Handle EROFS errors

A EROFS will be raised when trying to write a dir onto a read-only
filesystem.  However, if the dir already is there, then it should be
treated like an EEXIST (since EROFS can be raised by trying to create a
dir over a mount point, where the mount point is not read-only, but you
still can't just clobber over it).

If the dir doesn't already exist, then the EROFS will be accurately
reported as the reason why it could not be created.

(Copied from 17ee84f)

lib/mkdirp-manual.js

@@ -16,7 +16,7 @@ const mkdirpManual = (path, opts, made) => {
     if (er.code === 'ENOENT')
       return mkdirpManual(parent, opts)
         .then(made => mkdirpManual(path, opts, made))
-    if (er.code !== 'EEXIST')
+    if (er.code !== 'EEXIST' && er.code !== 'EROFS')
       throw er
     return opts.statAsync(path).then(st => {
       if (st.isDirectory())
@@ -50,7 +50,7 @@ const mkdirpManualSync = (path, opts, made) => {
   } catch (er) {
     if (er.code === 'ENOENT')
       return mkdirpManualSync(path, opts, mkdirpManualSync(parent, opts, made))
-    if (er.code !== 'EEXIST')
+    if (er.code !== 'EEXIST' && er.code !== 'EROFS')
       throw er
     try {
       if (!opts.statSync(path).isDirectory())

test/mkdirp-manual.js

@@ -62,6 +62,26 @@ t.test('mkdirpManual / just calls implementation', t => {
   t.end()
 })
 
+t.test('read-only file system, still succeed if dir exists', t => {
+  const dir = t.testdir({ foo: {} })
+  const opt = {
+    stat,
+    statAsync,
+    statSync,
+    mkdir,
+    mkdirAsync: () => Promise.reject(Object.assign(new Error('EROFS'), {
+      code: 'EROFS',
+    })),
+    mkdirSync: () => {
+      throw Object.assign(new Error('EROFS'), {
+        code: 'EROFS',
+      })
+    },
+  }
+  t.equal(mkdirpManualSync(`${dir}/foo`, opt), undefined)
+  return mkdirpManual(`${dir}/foo`, opt).then(made => t.equal(made, undefined))
+})
+
 t.test('recurse and return first dir made', t => {
   const dir = t.testdir()
   const opt = {