ln: add WASI support via symlink_path (#11713)

.github/workflows/wasi.yml

@@ -50,7 +50,7 @@ jobs:
         # Tests incompatible with WASI are annotated with
         # #[cfg_attr(wasi_runner, ignore)] in the test source files.
         # TODO: add integration tests for these tools as WASI support is extended:
-        #   arch b2sum cat cksum cp csplit date dir dircolors fmt join ln
+        #   arch b2sum cat cksum cp csplit date dir dircolors fmt join
         #   ls md5sum mkdir mv nproc pathchk pr printenv ptx pwd readlink
         #   realpath rm rmdir seq sha1sum sha224sum sha256sum sha384sum
         #   sha512sum shred sleep sort split tail touch tsort uname uniq
@@ -61,7 +61,7 @@ jobs:
           test_base32:: test_base64:: test_basenc:: test_basename:: \
           test_comm:: test_cut:: test_dirname:: test_echo:: \
           test_expand:: test_factor:: test_false:: test_fold:: \
-          test_head:: test_link:: test_nl:: test_numfmt:: \
+          test_head:: test_link:: test_ln:: test_nl:: test_numfmt:: \
           test_od:: test_paste:: test_printf:: test_shuf:: test_sum:: \
           test_tee:: test_tr:: test_true:: test_truncate:: \
           test_unexpand:: test_unlink:: test_wc::

Cargo.lock

@@ -3681,6 +3681,7 @@ version = "0.9.0"
 dependencies = [
  "clap",
  "fluent",
+ "rustix",
  "thiserror 2.0.18",
  "uucore",
 ]

docs/src/wasi-test-gaps.md

@@ -6,7 +6,7 @@ To find all annotated tests: `grep -rn 'wasi_runner, ignore' tests/`
 
 ## Tools not yet covered by integration tests
 
-arch, b2sum, cat, cksum, cp, csplit, date, dir, dircolors, fmt, join, ln, ls, md5sum, mkdir, mv, nproc, pathchk, pr, printenv, ptx, pwd, readlink, realpath, rm, rmdir, seq, sha1sum, sha224sum, sha256sum, sha384sum, sha512sum, shred, sleep, sort, split, tail, touch, tsort, uname, uniq, vdir, yes
+arch, b2sum, cat, cksum, cp, csplit, date, dir, dircolors, fmt, join, ls, md5sum, mkdir, mv, nproc, pathchk, pr, printenv, ptx, pwd, readlink, realpath, rm, rmdir, seq, sha1sum, sha224sum, sha256sum, sha384sum, sha512sum, shred, sleep, sort, split, tail, touch, tsort, uname, uniq, vdir, yes
 
 ## WASI sandbox: host paths not visible
 
@@ -31,3 +31,7 @@ WASI does not support spawning child processes. Tests that shell out to other co
 ## WASI: stdin file position not preserved through wasmtime
 
 When stdin is a seekable file, wasmtime does not preserve the file position between the host and guest. Tests that validate stdin offset behavior after `head` reads are skipped.
+
+## WASI: read_link on absolute paths fails under wasmtime via spawned test harness
+
+`fs::read_link` on an absolute path inside the sandbox (e.g. `/file2`) returns `EPERM` when the WASI binary is launched through `std::process::Command` from the test harness, even though the same call works when wasmtime is invoked directly. This breaks `uucore::fs::canonicalize` for symlink sources, so tests that rely on following a symlink to compute a relative path are skipped.

src/uu/ln/Cargo.toml

@@ -25,6 +25,9 @@ uucore = { workspace = true, features = ["backup-control", "fs"] }
 thiserror = { workspace = true }
 fluent = { workspace = true }
 
+[target.'cfg(target_os = "wasi")'.dependencies]
+rustix = { workspace = true, features = ["fs"] }
+
 [[bin]]
 name = "ln"
 path = "src/main.rs"

src/uu/ln/src/ln.rs

@@ -524,9 +524,6 @@ pub fn symlink<P1: AsRef<Path>, P2: AsRef<Path>>(src: P1, dst: P2) -> io::Result
 }
 
 #[cfg(target_os = "wasi")]
-fn symlink<P1: AsRef<Path>, P2: AsRef<Path>>(_src: P1, _dst: P2) -> io::Result<()> {
-    Err(io::Error::new(
-        io::ErrorKind::Unsupported,
-        "symlinks not supported on this platform",
-    ))
+pub fn symlink<P1: AsRef<Path>, P2: AsRef<Path>>(src: P1, dst: P2) -> io::Result<()> {
+    rustix::fs::symlink(src.as_ref(), dst.as_ref()).map_err(io::Error::from)
 }

src/uucore/src/lib/features/backup_control.rs

@@ -91,6 +91,7 @@ use std::{
     error::Error,
     ffi::{OsStr, OsString},
     fmt::{Debug, Display},
+    fs,
     path::{Path, PathBuf},
 };
 
@@ -442,7 +443,11 @@ fn numbered_backup_path(path: &Path) -> PathBuf {
     let mut i: u64 = 1;
     loop {
         let new_path = simple_backup_path(path, OsString::from(format!(".~{i}~")));
-        if !new_path.exists() {
+        // Use `symlink_metadata` rather than `exists()` so that a dangling
+        // symlink still counts as an existing backup (avoiding a silent
+        // overwrite), and so we do not report a live symlink as missing when
+        // the target cannot be stat'd.
+        if fs::symlink_metadata(&new_path).is_err() {
             return new_path;
         }
         i += 1;
@@ -451,7 +456,7 @@ fn numbered_backup_path(path: &Path) -> PathBuf {
 
 fn existing_backup_path<S: AsRef<OsStr>>(path: &Path, suffix: S) -> PathBuf {
     let test_path = simple_backup_path(path, OsString::from(".~1~"));
-    if test_path.exists() {
+    if fs::symlink_metadata(&test_path).is_ok() {
         return numbered_backup_path(path);
     }
     simple_backup_path(path, suffix.as_ref())

tests/by-util/test_ln.rs

@@ -501,6 +501,7 @@ fn test_symlink_implicit_target_dir() {
 }
 
 #[test]
+#[cfg_attr(wasi_runner, ignore = "WASI sandbox: host paths not visible")]
 fn test_symlink_to_dir_2args() {
     let (at, mut ucmd) = at_and_ucmd!();
     let filename = "test_symlink_to_dir_2args_file";
@@ -754,6 +755,10 @@ fn test_relative_dst_already_symlink() {
 }
 
 #[test]
+#[cfg_attr(
+    wasi_runner,
+    ignore = "WASI: read_link on absolute paths fails under wasmtime via spawned test harness"
+)]
 fn test_relative_src_already_symlink() {
     let (at, mut ucmd) = at_and_ucmd!();
     at.touch("file1");
@@ -967,6 +972,7 @@ fn test_ln_seen_file() {
 
 #[test]
 #[cfg(target_os = "linux")]
+#[cfg_attr(wasi_runner, ignore = "WASI: argv/filenames must be valid UTF-8")]
 fn test_ln_non_utf8_paths() {
     use std::ffi::OsStr;
     use std::os::unix::ffi::OsStrExt;