Change resolve behavior (#249)

Use default resolver when custom resolve does not return an
absolute path.

README.md

@@ -163,10 +163,12 @@ files).
 Type: `Function`  
 Default: `null`
 
-You can overwrite the default path resolving way by setting this option.
-This function gets `(id, basedir, importOptions)` arguments and returns full
-path, array of paths or promise resolving paths.
-You can use [resolve](https://github.com/substack/node-resolve) for that.
+You can provide a custom path resolver with this option. This function gets
+`(id, basedir, importOptions)` arguments and should return a path, an array of
+paths or a promise resolving to the path(s). If you do not return an absolute
+path, your path will be resolved to an absolute path using the default
+resolver.
+You can use [resolve](https://github.com/substack/node-resolve) for this.
 
 #### `load`
 

index.js

@@ -227,11 +227,18 @@ function resolveImportId(
     : options.root
 
   return Promise.resolve(options.resolve(stmt.uri, base, options))
-  .then(function(resolved) {
-    if (!Array.isArray(resolved)) {
-      resolved = [ resolved ]
+  .then(function(paths) {
+    if (!Array.isArray(paths)) {
+      paths = [ paths ]
     }
 
+    return Promise.all(paths.map(function(file) {
+      // Ensure that each path is absolute:
+      if (!path.isAbsolute(file)) return resolveId(file, base, options)
+      return file
+    }))
+  })
+  .then(function(resolved) {
     // Add dependency messages:
     resolved.forEach(function(file) {
       result.messages.push({

test/custom-resolve.js

@@ -1,5 +1,7 @@
 import test from "ava"
 import compareFixtures from "./helpers/compare-fixtures"
+import postcss from "postcss"
+import atImport from ".."
 import path from "path"
 
 test.serial("should accept file", t => {
@@ -43,3 +45,23 @@ test.serial("should accept promised array of files", t => {
     },
   })
 })
+
+test(
+  "should apply default resolver when custom doesn't return an absolute path",
+  function(t) {
+    return postcss()
+    .use(atImport({
+      resolve: path => {
+        return path.replace("foo", "imports/bar")
+      },
+      load: p => {
+        t.is(p, path.resolve("fixtures/imports", "bar.css"))
+        return "/* comment */"
+      },
+    }))
+    .process(`@import "foo.css";`, { from: "fixtures/custom-resolve-file" })
+    .then(result => {
+      t.is(result.css, "/* comment */")
+    })
+  }
+)