libutil: Use openFileEnsureBeneathNoSymlinks in RestoreSink::createRegularFile

Add more assertions for preconditions of openFileEnsureBeneathNoSymlinks to prevent
misuse. Also start using it for regular file creation as well.

Author: xokdvium

src/libutil-tests/file-system.cc

@@ -351,6 +351,10 @@ TEST(openFileEnsureBeneathNoSymlinks, works)
             sink.createDirectory(
                 CanonPath("a/b/c/f"), [](FileSystemObjectSink & dirSink, const CanonPath & relPath) {}),
             SymlinkNotAllowed);
+        ASSERT_THROW(
+            sink.createRegularFile(
+                CanonPath("a/b/c/regular"), [](CreateRegularFileSink & crf) { crf("some contents"); }),
+            SymlinkNotAllowed);
     }
 
     AutoCloseFD dirFd = openDirectory(tmpDir);

src/libutil/fs-sink.cc

@@ -170,7 +170,7 @@ void RestoreSink::createRegularFile(const CanonPath & path, std::function<void(C
             constexpr int flags = O_CREAT | O_EXCL | O_WRONLY | O_CLOEXEC;
             if (!dirFd)
                 return ::open(p.c_str(), flags, 0666);
-            return ::openat(dirFd.get(), path.rel_c_str(), flags, 0666);
+            return unix::openFileEnsureBeneathNoSymlinks(dirFd.get(), path, flags, 0666);
         }();
 #endif
         ;

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

@@ -263,6 +263,8 @@ struct SymlinkNotAllowed : public Error
  * @param flags O_* flags
  * @param mode Mode for O_{CREAT,TMPFILE}
  *
+ * @pre path.isRoot() is false
+ *
  * @throws SymlinkNotAllowed if any path components
  */
 Descriptor openFileEnsureBeneathNoSymlinks(Descriptor dirFd, const CanonPath & path, int flags, mode_t mode = 0);

src/libutil/unix/file-descriptor.cc

@@ -269,6 +269,7 @@ openFileEnsureBeneathNoSymlinksIterative(Descriptor dirFd, const CanonPath & pat
 {
     AutoCloseFD parentFd;
     auto nrComponents = std::ranges::distance(path);
+    assert(nrComponents >= 1);
     auto components = std::views::take(path, nrComponents - 1); /* Everything but last component */
     auto getParentFd = [&]() { return parentFd ? parentFd.get() : dirFd; };
 
@@ -320,6 +321,8 @@ openFileEnsureBeneathNoSymlinksIterative(Descriptor dirFd, const CanonPath & pat
 
 Descriptor unix::openFileEnsureBeneathNoSymlinks(Descriptor dirFd, const CanonPath & path, int flags, mode_t mode)
 {
+    assert(!path.rel().starts_with('/')); /* Just in case the invariant is somehow broken. */
+    assert(!path.isRoot());
 #ifdef __linux__
     auto maybeFd = linux::openat2(
         dirFd, path.rel_c_str(), flags, static_cast<uint64_t>(mode), RESOLVE_BENEATH | RESOLVE_NO_SYMLINKS);