fix cicle required modules

lib/mock.js

@@ -29,6 +29,8 @@ are the same used in ${filename} require calls`)
       this.mocks.set(mockFilePath, mocks[key])
     }
 
+    const seen = new Map()
+
     class MockedModule extends Module {
       require (id) {
         const requiredFilePath = Module._resolveFilename(id, this)
@@ -45,7 +47,11 @@ are the same used in ${filename} require calls`)
         if (!isRelative && !isAbsolute(id))
           return super.require(id)
 
+        if (seen.has(requiredFilePath))
+          return seen.get(requiredFilePath).exports
+
         const unmockedModule = new MockedModule(requiredFilePath, this)
+        seen.set(requiredFilePath, unmockedModule)
         unmockedModule.load(requiredFilePath)
         return unmockedModule.exports
       }

test/test.js

@@ -1488,5 +1488,34 @@ t.test('require defining mocks', t => {
     )
   })
 
+  t.test('should support cicle require', t => {
+    const f = t.testdir({
+      lib: {
+        'a.js':
+          `module.exports = require('./b.js')`,
+        'b.js':
+          `const c = require('./c.js')
+          const b = () => 'b and ' + c
+          b.extra = 'BBB'
+          module.exports = () => 'b and ' + c`,
+        'c.js':
+          `const b = require('./b.js')
+          module.exports = () => 'c and ' + b.extra`,
+      },
+      'test.js':
+        `const t = require('../..'); // tap
+        t.test('mocking cicled required deps', t => {
+          const a = t.mock('./lib/a.js', {})
+          t.ok('should not explode')
+          t.end()
+        })`,
+    })
+    return t.spawn(
+      process.execPath,
+      [ path.resolve(f, 'test.js') ],
+      { cwd: f },
+    )
+  })
+
   t.end()
 })