core: Enable direct I/O optimistically

...but keep going if the filesystem does not support it. Fixes Limbo on
older Linux kernels that have io_uring but don't support direct I/O on
all filesystems, such as tmpfs or encryptfs.

Cargo.lock

@@ -205,6 +205,12 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
+[[package]]
+name = "cfg_aliases"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "613afe47fcd5fac7ccf1db93babcb082c5994d996f20b8b159f2ad1658eb5724"
+
 [[package]]
 name = "cfg_block"
 version = "0.1.1"
@@ -972,6 +978,7 @@ dependencies = [
  "libc",
  "log",
  "mimalloc",
+ "nix 0.29.0",
  "ordered-multimap",
  "polling",
  "pprof",
@@ -1076,6 +1083,18 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "nix"
+version = "0.29.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "71e2746dc3a24dd78b3cfcb7be93368c6de9963d30f43a6a73998a9cf4b17b46"
+dependencies = [
+ "bitflags 2.6.0",
+ "cfg-if",
+ "cfg_aliases",
+ "libc",
+]
+
 [[package]]
 name = "num-format"
 version = "0.4.4"
@@ -1274,7 +1293,7 @@ dependencies = [
  "inferno",
  "libc",
  "log",
- "nix",
+ "nix 0.26.4",
  "once_cell",
  "parking_lot",
  "smallvec",
@@ -1525,7 +1544,7 @@ dependencies = [
  "libc",
  "log",
  "memchr",
- "nix",
+ "nix 0.26.4",
  "radix_trie",
  "scopeguard",
  "unicode-segmentation",

core/Cargo.toml

@@ -33,6 +33,7 @@ cfg_block = "0.1.1"
 fallible-iterator = "0.3.0"
 libc = "0.2.155"
 log = "0.4.20"
+nix = { version = "0.29.0", features = ["fs"] }
 ordered-multimap = "0.7.1"
 sieve-cache = "0.1.4"
 sqlite3-parser = "0.11.0"

core/io/linux.rs

@@ -3,7 +3,7 @@ use anyhow::Result;
 use libc::iovec;
 use log::trace;
 use std::cell::{Ref, RefCell};
-use std::os::unix::fs::OpenOptionsExt;
+use nix::fcntl::{self, FcntlArg, OFlag};
 use std::os::unix::io::AsRawFd;
 use std::rc::Rc;
 
@@ -52,8 +52,11 @@ impl IO for LinuxIO {
         let file = std::fs::File::options()
             .read(true)
             .write(true)
-            .custom_flags(libc::O_DIRECT)
             .open(path)?;
+        // Let's attempt to enable direct I/O. Not all filesystems support it
+        // so ignore any errors.
+        let fd = file.as_raw_fd();
+        let _= nix::fcntl::fcntl(fd, FcntlArg::F_SETFL(OFlag::O_DIRECT));
         Ok(Rc::new(LinuxFile {
             io: self.inner.clone(),
             file,