ops: add fs::symlink function (#308)

This change adds the `symlink` function to tokio-uring, with the
identical signature and documentation as in tokio itself. I added one
unit test. I'm merrily doing blocking I/O inside the async block in the
unit test, which I _think_ should be fine because it's just a test.

src/fs/mod.rs

@@ -20,3 +20,6 @@ mod statx;
 pub use statx::is_dir_regfile;
 pub use statx::statx;
 pub use statx::StatxBuilder;
+
+mod symlink;
+pub use symlink::symlink;

src/fs/symlink.rs

@@ -0,0 +1,10 @@
+use crate::runtime::driver::op::Op;
+use std::io;
+use std::path::Path;
+
+/// Creates a new symbolic link on the filesystem.
+/// The dst path will be a symbolic link pointing to the src path.
+/// This is an async version of std::os::unix::fs::symlink.
+pub async fn symlink<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Result<()> {
+    Op::symlink(src, dst)?.await
+}

src/io/mod.rs

@@ -43,6 +43,8 @@ pub(crate) use socket::Socket;
 
 mod statx;
 
+mod symlink;
+
 mod unlink_at;
 
 mod util;

src/io/symlink.rs

@@ -0,0 +1,45 @@
+use crate::runtime::driver::op::{Completable, CqeResult, Op};
+use crate::runtime::CONTEXT;
+
+use super::util::cstr;
+
+use std::ffi::CString;
+use std::io;
+use std::path::Path;
+
+pub(crate) struct Symlink {
+    pub(crate) _from: CString,
+    pub(crate) _to: CString,
+}
+
+impl Op<Symlink> {
+    pub(crate) fn symlink<P: AsRef<Path>, Q: AsRef<Path>>(
+        from: P,
+        to: Q,
+    ) -> io::Result<Op<Symlink>> {
+        use io_uring::{opcode, types};
+
+        let _from = cstr(from.as_ref())?;
+        let _to = cstr(to.as_ref())?;
+
+        CONTEXT.with(|x| {
+            x.handle().expect("Not in a runtime context").submit_op(
+                Symlink { _from, _to },
+                |symlink| {
+                    let from_ref = symlink._from.as_c_str().as_ptr();
+                    let to_ref = symlink._to.as_c_str().as_ptr();
+
+                    opcode::SymlinkAt::new(types::Fd(libc::AT_FDCWD), from_ref, to_ref).build()
+                },
+            )
+        })
+    }
+}
+
+impl Completable for Symlink {
+    type Output = io::Result<()>;
+
+    fn complete(self, cqe: CqeResult) -> Self::Output {
+        cqe.result.map(|_| ())
+    }
+}

tests/fs_symlink.rs

@@ -0,0 +1,28 @@
+#[path = "../src/future.rs"]
+#[allow(warnings)]
+mod future;
+
+use std::io::Write;
+use tokio_test::assert_ok;
+use tokio_uring::fs;
+
+use tempfile::tempdir;
+use tempfile::NamedTempFile;
+
+const TEST_PAYLOAD: &[u8] = b"I am data in the source file";
+
+#[test]
+fn test_create_symlink() {
+    tokio_uring::start(async {
+        let mut src_file = NamedTempFile::new().unwrap();
+        src_file.write_all(TEST_PAYLOAD).unwrap();
+
+        let dst_enclosing_dir = tempdir().unwrap();
+
+        assert_ok!(fs::symlink(src_file.path(), dst_enclosing_dir.path().join("abc")).await);
+
+        let content = std::fs::read(dst_enclosing_dir.path().join("abc")).unwrap();
+
+        assert_eq!(content, TEST_PAYLOAD);
+    });
+}