Changes requested by @asomers on nix-rust#1231

src/fcntl.rs

@@ -180,7 +180,7 @@ pub fn renameat<P1: ?Sized + NixPath, P2: ?Sized + NixPath>(old_dirfd: Option<Ra
     Errno::result(res).map(drop)
 }
 
-fn wrap_readlink_result(v: &mut Vec<u8>, res: ssize_t) -> Result<OsString> {
+fn wrap_readlink_result(mut v: Vec<u8>, res: ssize_t) -> Result<OsString> {
     match Errno::result(res) {
         Err(err) => Err(err),
         Ok(len) => {
@@ -191,48 +191,60 @@ fn wrap_readlink_result(v: &mut Vec<u8>, res: ssize_t) -> Result<OsString> {
     }
 }
 
-fn inner_readlink<P: ?Sized + NixPath>(dirfd: Option<RawFd>, path: &P) -> Result<OsString> {
-    let mut v = Vec::with_capacity(libc::PATH_MAX as usize);
-    // simple case: result is strictly less than `PATH_MAX`
-    let res = path.with_nix_path(|cstr| {
+fn readlink_maybe_at<P: ?Sized + NixPath>(dirfd: Option<RawFd>, path: &P,
+                                          v: &mut Vec<u8>)
+                                          -> Result<libc::ssize_t> {
+    path.with_nix_path(|cstr| {
         unsafe {
             match dirfd  {
-                Some(dirfd) => libc::readlinkat(dirfd, cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, v.capacity() as size_t),
-                None => libc::readlink(cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, v.capacity() as size_t),
+                Some(dirfd) => libc::readlinkat(dirfd, cstr.as_ptr(),
+                                                v.as_mut_ptr() as *mut c_char,
+                                                v.capacity() as size_t),
+                None => libc::readlink(cstr.as_ptr(),
+                                       v.as_mut_ptr() as *mut c_char,
+                                       v.capacity() as size_t),
             }
         }
-    })?;
+    })
+}
+
+fn inner_readlink<P: ?Sized + NixPath>(dirfd: Option<RawFd>, path: &P)
+                                       -> Result<OsString> {
+    let mut v = Vec::with_capacity(libc::PATH_MAX as usize);
+    // simple case: result is strictly less than `PATH_MAX`
+    let res = readlink_maybe_at(dirfd, path, &mut v)?;
     let len = Errno::result(res)?;
     debug_assert!(len >= 0);
     if (len as usize) < v.capacity() {
-        return wrap_readlink_result(&mut v, res);
+        return wrap_readlink_result(v, res);
     }
     // Uh oh, the result is too long...
     // Let's try to ask lstat how many bytes to allocate.
-    let reported_size = super::sys::stat::lstat(path).and_then(|x| Ok(x.st_size)).unwrap_or(0);
-    let mut try_size = if reported_size > 0 { reported_size as usize + 1 }
-    // If lstat doesn't cooperate, or reports an error, be a little less precise.
-    else { (libc::PATH_MAX as usize).max(128) << 1 };
+    let reported_size = super::sys::stat::lstat(path)
+        .and_then(|x| Ok(x.st_size)).unwrap_or(0);
+    let mut try_size = if reported_size > 0 {
+        // Note: even if `lstat`'s apparently valid answer turns out to be
+        // wrong, we will still read the full symlink no matter what.
+        reported_size as usize + 1
+    } else {
+        // If lstat doesn't cooperate, or reports an error, be a little less
+        // precise.
+        (libc::PATH_MAX as usize).max(128) << 1
+    };
     loop {
         v.reserve_exact(try_size);
-        let res = path.with_nix_path(|cstr| {
-            unsafe {
-                match dirfd {
-                    Some(dirfd) => libc::readlinkat(dirfd, cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, v.capacity() as size_t),
-                    None => libc::readlink(cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, v.capacity() as size_t),
-                }
-            }
-        })?;
+        let res = readlink_maybe_at(dirfd, path, &mut v)?;
         let len = Errno::result(res)?;
         debug_assert!(len >= 0);
         if (len as usize) < v.capacity() {
-            break wrap_readlink_result(&mut v, res);
+            break wrap_readlink_result(v, res);
         }
         else {
             // Ugh! Still not big enough!
             let next_size = try_size << 1;
             if next_size < try_size {
-                // It's absurd that this would happen, but handle it sanely anyway.
+                // It's absurd that this would happen, but handle it sanely
+                // anyway.
                 break Err(super::Error::Sys(Errno::ENAMETOOLONG))
             }
             else {
@@ -247,7 +259,8 @@ pub fn readlink<P: ?Sized + NixPath>(path: &P) -> Result<OsString> {
 }
 
 
-pub fn readlinkat<P: ?Sized + NixPath>(dirfd: RawFd, path: &P) -> Result<OsString> {
+pub fn readlinkat<P: ?Sized + NixPath>(dirfd: RawFd, path: &P)
+                                       -> Result<OsString> {
     inner_readlink(Some(dirfd), path)
 }