Basic fcntl-style locking. (#555)

Add an `fcntl_lock` function implementing fcntl-style
process-associated locking. Currently this only supports locking the
entire file.

Author: sunfishcode

src/backend/libc/fs/syscalls.rs

@@ -68,7 +68,7 @@ use crate::fs::Advice;
     target_os = "redox",
 )))]
 use crate::fs::FallocateFlags;
-#[cfg(not(any(target_os = "solaris", target_os = "wasi")))]
+#[cfg(not(target_os = "wasi"))]
 use crate::fs::FlockOperation;
 #[cfg(any(target_os = "android", target_os = "freebsd", target_os = "linux"))]
 use crate::fs::MemfdFlags;
@@ -829,6 +829,40 @@ pub(crate) fn fcntl_add_seals(fd: BorrowedFd<'_>, seals: SealFlags) -> io::Resul
     unsafe { ret(c::fcntl(borrowed_fd(fd), c::F_ADD_SEALS, seals.bits())) }
 }
 
+#[cfg(not(any(
+    target_os = "emscripten",
+    target_os = "fuchsia",
+    target_os = "redox",
+    target_os = "wasi"
+)))]
+#[inline]
+pub(crate) fn fcntl_lock(fd: BorrowedFd<'_>, operation: FlockOperation) -> io::Result<()> {
+    use c::{flock, F_RDLCK, F_SETLK, F_SETLKW, F_UNLCK, F_WRLCK, SEEK_SET};
+
+    let (cmd, l_type) = match operation {
+        FlockOperation::LockShared => (F_SETLKW, F_RDLCK),
+        FlockOperation::LockExclusive => (F_SETLKW, F_WRLCK),
+        FlockOperation::Unlock => (F_SETLKW, F_UNLCK),
+        FlockOperation::NonBlockingLockShared => (F_SETLK, F_RDLCK),
+        FlockOperation::NonBlockingLockExclusive => (F_SETLK, F_WRLCK),
+        FlockOperation::NonBlockingUnlock => (F_SETLK, F_UNLCK),
+    };
+
+    unsafe {
+        let mut lock: flock = core::mem::zeroed();
+        lock.l_type = l_type as _;
+
+        // When `l_len` is zero, this locks all the bytes from
+        // `l_whence`/`l_start` to the end of the file, even as the
+        // file grows dynamically.
+        lock.l_whence = SEEK_SET as _;
+        lock.l_start = 0;
+        lock.l_len = 0;
+
+        ret(c::fcntl(borrowed_fd(fd), cmd, &lock))
+    }
+}
+
 pub(crate) fn seek(fd: BorrowedFd<'_>, pos: SeekFrom) -> io::Result<u64> {
     let (whence, offset): (c::c_int, libc_off_t) = match pos {
         SeekFrom::Start(pos) => {

src/backend/libc/fs/types.rs

@@ -785,10 +785,11 @@ bitflags! {
     }
 }
 
-/// `LOCK_*` constants for use with [`flock`]
+/// `LOCK_*` constants for use with [`flock`] and [`fcntl_lock`].
 ///
 /// [`flock`]: crate::fs::flock
-#[cfg(not(any(target_os = "solaris", target_os = "wasi")))]
+/// [`fcntl_lock`]: crate::fs::fcntl_lock
+#[cfg(not(target_os = "wasi"))]
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 #[repr(i32)]
 pub enum FlockOperation {

src/backend/linux_raw/fs/syscalls.rs

@@ -1022,6 +1022,58 @@ pub(crate) fn fcntl_add_seals(fd: BorrowedFd<'_>, seals: SealFlags) -> io::Resul
     }
 }
 
