fix: fallback for memfd (kernel 3.17) (#204)

Signed-off-by: usamoi <[email protected]>

Author: usamoi

Diff for: Cargo.lock

@@ -33,6 +33,7 @@ pgrx = { git = "https://github.com/tensorchord/pgrx.git", rev = "7c30e2023876c1e
 openai_api_rust = { git = "https://github.com/tensorchord/openai-api.git", rev = "228d54b6002e98257b3c81501a054942342f585f" }
 env_logger = "0.10.0"
 toml = "0.8.8"
+rand = "0.8.5"
 
 [dev-dependencies]
 pgrx-tests = { git = "https://github.com/tensorchord/pgrx.git", rev = "7c30e2023876c1efce613756f5ec81f3ab05696b" }

Diff for: Cargo.toml

@@ -3,7 +3,7 @@
 //! Provides an easy-to-use extension for vector similarity search.
 #![feature(offset_of)]
 #![feature(arbitrary_self_types)]
-#![feature(try_blocks)]
+#![feature(lazy_cell)]
 
 mod bgworker;
 mod datatype;

Diff for: src/lib.rs

@@ -1,6 +1,18 @@
 use rustix::fd::{AsFd, OwnedFd};
 use rustix::mm::{MapFlags, ProtFlags};
+use std::io::ErrorKind;
 use std::sync::atomic::AtomicU32;
+use std::sync::LazyLock;
+
+#[cfg(target_os = "linux")]
+static SUPPORT_MEMFD: LazyLock<bool> = LazyLock::new(|| {
+    use rustix::fs::MemfdFlags;
+    match rustix::fs::memfd_create(".memfd.VECTORS.SUPPORT", MemfdFlags::empty()) {
+        Ok(_) => true,
+        Err(e) if e.kind() == ErrorKind::Unsupported => false,
+        Err(_) => false,
+    }
+});
 
 #[cfg(target_os = "linux")]
 pub unsafe fn futex_wait(futex: &AtomicU32, value: u32) {
@@ -28,11 +40,30 @@ pub unsafe fn futex_wake(futex: &AtomicU32) {
 
 #[cfg(target_os = "linux")]
 pub fn memfd_create() -> std::io::Result<OwnedFd> {
-    use rustix::fs::MemfdFlags;
-    Ok(rustix::fs::memfd_create(
-        format!(".memfd.VECTORS.{:x}", std::process::id()),
-        MemfdFlags::empty(),
-    )?)
+    if *SUPPORT_MEMFD {
+        use rustix::fs::MemfdFlags;
+        Ok(rustix::fs::memfd_create(
+            format!(".memfd.VECTORS.{:x}", std::process::id()),
+            MemfdFlags::empty(),
+        )?)
+    } else {
+        use rustix::fs::Mode;
+        use rustix::fs::OFlags;
+        // POSIX fcntl locking do not support shmem, so we use a regular file here.
+        // reference: https://man7.org/linux/man-pages/man3/fcntl.3p.html
+        let name = format!(
+            ".shm.VECTORS.{:x}.{:x}",
+            std::process::id(),
+            rand::random::<u32>()
+        );
+        let fd = rustix::fs::open(
+            &name,
+            OFlags::RDWR | OFlags::CREATE | OFlags::EXCL,
+            Mode::RUSR | Mode::WUSR,
+        )?;
+        rustix::fs::unlink(&name)?;
+        Ok(fd)
+    }
 }
 
 #[cfg(target_os = "linux")]