Do not set File._errno when reading less than requested bytes. (#2785)

When requesting more bytes than available from a File the _errno was set to
although reading less than available bytes (> 0) is no error condition.
If anything then FileEOF should be set in this case to signal that we reached the end of file.
Nonetheless i kept it as it is right now, as i think it is more reliable to detect FileEOF when reading 0 bytes.

packages/files/_test.pony

@@ -40,6 +40,7 @@ actor Main is TestList
     test(_TestFileMixedWriteQueue)
     test(_TestFileWritevLarge)
     test(_TestFileFlush)
+    test(_TestFileReadMore)
 
 primitive _FileHelper
   fun make_files(h: TestHelper, files: Array[String]): FilePath ? =>
@@ -811,3 +812,27 @@ class iso _TestFileFlush is UnitTest
     else
       h.fail("Unhandled error!")
     end
+
+class iso _TestFileReadMore is UnitTest
+  fun name(): String => "files/File.read-more"
+  fun apply(h: TestHelper)? =>
+    let path = FilePath(h.env.root as AmbientAuth, "tmp-read-more")?
+    with file = CreateFile(path) as File do
+      h.assert_true(file.write("foobar"))
+    end
+
+    with read_file = OpenFile(path) as File do
+      let content = read_file.read(10)
+      h.assert_eq[USize](6, content.size())
+      h.assert_is[FileErrNo](
+        read_file.errno(),
+        FileOK,
+        "File errno is not OK after reading fewer bytes than requested")
+      h.assert_eq[USize](0, read_file.read(10).size())
+      h.assert_is[FileErrNo](
+        read_file.errno(),
+        FileEOF,
+        "File errno is not EOF after reading past the last byte")
+    end
+    path.remove()
+

packages/files/file.pony

@@ -250,14 +250,11 @@ class File
         end)
           .isize()
 
-      if r < bytes_to_read.isize() then
-        _errno =
-          if r == 0 then
-             FileEOF
-           else
-             _get_error() // error
-             error
-           end
+      match r
+      | 0  => _errno = FileEOF
+      | -1 =>
+        _errno = _get_error()
+        error
       else
         // truncate at offset in order to adjust size of string after ffi call
         // and to avoid scanning full array via recalc
@@ -312,13 +309,9 @@ class File
         end)
           .isize()
 
-      if r < len.isize() then
-        _errno =
-          if r == 0 then
-             FileEOF
-           else
-             _get_error() // error
-           end
+      match r
+      | 0  => _errno = FileEOF
+      | -1 => _errno = _get_error()
       end
 
       result.truncate(r.usize())
@@ -341,13 +334,9 @@ class File
         @read(_fd, result.cpointer(), result.space())
       end).isize()
 
-      if r < len.isize() then
-        _errno =
-          if r == 0 then
-             FileEOF
-           else
-             _get_error() // error
-           end
+      match r
+      | 0  => _errno = FileEOF
+      | -1 => _errno = _get_error()
       end
 
       result.truncate(r.usize())