libc: io: implement file io buffering and internal locking for concurrency

lib/libc/io/stdio/fgets.c

@@ -7,7 +7,9 @@ char *fgets(
 	int count,
 	struct __opaque_file *restrict stream)
 {
+	__libc_file_lock(stream);
 	if (stream->f_flags & (FILE_EOF | FILE_ERR)) {
+		__libc_file_unlock(stream);
 		return NULL;
 	}
 
@@ -28,13 +30,16 @@ char *fgets(
 		}
 	}
 
-	if (ferr(stream)) {
+	if (ferror(stream)) {
+		__libc_file_unlock(stream);
 		return NULL;
 	}
 
 	if (feof(stream) && i == 0) {
+		__libc_file_unlock(stream);
 		return NULL;
 	}
 
+	__libc_file_unlock(stream);
 	return str;
 }