fix readlink/readlinkat to return too long only when it is long

CHANGELOG.md

@@ -12,8 +12,16 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ```rust
   use nix::fcntl::{readlink, readlinkat};
 
-  readlink(&path);
-  readlinkat(dirfd, &path);
+  // the buffer argument of `readlink` and `readlinkat` has been removed,
+  // and the return value is now an owned type (`OsString`).
+  // Existing code can be updated by removing the buffer argument
+  // and removing any clone or similar operation on the output
+
+  // old code `readlink(&path, &mut buf)` can be replaced with the following
+  readlink(&path); // this returns OsString
+
+  // old code `readlinkat(dirfd, &path, &mut buf)` can be replaced with the following
+  readlinkat(dirfd, &path); // this returns OsString
   ```
 
 ### Fixed

src/fcntl.rs

@@ -188,21 +188,19 @@ fn wrap_readlink_result(v: &mut Vec<u8>, res: ssize_t) -> Result<OsString> {
 }
 
 pub fn readlink<'a, P: ?Sized + NixPath>(path: &P) -> Result<OsString> {
-    let len = libc::PATH_MAX as usize;
-    let mut v = Vec::with_capacity(len);
+    let mut v = Vec::with_capacity(libc::PATH_MAX as usize);
     let res = path.with_nix_path(|cstr| {
-        unsafe { libc::readlink(cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, len as size_t) }
+        unsafe { libc::readlink(cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, v.capacity() as size_t) }
     })?;
 
     wrap_readlink_result(&mut v, res)
 }
 
 
 pub fn readlinkat<'a, P: ?Sized + NixPath>(dirfd: RawFd, path: &P) -> Result<OsString> {
-    let len = libc::PATH_MAX as usize;
-    let mut v = Vec::with_capacity(len);
+    let mut v = Vec::with_capacity(libc::PATH_MAX as usize);
     let res = path.with_nix_path(|cstr| {
-        unsafe { libc::readlinkat(dirfd, cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, len as size_t) }
+        unsafe { libc::readlinkat(dirfd, cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, v.capacity() as size_t) }
     })?;
 
     wrap_readlink_result(&mut v, res)