Fix FS.mkdirTree with relative paths (#23902)

Split out from #23897

Author: sbc100

src/lib/libfs.js

@@ -748,7 +748,8 @@ FS.staticInit();
       var d = '';
       for (var dir of dirs) {
         if (!dir) continue;
-        d += '/' + dir;
+        if (d || PATH.isAbs(path)) d += '/';
+        d += dir;
         try {
           FS.mkdir(d, mode);
         } catch(e) {

src/lib/libwasmfs.js

@@ -542,7 +542,8 @@ addToLibrary({
     var d = '';
     for (var dir of dirs) {
       if (!dir) continue;
-      d += '/' + dir;
+      if (d || PATH.isAbs(path)) d += '/';
+      d += dir;
       try {
         FS_mkdir(d, mode);
       } catch(e) {

test/fs/test_fs_js_api.c

@@ -381,20 +381,19 @@ void test_fs_mmap() {
 
 void test_fs_mkdirTree() {
   EM_ASM(
-    FS.mkdirTree("/test1/test2/test3");
-
+    FS.mkdirTree("/test1/test2/test3"); // Abs path
     FS.mkdirTree("/readable", 0400 /* S_IRUSR */);
   );
 
   struct stat s;
-  stat("/test1", &s);
+  assert(stat("/test1", &s) == 0);
   assert(S_ISDIR(s.st_mode));
-  stat("/test1/test2", &s);
+  assert(stat("/test1/test2", &s) == 0);
   assert(S_ISDIR(s.st_mode));
-  stat("/test1/test2/test3", &s);
+  assert(stat("/test1/test2/test3", &s) == 0);
   assert(S_ISDIR(s.st_mode));
 
-  stat("/readable", &s);
+  assert(stat("/readable", &s) == 0);
   assert(s.st_mode & 0400 /* S_IRUSR */);
 
   EM_ASM(
@@ -407,8 +406,22 @@ void test_fs_mkdirTree() {
     assert(ex.name === "ErrnoError" && ex.errno === 2 /* EACCES */);
   );
 
+  chdir("test1");
+  EM_ASM(
+    FS.mkdirTree("foo/bar"); // Relative path
+  );
+  chdir("/");
+
+  assert(stat("/test1/foo", &s) == 0);
+  assert(S_ISDIR(s.st_mode));
+
+  assert(stat("/test1/foo/bar", &s) == 0);
+  assert(S_ISDIR(s.st_mode));
+
   remove("/test1/test2/test3");
   remove("/test1/test2");
+  remove("/test1/foo/bar");
+  remove("/test1/foo");
   remove("/test1");
   remove("/readable");
 }