libutil: Implement createAnonymousTempFile

There are a lot of cases where we don't care about having
the temporary file linked anywhere at all -- just a descriptor is more
than enough.

src/libutil-tests/file-system.cc

@@ -381,4 +381,21 @@ TEST(openFileEnsureBeneathNoSymlinks, works)
 
 #endif
 
+/* ----------------------------------------------------------------------------
+ * createAnonymousTempFile
+ * --------------------------------------------------------------------------*/
+
+TEST(createAnonymousTempFile, works)
+{
+    auto fd = createAnonymousTempFile();
+    writeFull(fd.get(), "test");
+    lseek(fd.get(), 0, SEEK_SET);
+    FdSource source{fd.get()};
+    EXPECT_EQ(source.drain(), "test");
+    lseek(fd.get(), 0, SEEK_END);
+    writeFull(fd.get(), "test");
+    lseek(fd.get(), 0, SEEK_SET);
+    EXPECT_EQ(source.drain(), "testtest");
+}
+
 } // namespace nix

src/libutil/file-system.cc

@@ -713,11 +713,31 @@ std::filesystem::path createTempDir(const std::filesystem::path & tmpRoot, const
     }
 }
 
+AutoCloseFD createAnonymousTempFile()
+{
+    AutoCloseFD fd;
+#ifdef O_TMPFILE
+    fd = ::open(defaultTempDir().c_str(), O_TMPFILE | O_CLOEXEC | O_RDWR, S_IWUSR | S_IRUSR);
+    if (!fd)
+        throw SysError("creating anonymous temporary file");
+#else
+    auto [fd2, path] = createTempFile("nix-anonymous");
+    if (!fd2)
+        throw SysError("creating temporary file '%s'", path);
+    fd = std::move(fd2);
+#  ifndef _WIN32
+    unlink(requireCString(path)); /* We only care about the file descriptor. */
+#  endif
+#endif
+    return fd;
+}
+
 std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
 {
     Path tmpl(defaultTempDir() + "/" + prefix + ".XXXXXX");
     // Strictly speaking, this is UB, but who cares...
     // FIXME: use O_TMPFILE.
+    // FIXME: Windows should use FILE_ATTRIBUTE_TEMPORARY | FILE_FLAG_DELETE_ON_CLOSE
     AutoCloseFD fd = toDescriptor(mkstemp((char *) tmpl.c_str()));
     if (!fd)
         throw SysError("creating temporary file '%s'", tmpl);

src/libutil/include/nix/util/file-system.hh

@@ -337,6 +337,12 @@ typedef std::unique_ptr<DIR, DIRDeleter> AutoCloseDir;
 std::filesystem::path
 createTempDir(const std::filesystem::path & tmpRoot = "", const std::string & prefix = "nix", mode_t mode = 0755);
 
+/**
+ * Create an anonymous readable/writable temporary file, returning a file handle.
+ * On UNIX there resulting file isn't linked to any path on the filesystem.
+ */
+AutoCloseFD createAnonymousTempFile();
+
 /**
  * Create a temporary file, returning a file handle and its path.
  */