Check if project is missing from vendor during ensure

If a the vendor directory already exists, and the lock file hasn't
changed, even though a project may be missing from the vendor
directory, dep ensure would not add it.

This change checks if we're not already writing the vendor directory
whether any of the projects are missing from the vendor directory
and if they are, ensures the writer writes out the vendor directory.

Fixes golang#883.

cmd/dep/ensure.go

@@ -163,6 +163,23 @@ func (cmd *ensureCommand) Run(ctx *dep.Ctx, args []string) error {
 	}
 
 	newLock := dep.LockFromSolution(solution)
+
+	// check if any projects are missing from vendor, if they are
+	// we'll need to write the vendor directory.
+	if writeV != dep.VendorAlways {
+		for _, proj := range newLock.Projects() {
+			projVendorPath := filepath.Join(p.AbsRoot, "vendor", string(proj.Ident().ProjectRoot))
+			projInVendor, err := fs.IsNonEmptyDir(projVendorPath)
+			if err != nil {
+				return errors.Wrapf(err, "ensure %v is a directory or remove it")
+			}
+			if !projInVendor {
+				writeV = dep.VendorAlways
+				break
+			}
+		}
+	}
+
 	sw, err := dep.NewSafeWriter(nil, p.Lock, newLock, writeV)
 	if err != nil {
 		return err