System.Directory.nextDirEntry

* add `nextDirEntry` which returns `Maybe String`, so `Nothing` on
  the end of directory unlike `dirEntry` which returns unspecified error
  on the end of directory
* `dirEntry` is deprecated now, but not removed because compiler depends on it
* native implementation of `dirEntry` is patched to explicitly reset `errno`
  before the `readdir` call: without it end of directory and error were
  indistinguishable
* test added

Author: stepancheg

libs/base/System/Directory.idr

@@ -1,5 +1,6 @@
 module System.Directory
 
+import System.Errno
 import public System.File
 
 %default total
@@ -97,9 +98,20 @@ removeDir : HasIO io => String -> io ()
 removeDir dirName = primIO (prim__removeDir dirName)
 
 export
-dirEntry : HasIO io => Directory -> io (Either FileError String)
-dirEntry (MkDir d)
+nextDirEntry : HasIO io => Directory -> io (Either FileError (Maybe String))
+nextDirEntry (MkDir d)
     = do res <- primIO (prim__dirEntry d)
          if prim__nullPtr res /= 0
-            then returnError
-            else ok (prim__getString res)
+            then if !(getErrno) /= 0
+                    then returnError
+                    else pure $ Right Nothing
+            else pure $ Right (Just (prim__getString res))
+
+-- This function is deprecated; to be removed after the next version bump
+export
+dirEntry : HasIO io => Directory -> io (Either FileError String)
+dirEntry d = do r <- nextDirEntry d
+                pure $ case r of
+                         Left e         => Left e
+                         Right (Just n) => Right n
+                         Right Nothing  => Left FileNotFound

support/c/idris_directory.c

@@ -60,6 +60,10 @@ int idris2_removeDir(char* path) {
 
 char* idris2_nextDirEntry(void* d) {
     DirInfo* di = (DirInfo*)d;
+    // `readdir` keeps `errno` unchanged on end of stream
+    // so we need to reset `errno` to distinguish between
+    // end of stream and failure.
+    errno = 0;
     struct dirent* de = readdir(di->dirptr);
 
     if (de == NULL) {

tests/Main.idr

@@ -298,6 +298,7 @@ baseLibraryTests = MkTestPool "Base library" [Chez, Node] Nothing
   , "data_bits001"
   , "data_string_lines001"
   , "data_string_unlines001"
+  , "system_directory"
   , "system_errno"
   , "system_info001"
   , "system_signal001", "system_signal002", "system_signal003", "system_signal004"

tests/base/system_directory/ReadDir.idr

@@ -0,0 +1,22 @@
+import System
+import System.Directory
+
+panic : String -> IO a
+panic s = do putStrLn s
+             exitFailure
+
+collectEntries : Directory -> IO (List String)
+collectEntries d = do Right (Just n) <- nextDirEntry d
+                        | Right Nothing => pure []
+                        | Left e        => panic (show e)
+                      ns <- collectEntries d
+                      if n == "." || n == ".."
+                         then pure ns
+                         else pure (n :: ns)
+
+main : IO ()
+main = do Right d <- openDir "dir"
+            | Left e => panic (show e)
+          ["a"] <- collectEntries d
+            | x => panic ("wrong entries: " ++ (show x))
+          pure ()

tests/base/system_directory/dir/a

@@ -0,0 +1,2 @@
+1/1: Building ReadDir (ReadDir.idr)
+Main> Main> Bye for now!

tests/base/system_directory/expected

@@ -0,0 +1,2 @@
+:exec main
+:q

tests/base/system_directory/input

@@ -0,0 +1,3 @@
+rm -rf build
+
+$1 --no-color --console-width 0 --no-banner ReadDir.idr < input