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/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);