Fix bug where monorepo packages were not configured

This fixes a bug, where deep `package.json`s without config,
which were previously ignored and higher `package.json`s used,
were instead seen as an unconfigured project.

This bug was introduced in 9.1.0.

Related-to: GH-64.
Related-to: 7f91757.

lib/find-up.js

@@ -7,7 +7,7 @@ import {wrap} from 'trough'
 const debug = createDebug('unified-engine:find-up')
 
 /**
- * @template Value
+ * @template {object & {filePath?: string}} Value
  */
 export class FindUp {
   /**
@@ -193,9 +193,11 @@ export class FindUp {
                 error.message
               )
             )
-          } else {
+          } else if (result && result.filePath) {
             debug('Read file `%s`', fp)
             found(null, result)
+          } else {
+            next()
           }
         })(buf, fp)
       }

test/configuration.js

@@ -12,7 +12,7 @@ import {spy} from './util/spy.js'
 const fixtures = new URL('fixtures/', import.meta.url)
 
 test('configuration', (t) => {
-  t.plan(15)
+  t.plan(16)
 
   t.test('should fail fatally when custom rc files are missing', (t) => {
     const stderr = spy()
@@ -483,4 +483,32 @@ test('configuration', (t) => {
       }
     )
   })
+
+  t.test('should ignore unconfigured `packages.json`', (t) => {
+    const stderr = spy()
+
+    t.plan(1)
+
+    engine(
+      {
+        processor: noop(),
+        cwd: new URL('config-monorepo-package/', fixtures),
+        streamError: stderr.stream,
+        files: ['.'],
+        packageField: 'fooConfig',
+        extensions: ['txt']
+      },
+      (error, code) => {
+        const actual = stderr().split('\n').slice(0, 4).join('\n')
+        const expected = [
+          'packages' + sep + 'deep' + sep + 'one.txt',
+          '  1:1  error  Error: Cannot parse file `package.json`',
+          'Cannot import `plugin.js`',
+          'Error: Boom!'
+        ].join('\n')
+
+        t.deepEqual([error, code, actual], [null, 1, expected], 'should report')
+      }
+    )
+  })
 })

test/fixtures/config-monorepo-package/package.json

@@ -0,0 +1,8 @@
+{
+  "type": "module",
+  "fooConfig": {
+    "plugins": [
+      "./plugin.js"
+    ]
+  }
+}

test/fixtures/config-monorepo-package/packages/deep/package.json

@@ -0,0 +1,4 @@
+{
+  "type": "module",
+  "#": "this `package.json` does not have a config field"
+}

test/fixtures/config-monorepo-package/plugin.js

@@ -0,0 +1 @@
+throw new Error('Boom!')