+#[inline]
+pub(crate) fn fcntl_lock(fd: BorrowedFd<'_>, operation: FlockOperation) -> io::Result<()> {
+    #[cfg(target_pointer_width = "64")]
+    use linux_raw_sys::general::{flock, F_SETLK, F_SETLKW};
+    #[cfg(target_pointer_width = "32")]
+    use linux_raw_sys::general::{flock64 as flock, F_SETLK64 as F_SETLK, F_SETLKW64 as F_SETLKW};
+    use linux_raw_sys::general::{F_RDLCK, F_UNLCK, F_WRLCK};
+
+    let (cmd, l_type) = match operation {
+        FlockOperation::LockShared => (F_SETLKW, F_RDLCK),
+        FlockOperation::LockExclusive => (F_SETLKW, F_WRLCK),
+        FlockOperation::Unlock => (F_SETLKW, F_UNLCK),
+        FlockOperation::NonBlockingLockShared => (F_SETLK, F_RDLCK),
+        FlockOperation::NonBlockingLockExclusive => (F_SETLK, F_WRLCK),
+        FlockOperation::NonBlockingUnlock => (F_SETLK, F_UNLCK),
+    };
+
+    unsafe {
+        let lock = flock {
+            l_type: l_type as _,
+
+            // When `l_len` is zero, this locks all the bytes from
+            // `l_whence`/`l_start` to the end of the file, even as the
+            // file grows dynamically.
+            l_whence: SEEK_SET as _,
+            l_start: 0,
+            l_len: 0,
+
+            ..core::mem::zeroed()
+        };
+
+        #[cfg(target_pointer_width = "32")]
+        {
+            ret(syscall_readonly!(
+                __NR_fcntl64,
+                fd,
+                c_uint(cmd),
+                by_ref(&lock)
+            ))
+        }
+        #[cfg(target_pointer_width = "64")]
+        {
+            ret(syscall_readonly!(
+                __NR_fcntl,
+                fd,
+                c_uint(cmd),
+                by_ref(&lock)
+            ))
+        }
+    }
+}
+
 #[inline]
 pub(crate) fn rename(oldname: &CStr, newname: &CStr) -> io::Result<()> {
     #[cfg(target_arch = "riscv64")]

src/backend/linux_raw/fs/types.rs

@@ -518,9 +518,10 @@ bitflags! {
     }
 }
 
-/// `LOCK_*` constants for use with [`flock`]
+/// `LOCK_*` constants for use with [`flock`] and [`fcntl_lock`].
 ///
 /// [`flock`]: crate::fs::flock
+/// [`fcntl_lock`]: crate::fs::fcntl_lock
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 #[repr(u32)]
 pub enum FlockOperation {

src/fs/fcntl.rs

@@ -3,6 +3,13 @@
 //! a type-safe API, rustix makes them all separate functions so that they
 //! can have dedicated static type signatures.
 
+#[cfg(not(any(
+    target_os = "emscripten",
+    target_os = "fuchsia",
+    target_os = "redox",
+    target_os = "wasi"
+)))]
+use crate::fs::FlockOperation;
 use crate::{backend, io};
 use backend::fd::AsFd;
 use backend::fs::types::OFlags;
@@ -85,3 +92,32 @@ pub use backend::fs::types::SealFlags;
 pub fn fcntl_add_seals<Fd: AsFd>(fd: Fd, seals: SealFlags) -> io::Result<()> {
     backend::fs::syscalls::fcntl_add_seals(fd.as_fd(), seals)
 }
+
+/// `fcntl(fd, F_SETLK)`—Acquire or release an `fcntl`-style lock.
+///
+/// This function doesn't currently have an offset or len; it currently always
+/// sets the `l_len` field to 0, which is a special case that means the entire
+/// file should be locked.
+///
+/// Unlike `flock`-style locks, `fcntl`-style locks are process-associated,
+/// meaning that they don't guard against being acquired by two threads in
+/// the same process.
+///
+/// # References
+///  - [POSIX]
+///  - [Linux]
+///
+/// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/fcntl.html
+/// [Linux]: https://man7.org/linux/man-pages/man2/fcntl.2.html
+#[cfg(not(any(
+    target_os = "emscripten",
+    target_os = "fuchsia",
+    target_os = "redox",
+    target_os = "wasi"
+)))]
+#[inline]
+#[doc(alias = "F_SETLK")]
+#[doc(alias = "F_SETLKW")]
+pub fn fcntl_lock<Fd: AsFd>(fd: Fd, operation: FlockOperation) -> io::Result<()> {
+    backend::fs::syscalls::fcntl_lock(fd.as_fd(), operation)
+}

