Add os.read_at_least and os_read_full utility procedures.

core/os/os.odin

@@ -55,6 +55,25 @@ write_encoded_rune :: proc(fd: Handle, r: rune) {
 	write_byte(fd, '\'')
 }
 
+read_at_least :: proc(fd: Handle, buf: []byte, min: int) -> (n: int, err: Errno) {
+	if len(buf) < min {
+		return 0, -1
+	}
+	for n < min && err == 0 {
+		nn: int
+		nn, err = read(fd, buf[n:])
+		n += nn
+	}
+	if n >= min {
+		err = 0
+	}
+	return
+}
+
+read_full :: proc(fd: Handle, buf: []byte) -> (n: int, err: Errno) {
+	return read_at_least(fd, buf, len(buf))
+}
+
 
 file_size_from_path :: proc(path: string) -> i64 {
 	fd, err := open(path, O_RDONLY, 0)
@@ -100,7 +119,7 @@ read_entire_file_from_handle :: proc(fd: Handle, allocator := context.allocator)
 	return nil, false
 	}
 
-    bytes_read, read_err := read(fd, data)
+	bytes_read, read_err := read_full(fd, data)
 	if read_err != ERROR_NONE {
 		delete(data)
 		return nil, false