Avoid calling strerror[_r] in PQcancel().

tglsfdc · tglsfdc · commit 38f099ef935b · 2022-01-17T12:52:44.000-05:00
PQcancel() is supposed to be safe to call from a signal handler, and indeed psql uses it that way. All of the library functions it uses are specified to be async-signal-safe by POSIX ... except for strerror. Neither plain strerror nor strerror_r are considered safe. When this code was written, back in the dark ages, we probably figured "oh, strerror will just index into a constant array of strings" ... but in any locale except C, that's unlikely to be true. Probably the reason we've not heard complaints is that (a) this error-handling code is unlikely to be reached in normal use, and (b) in many scenarios, localized error strings would already have been loaded, after which maybe it's safe to call strerror here. Still, this is clearly unacceptable. The best we can do without relying on strerror is to print the decimal value of errno, so make it do that instead. (This is probably not much loss of user-friendliness, given that it is hard to get a failure here.) Back-patch to all supported branches. Discussion: https://postgr.es/m/2937814.1641960929@sss.pgh.pa.us
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
@@ -4292,7 +4292,6 @@ internal_cancel(SockAddr *raddr, int be_pid, int be_key,
 {
 	int			save_errno = SOCK_ERRNO;
 	pgsocket	tmpsock = PGINVALID_SOCKET;
-	char		sebuf[PG_STRERROR_R_BUFLEN];
 	int			maxlen;
 	struct
 	{
@@ -4371,8 +4370,25 @@ internal_cancel(SockAddr *raddr, int be_pid, int be_key,
 	maxlen = errbufsize - strlen(errbuf) - 2;
 	if (maxlen >= 0)
 	{
-		strncat(errbuf, SOCK_STRERROR(SOCK_ERRNO, sebuf, sizeof(sebuf)),
-				maxlen);
+		/*
+		 * We can't invoke strerror here, since it's not signal-safe.  Settle
+		 * for printing the decimal value of errno.  Even that has to be done
+		 * the hard way.
+		 */
+		int			val = SOCK_ERRNO;
+		char		buf[32];
+		char	   *bufp;
+
+		bufp = buf + sizeof(buf) - 1;
+		*bufp = '\0';
+		do
+		{
+			*(--bufp) = (val % 10) + '0';
+			val /= 10;
+		} while (val > 0);
+		bufp -= 6;
+		memcpy(bufp, "error ", 6);
+		strncat(errbuf, bufp, maxlen);
 		strcat(errbuf, "\n");
 	}
 	if (tmpsock != PGINVALID_SOCKET)