src/fs/fcntl_darwin.rs renamed to src/fs/fcntl_apple.rs

@@ -8,7 +8,7 @@ use crate::process::{Gid, Uid};
 use crate::{backend, io};
 use backend::fd::{AsFd, BorrowedFd};
 
-#[cfg(not(any(target_os = "solaris", target_os = "wasi")))]
+#[cfg(not(target_os = "wasi"))]
 pub use backend::fs::types::FlockOperation;
 
 #[cfg(not(any(

src/fs/fd.rs

@@ -21,7 +21,7 @@ mod dir;
 mod fadvise;
 pub(crate) mod fcntl;
 #[cfg(apple)]
-mod fcntl_darwin;
+mod fcntl_apple;
 #[cfg(apple)]
 mod fcopyfile;
 pub(crate) mod fd;
@@ -66,7 +66,7 @@ pub use dir::{Dir, DirEntry};
 pub use fadvise::{fadvise, Advice};
 pub use fcntl::*;
 #[cfg(apple)]
-pub use fcntl_darwin::{fcntl_fullfsync, fcntl_rdadvise};
+pub use fcntl_apple::{fcntl_fullfsync, fcntl_rdadvise};
 #[cfg(apple)]
 pub use fcopyfile::*;
 pub use fd::*;

src/fs/mod.rs

@@ -0,0 +1,33 @@
+#[test]
+fn test_fcntl_lock() {
+    use rustix::fs::{fcntl_lock, FlockOperation};
+
+    let f = tempfile::tempfile().unwrap();
+    fcntl_lock(&f, FlockOperation::LockExclusive).unwrap();
+    fcntl_lock(&f, FlockOperation::Unlock).unwrap();
+    let g = tempfile::tempfile().unwrap();
+    fcntl_lock(&g, FlockOperation::LockExclusive).unwrap();
+    fcntl_lock(&g, FlockOperation::Unlock).unwrap();
+    drop(f);
+    drop(g);
+
+    let f = tempfile::tempfile().unwrap();
+    fcntl_lock(&f, FlockOperation::LockShared).unwrap();
+    let g = tempfile::tempfile().unwrap();
+    fcntl_lock(&g, FlockOperation::LockShared).unwrap();
+    fcntl_lock(&f, FlockOperation::Unlock).unwrap();
+    fcntl_lock(&g, FlockOperation::Unlock).unwrap();
+    drop(f);
+    drop(g);
+
+    let f = tempfile::tempfile().unwrap();
+    fcntl_lock(&f, FlockOperation::LockShared).unwrap();
+    fcntl_lock(&f, FlockOperation::LockExclusive).unwrap();
+    fcntl_lock(&f, FlockOperation::Unlock).unwrap();
+    let g = tempfile::tempfile().unwrap();
+    fcntl_lock(&g, FlockOperation::LockShared).unwrap();
+    fcntl_lock(&g, FlockOperation::LockExclusive).unwrap();
+    fcntl_lock(&g, FlockOperation::Unlock).unwrap();
+    drop(f);
+    drop(g);
+}

tests/fs/fcntl_lock.rs

@@ -9,6 +9,13 @@
 mod cwd;
 mod dir;
 mod fcntl;
+#[cfg(not(any(
+    target_os = "emscripten",
+    target_os = "fuchsia",
+    target_os = "redox",
+    target_os = "wasi"
+)))]
+mod fcntl_lock;
 mod file;
 #[cfg(not(target_os = "wasi"))]
 mod flock;