xap_helper: avoid strerror(3) inside signal handler

It's not async-signal-safe and the glibc implementation uses
malloc via asnprintf.  Practically it's not a problem unless the
kernel OOMs and the write(2) fails to the self-pipe.

diff --git a/lib/PublicInbox/xap_helper.h b/lib/PublicInbox/xap_helper.h
index b6b517d5932527f38a038d01ced8041f9abcf225..1d8437c9a6f8f53549c1c6df674b555962d69485 100644 (file)
--- a/lib/PublicInbox/xap_helper.h
+++ b/lib/PublicInbox/xap_helper.h
@@ -980,7 +980,8 @@ static void sigp(int sig) // parent signal handler
 {
        static const char eagain[] = "signals coming in too fast";
        static const char bad_sig[] = "BUG: bad sig\n";
-       static const char write_err[] = "BUG: sigp write: ";
+       static const char write_errno[] = "BUG: sigp write (errno)";
+       static const char write_zero[] = "BUG: sigp write wrote zero bytes";
        char c = 0;
 
        switch (sig) {
@@ -992,23 +993,17 @@ static void sigp(int sig) // parent signal handler
                _exit(EXIT_FAILURE);
        }
        ssize_t w = write(pipefds[1], &c, 1);
-       if (w == sizeof(c)) return;
-       int e = 0;
-       if (w < 0) {
-               e = errno;
-               if (e == EAGAIN) {
-                       write(STDERR_FILENO, eagain, sizeof(eagain) - 1);
-                       return;
-               }
+       if (w > 0) return;
+       if (w < 0 && errno == EAGAIN) {
+               write(STDERR_FILENO, eagain, sizeof(eagain) - 1);
+               return;
+       } else if (w == 0) {
+               write(STDERR_FILENO, write_zero, sizeof(write_zero) - 1);
+       } else {
+               // strerror isn't technically async-signal-safe, and
+               // strerrordesc_np+strerrorname_np isn't portable
+               write(STDERR_FILENO, write_errno, sizeof(write_errno) - 1);
        }
-       struct iovec iov[3];
-       iov[0].iov_base = (void *)write_err;
-       iov[0].iov_len = sizeof(write_err) - 1;
-       iov[1].iov_base = (void *)(e ? strerror(e) : "zero write");
-       iov[1].iov_len = strlen((const char *)iov[1].iov_base);
-       iov[2].iov_base = (void *)"\n";
-       iov[2].iov_len = 1;
-       (void)writev(STDERR_FILENO, iov, MY_ARRAY_SIZE(iov));
        _exit(EXIT_FAILURE);
 }