Be more robust when strerror() doesn't give a useful result.

glibc, at least, is capable of returning "???" instead of anything useful
if it doesn't like the setting of LC_CTYPE.  If this happens, or in the
previously-known case of strerror() returning an empty string, try to
print the C macro name for the error code ("EACCES" etc).  Only if we
don't have the error code in our compiled-in list of popular error codes
(which covers most though not quite all of what's called out in the POSIX
spec) will we fall back to printing a numeric error code.  This should
simplify debugging.

Note that this functionality is currently only provided for %m in backend
ereport/elog messages.  That may be sufficient, since we don't fool with the
locale environment in frontend clients, but it's foreseeable that we might
want similar code in libpq for instance.

There was some talk of back-patching this, but let's see how the buildfarm
likes it first.  It seems likely that at least some of the POSIX-defined
error code symbols don't exist on all platforms.  I don't want to clutter
the entire list with #ifdefs, but we may need more than are here now.

MauMau, edited by me

src/backend/utils/error/elog.c

@@ -173,6 +173,7 @@ static void send_message_to_server_log(ErrorData *edata);
 static void send_message_to_frontend(ErrorData *edata);
 static char *expand_fmt_string(const char *fmt, ErrorData *edata);
 static const char *useful_strerror(int errnum);
+static const char *get_errno_symbol(int errnum);
 static const char *error_severity(int elevel);
 static void append_with_tabs(StringInfo buf, const char *str);
 static bool is_log_level_output(int elevel, int log_min_level);
@@ -3206,7 +3207,7 @@ expand_fmt_string(const char *fmt, ErrorData *edata)
 static const char *
 useful_strerror(int errnum)
 {
-	/* this buffer is only used if errno has a bogus value */
+	/* this buffer is only used if strerror() and get_errno_symbol() fail */
 	static char errorstr_buf[48];
 	const char *str;
 
@@ -3218,10 +3219,16 @@ useful_strerror(int errnum)
 	str = strerror(errnum);
 
 	/*
-	 * Some strerror()s return an empty string for out-of-range errno. This is
-	 * ANSI C spec compliant, but not exactly useful.
+	 * Some strerror()s return an empty string for out-of-range errno.	This
+	 * is ANSI C spec compliant, but not exactly useful.  Also, we may get
+	 * back strings of question marks if libc cannot transcode the message to
+	 * the codeset specified by LC_CTYPE.  If we get nothing useful, first try
+	 * get_errno_symbol(), and if that fails, print the numeric errno.
 	 */
-	if (str == NULL || *str == '\0')
+	if (str == NULL || *str == '\0' || *str == '?')
+		str = get_errno_symbol(errnum);
+
+	if (str == NULL)
 	{
 		snprintf(errorstr_buf, sizeof(errorstr_buf),
 		/*------
@@ -3234,6 +3241,152 @@ useful_strerror(int errnum)
 	return str;
 }
 
+/*
+ * Returns a symbol (e.g. "ENOENT") for an errno code.
+ * Returns NULL if the code is unrecognized.
+ */
+static const char *
+get_errno_symbol(int errnum)
+{
+	switch (errnum)
+	{
+		case E2BIG:
+			return "E2BIG";
+		case EACCES:
+			return "EACCES";
+		case EADDRINUSE:
+			return "EADDRINUSE";
+		case EADDRNOTAVAIL:
+			return "EADDRNOTAVAIL";
+		case EAFNOSUPPORT:
+			return "EAFNOSUPPORT";
+#ifdef EAGAIN
+		case EAGAIN:
+			return "EAGAIN";
+#endif
+		case EALREADY:
+			return "EALREADY";
+		case EBADF:
+			return "EBADF";
+		case EBADMSG:
+			return "EBADMSG";
+		case EBUSY:
+			return "EBUSY";
+		case ECHILD:
+			return "ECHILD";
+		case ECONNABORTED:
+			return "ECONNABORTED";
+		case ECONNREFUSED:
+			return "ECONNREFUSED";
+#ifdef ECONNRESET
+		case ECONNRESET:
+			return "ECONNRESET";
+#endif
+		case EDEADLK:
+			return "EDEADLK";
+		case EDOM:
+			return "EDOM";
+		case EEXIST:
+			return "EEXIST";
+		case EFAULT:
+			return "EFAULT";
+		case EFBIG:
+			return "EFBIG";
+		case EHOSTUNREACH:
+			return "EHOSTUNREACH";
+		case EIDRM:
+			return "EIDRM";
+		case EINPROGRESS:
+			return "EINPROGRESS";
+		case EINTR:
+			return "EINTR";
+		case EINVAL:
+			return "EINVAL";
+		case EIO:
+			return "EIO";
+		case EISCONN:
+			return "EISCONN";
+		case EISDIR:
+			return "EISDIR";
+		case ELOOP:
+			return "ELOOP";
+		case EMFILE:
+			return "EMFILE";
+		case EMLINK:
+			return "EMLINK";
+		case EMSGSIZE:
+			return "EMSGSIZE";
+		case ENAMETOOLONG:
+			return "ENAMETOOLONG";
+		case ENFILE:
+			return "ENFILE";
+		case ENOBUFS:
+			return "ENOBUFS";
+		case ENODEV:
+			return "ENODEV";
+		case ENOENT:
+			return "ENOENT";
+		case ENOEXEC:
+			return "ENOEXEC";
+		case ENOMEM:
+			return "ENOMEM";
+		case ENOSPC:
+			return "ENOSPC";
+		case ENOSYS:
+			return "ENOSYS";
+		case ENOTCONN:
+			return "ENOTCONN";
+		case ENOTDIR:
+			return "ENOTDIR";
+#if defined(ENOTEMPTY) && (ENOTEMPTY != EEXIST) /* same code on AIX */
+		case ENOTEMPTY:
+			return "ENOTEMPTY";
+#endif
+		case ENOTSOCK:
+			return "ENOTSOCK";
+#ifdef ENOTSUP
+		case ENOTSUP:
+			return "ENOTSUP";
+#endif
+		case ENOTTY:
+			return "ENOTTY";
+		case ENXIO:
+			return "ENXIO";
+#if defined(EOPNOTSUPP) && (!defined(ENOTSUP) || (EOPNOTSUPP != ENOTSUP))
+		case EOPNOTSUPP:
+			return "EOPNOTSUPP";
+#endif
+		case EOVERFLOW:
+			return "EOVERFLOW";
+		case EPERM:
+			return "EPERM";
+		case EPIPE:
+			return "EPIPE";
+		case EPROTONOSUPPORT:
+			return "EPROTONOSUPPORT";
+		case ERANGE:
+			return "ERANGE";
+#ifdef EROFS
+		case EROFS:
+			return "EROFS";
+#endif
+		case ESRCH:
+			return "ESRCH";
+		case ETIMEDOUT:
+			return "ETIMEDOUT";
+		case ETXTBSY:
+			return "ETXTBSY";
+#if defined(EWOULDBLOCK) && (!defined(EAGAIN) || (EWOULDBLOCK != EAGAIN))
+		case EWOULDBLOCK:
+			return "EWOULDBLOCK";
+#endif
+		case EXDEV:
+			return "EXDEV";
+	}
+
+	return NULL;
+}
+
 
 /*
  * error_severity --- get localized string representing elevel