primop: add readFileType, optimize readDir

Allows checking directory entry type of a single file/directory.

This was added to optimize the use of `builtins.readDir` on some
filesystems and operating systems which cannot detect this information
using POSIX's `readdir`.

Previously `builtins.readDir` would eagerly use system calls to lookup
these filetypes using other interfaces; this change makes these
operations lazy in the attribute values for each file with application
of `builtins.readFileType`.

1 files changed, 8 insertions, 0 deletions

diff --git a/doc/manual/src/release-notes/rl-next.md b/doc/manual/src/release-notes/rl-next.md
index 78ae99f4b..56507d731 100644
--- a/doc/manual/src/release-notes/rl-next.md
+++ b/doc/manual/src/release-notes/rl-next.md
@@ -1,2 +1,10 @@
 # Release X.Y (202?-??-??)
 
+* A new function `builtins.readFileType` is available. It is similar to
+  `builtins.readDir` but acts on a single file or directory.
+
+* The `builtins.readDir` function has been optimized when encountering unknown
+  file types from POSIX's `readdir`. In such cases the type of each file is/was
+  discovered by making multiple syscalls. This change makes these operations
+  lazy such that these lookups will only be performed if the attribute is used.
+  This optimization effects a minority of filesystems and operating systems.