wasi-common: change behavior of path_readlink to truncate on too-small buffers (#6225)

this is the same behavior as exists in posix readlink(2)

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

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

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

@@ -801,11 +801,11 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
             .into_string()
             .map_err(|_| Error::illegal_byte_sequence().context("link contents"))?;
         let link_bytes = link.as_bytes();
-        let link_len = link_bytes.len();
-        if link_len > buf_len as usize {
-            return Err(Error::range());
-        }
-        buf.as_array(link_len as u32).copy_from_slice(link_bytes)?;
+        // Like posix readlink(2), silently truncate links when they are larger than the
+        // destination buffer:
+        let link_len = std::cmp::min(link_bytes.len(), buf_len as usize);
+        buf.as_array(link_len as u32)
+            .copy_from_slice(&link_bytes[..link_len])?;
         Ok(link_len as types::Size)
     }