readlink: get rid of weird partial-buffer semantics carried over from posix

but follow posix in returning ERANGE when the result is too big

crates/test-programs/wasi-tests/src/bin/readlink.rs

@@ -10,7 +10,7 @@ unsafe fn test_readlink(dir_fd: wasi::Fd) {
 
     // Read link into the buffer
     let buf = &mut [0u8; 10];
-    let mut bufused = wasi::path_readlink(dir_fd, "symlink", buf.as_mut_ptr(), buf.len())
+    let bufused = wasi::path_readlink(dir_fd, "symlink", buf.as_mut_ptr(), buf.len())
         .expect("readlink should succeed");
     assert_eq!(bufused, 6, "should use 6 bytes of the buffer");
     assert_eq!(&buf[..6], b"target", "buffer should contain 'target'");
@@ -22,10 +22,14 @@ unsafe fn test_readlink(dir_fd: wasi::Fd) {
 
     // Read link into smaller buffer than the actual link's length
     let buf = &mut [0u8; 4];
-    bufused = wasi::path_readlink(dir_fd, "symlink", buf.as_mut_ptr(), buf.len())
-        .expect("readlink should succeed");
-    assert_eq!(bufused, 4, "should use all 4 bytes of the buffer");
-    assert_eq!(buf, b"targ", "buffer should contain 'targ'");
+    let err = wasi::path_readlink(dir_fd, "symlink", buf.as_mut_ptr(), buf.len())
+        .err()
+        .expect("readlink with too-small buffer should fail");
+    assert_eq!(
+        err.raw_error(),
+        wasi::ERRNO_RANGE,
+        "readlink with too-small buffer should give ERANGE"
+    );
 
     // Clean up.
     wasi::path_unlink_file(dir_fd, "target").expect("removing a file");

crates/test-programs/wasi-tests/src/bin/readlink_no_buffer.rs

@@ -1,41 +0,0 @@
-use std::{env, process};
-use wasi_tests::open_scratch_directory;
-
-unsafe fn test_readlink_no_buffer(dir_fd: wasi::Fd) {
-    // First create a dangling symlink.
-    wasi::path_symlink("target", dir_fd, "symlink").expect("creating a symlink");
-
-    // Readlink it into a non-existent buffer.
-    let bufused = wasi::path_readlink(dir_fd, "symlink", (&mut []).as_mut_ptr(), 0)
-        .expect("readlink with a 0-sized buffer should succeed");
-    assert_eq!(
-        bufused, 0,
-        "readlink with a 0-sized buffer should return 'bufused' 0"
-    );
-
-    // Clean up.
-    wasi::path_unlink_file(dir_fd, "symlink").expect("removing a file");
-}
-
-fn main() {
-    let mut args = env::args();
-    let prog = args.next().unwrap();
-    let arg = if let Some(arg) = args.next() {
-        arg
-    } else {
-        eprintln!("usage: {} <scratch directory>", prog);
-        process::exit(1);
-    };
-
-    // Open scratch directory
-    let dir_fd = match open_scratch_directory(&arg) {
-        Ok(dir_fd) => dir_fd,
-        Err(err) => {
-            eprintln!("{}", err);
-            process::exit(1)
-        }
-    };
-
-    // Run the tests.
-    unsafe { test_readlink_no_buffer(dir_fd) }
-}

crates/wasi-c2/TEST_FAILURES

@@ -23,10 +23,6 @@ wasi_tests::path_symlink_trailing_slashes
     - unclear, path_symlink is giving a ENOTDIR when it expects an EEXIST...
 wasi_tests::poll_oneoff
     - no sched yet
-wasi_tests::readlink
-    - insists on small buffer semantics for readlink that don't seem right
-wasi_tests::readlink_no_buffer
-    - insists on small buffer semantics for readlink that don't seem right
 wasi_tests::remove_directory_trailing_slashes
     - apparently cap-std gives EINVAL when trying to remove dir with
       trailing slash?

crates/wasi-c2/src/error.rs

@@ -110,6 +110,9 @@ pub enum Error {
     /// Errno::Perm: Operation not permitted
     #[error("Perm: Operation not permitted")]
     Perm,
+    /// Errno::Range: Result too large
+    #[error("Range: Result too large")]
+    Range,
     /// Errno::Spipe: Invalid seek
     #[error("Spipe: Invalid seek")]
     Spipe,

crates/wasi-c2/src/snapshots/preview_1.rs

@@ -91,6 +91,7 @@ impl From<Error> for types::Errno {
             Error::Overflow => Errno::Overflow,
             Error::Pipe => Errno::Pipe,
             Error::Perm => Errno::Perm,
+            Error::Range => Errno::Range,
             Error::Spipe => Errno::Spipe,
             Error::FileNotCapable { .. } => Errno::Notcapable,
             Error::DirNotCapable { .. } => Errno::Notcapable,
@@ -749,7 +750,7 @@ impl<'a> wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
         let link_bytes = link.as_bytes();
         let link_len = link_bytes.len();
         if link_len > buf_len as usize {
-            return Err(Error::Nametoolong);
+            return Err(Error::Range);
         }
         let mut buf = buf.as_array(link_len as u32).as_slice_mut()?;
         buf.copy_from_slice(link_bytes);