Suppress errno from wait() and stat()

main.c

@@ -480,8 +480,7 @@ static void run_subprocess(char **argv) {
 
   // Wait for the child process to finish.
   int status;
-  while (wait(&status) > 0);
-  if (status != 0)
+  if (wait(&status) <= 0 || status != 0)
     exit(1);
 }
 
@@ -669,8 +668,13 @@ static char *find_file(char *pattern) {
 
 // Returns true if a given file exists.
 bool file_exists(char *path) {
+  assert(!errno);
+
   struct stat st;
-  return !stat(path, &st);
+  bool found = !stat(path, &st);
+  if (!found && errno == ENOENT)
+    errno = 0;
+  return found;
 }
 
 static char *find_libpath(void) {

preprocess.c

@@ -1142,10 +1142,14 @@ static Token *counter_macro(Token *start) {
 // modification time of the current file. E.g.
 // "Fri Jul 24 01:32:50 2020"
 static Token *timestamp_macro(Token *start) {
+  assert(!errno);
+
   Token *tok;
   struct stat st;
   if (stat(start->file->name, &st) != 0) {
     tok = new_str_token("??? ??? ?? ??:??:?? ????", start);
+    if (errno == ENOENT)
+      errno = 0;
   } else {
     char buf[30];
     ctime_r(&st.st_mtime, buf);