fix NamedEvent ftok() race on Linux/macOS (#1001)

- `ftok()` internally re-`stat()`s the path and fails with `ENOENT` if another owner's destructor unlinks the backing file between our `open()` and `ftok()`; the held fd does not protect against this
- derive the IPC key via `fstat()` on the fd instead, using the same `(ino & 0xffff) | ((dev & 0xff) << 16) | (proj << 24)` formula that glibc and macOS `ftok()` compute internally
- fixes intermittent "Failed to create an SysV IPC key" failures on macOS, where the slower on-disk /tmp widens the race window

1 files changed, 8 insertions, 4 deletions

diff --git a/src/zencore/thread.cpp b/src/zencore/thread.cpp
index fd72afaa7..1f4004373 100644
--- a/src/zencore/thread.cpp
+++ b/src/zencore/thread.cpp
@@ -336,13 +336,17 @@ NamedEvent::NamedEvent(std::string_view EventName)
 	}
 	fchmod(Fd, 0666);
 
-	// Use the file path to generate an IPC key
-	key_t IpcKey = ftok(EventPath.c_str(), 1);
-	if (IpcKey < 0)
+	// Derive the IPC key via fstat() on the fd instead of ftok() on the path:
+	// another owner's destructor can unlink the backing file between our open()
+	// and ftok(), in which case ftok's internal stat() fails with ENOENT. The
+	// formula below matches what glibc/macOS ftok() compute internally.
+	struct stat IpcStat;
+	if (fstat(Fd, &IpcStat) < 0)
 	{
 		close(Fd);
-		ThrowLastError("Failed to create an SysV IPC key");
+		ThrowLastError("Failed to stat backing file for SysV IPC key");
 	}
+	const key_t IpcKey = key_t((IpcStat.st_ino & 0xffff) | ((IpcStat.st_dev & 0xff) << 16) | (1 << 24));
 
 	// Use the key to create/open the semaphore
 	int Sem = semget(IpcKey, 1, 0600 | IPC_CREAT);