info must have exactly two items.')
+        dirname, varname = fbinfo
+        self.do_subproject(dirname, kwargs)
+        return self.subprojects[dirname].get_variable_method([varname], {})
+
     def func_executable(self, node, args, kwargs):
         return self.build_target(node, args, kwargs, ExecutableHolder)
 

test cases/common/95 dep fallback/meson.build

@@ -0,0 +1,6 @@
+project('dep fallback', 'c')
+
+bob = dependency('boblib', fallback : ['boblib', 'bob_dep'])
+
+exe = executable('bobtester', 'tester.c', dependencies : bob)
+test('bobtester', exe)

test cases/common/95 dep fallback/subprojects/boblib/bob.c

@@ -0,0 +1,5 @@
+#include"bob.h"
+
+const char* get_bob() {
+    return "bob";
+}

test cases/common/95 dep fallback/subprojects/boblib/bob.h

@@ -0,0 +1,3 @@
+#pragma once
+
+const char* get_bob();

test cases/common/95 dep fallback/subprojects/boblib/meson.build

@@ -0,0 +1,7 @@
+project('bob', 'c')
+
+boblib = static_library('bob', 'bob.c')
+bobinc = include_directories('.')
+
+bob_dep = declare_dependency(link_with : boblib,
+  include_directories : bobinc)

test cases/common/95 dep fallback/tester.c

@@ -0,0 +1,13 @@
+#include"bob.h"
+#include<string.h>
+#include<stdio.h>
+
+int main(int argc, char **argv) {
+    if(strcmp("bob", get_bob()) == 0) {
+        printf("Bob is indeed bob.\n");
+    } else {
+        printf("ERROR: bob is not bob.\n");
+        return 1;
+    }
+    return 0;
+}