Avoid calling strerror[_r] in PQcancel().

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
index 57aee951830843f1d33f980d497081715388508c..88f54e89b388520d49f3a601bbef54f402fed81b 100644 (file)
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -4373,7 +4373,6 @@ 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
    {
@@ -4452,8 +4451,25 @@ cancel_errReturn:
    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)