Fix mkdir("a/b/..") should return EEXIST (#23136)

Before this change `mkdir("a/b/..")` surprisingly makes a directory
called `a/b/a`. It should raise EEXIST.

src/library_fs.js

@@ -182,7 +182,7 @@ FS.staticInit();
       // limit max consecutive symlinks to 40 (SYMLOOP_MAX).
       linkloop: for (var nlinks = 0; nlinks < 40; nlinks++) {
         // split the absolute path
-        var parts = path.split('/').filter((p) => !!p && (p !== '.'));
+        var parts = path.split('/').filter((p) => !!p);
 
         // start at the root
         var current = FS.root;
@@ -195,6 +195,10 @@ FS.staticInit();
             break;
           }
 
+          if (parts[i] === '.') {
+            continue;
+          }
+
           if (parts[i] === '..') {
             current_path = PATH.dirname(current_path);
             current = current.parent;
@@ -665,9 +669,12 @@ FS.staticInit();
       var lookup = FS.lookupPath(path, { parent: true });
       var parent = lookup.node;
       var name = PATH.basename(path);
-      if (!name || name === '.' || name === '..') {
+      if (!name) {
         throw new FS.ErrnoError({{{ cDefs.EINVAL }}});
       }
+      if (name === '.' || name === '..') {
+        throw new FS.ErrnoError({{{ cDefs.EEXIST }}});
+      }
       var errCode = FS.mayCreate(parent, name);
       if (errCode) {
         throw new FS.ErrnoError(errCode);

test/fs/test_fs_mkdir_dotdot.c

@@ -0,0 +1,27 @@
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+#include <assert.h>
+
+int main() {
+  assert(mkdir("test", 0777) == 0);
+  assert(mkdir("test/a", 0777) == 0);
+  assert(mkdir("test/a/..", 0777) == -1);
+  printf("error: %s\n", strerror(errno));
+  assert(errno == EEXIST);
+  assert(mkdir("test/a/.", 0777) == -1);
+  printf("error: %s\n", strerror(errno));
+  assert(errno == EEXIST);
+  assert(mkdir("test/a/b/..", 0777) == -1);
+  printf("error: %s\n", strerror(errno));
+  assert(errno == ENOENT);
+  assert(mkdir("test/a/b/.", 0777) == -1);
+  printf("error: %s\n", strerror(errno));
+  assert(errno == ENOENT);
+  printf("success\n");
+}

test/test_core.py

@@ -5858,6 +5858,10 @@ def test_fs_rename_on_existing(self):
       self.set_setting('FORCE_FILESYSTEM')
     self.do_runf('fs/test_fs_rename_on_existing.c', 'success')
 
+  @also_with_nodefs_both
+  def test_fs_mkdir_dotdot(self):
+    self.do_runf('fs/test_fs_mkdir_dotdot.c', 'success')
+
   def test_sigalrm(self):
     self.do_runf('test_sigalrm.c', 'Received alarm!')
     self.do_runf('test_sigalrm.c', 'Received alarm!', emcc_args=['-sEXIT_RUNTIME'])