[WasmFS] Implement /tmp (#17237)

Also refactor the setup of /dev/* to use the newer backend API since we're
touching that code anyway. For testing, add a WasmFS configuration to an
arbitrary test that depends on /tmp.

Fixes #17127.

Author: tlively

system/lib/wasmfs/wasmfs.cpp

@@ -75,16 +75,19 @@ std::shared_ptr<Directory> WasmFS::initRootDirectory() {
   // The root directory is its own parent.
   lockedRoot.setParent(rootDirectory);
 
-  auto devDirectory =
-    std::make_shared<MemoryDirectory>(S_IRUGO | S_IXUGO, rootBackend);
-  lockedRoot.mountChild("dev", devDirectory);
-  auto lockedDev = devDirectory->locked();
-
-  lockedDev.mountChild("stdin", SpecialFiles::getStdin());
-  lockedDev.mountChild("stdout", SpecialFiles::getStdout());
-  lockedDev.mountChild("stderr", SpecialFiles::getStderr());
-  lockedDev.mountChild("random", SpecialFiles::getRandom());
-  lockedDev.mountChild("urandom", SpecialFiles::getURandom());
+  auto devDir = lockedRoot.insertDirectory("dev", S_IRUGO | S_IXUGO);
+  assert(devDir);
+  {
+    auto lockedDev = devDir->locked();
+    lockedDev.mountChild("stdin", SpecialFiles::getStdin());
+    lockedDev.mountChild("stdout", SpecialFiles::getStdout());
+    lockedDev.mountChild("stderr", SpecialFiles::getStderr());
+    lockedDev.mountChild("random", SpecialFiles::getRandom());
+    lockedDev.mountChild("urandom", SpecialFiles::getURandom());
+  }
+
+  [[maybe_unused]] auto tmpDir = lockedRoot.insertDirectory("tmp", S_IRWXUGO);
+  assert(tmpDir);
 
   return rootDirectory;
 }

tests/test_other.py

@@ -4949,6 +4949,7 @@ def test_strftime_zZ(self):
   def test_strptime_symmetry(self):
     self.do_runf(test_file('strptime_symmetry.cpp'), 'TEST PASSED')
 
+  @also_with_wasmfs
   def test_truncate_from_0(self):
     create_file('src.cpp', r'''
 #include <cerrno>