Produce a more useful error message for over-length Unix socket paths.

The length of a socket path name is constrained by the size of struct
sockaddr_un, and there's not a lot we can do about it since that is a
kernel API.  However, it would be a good thing if we produced an
intelligible error message when the user specifies a socket path that's too
long --- and getaddrinfo's standard API is too impoverished to do this in
the natural way.  So insert explicit tests at the places where we construct
a socket path name.  Now you'll get an error that makes sense and even
tells you what the limit is, rather than something generic like
"Non-recoverable failure in name resolution".

Per trouble report from Jeremy Drake and a fix idea from Andrew Dunstan.

src/backend/libpq/pqcomm.c

@@ -493,6 +493,14 @@ static int
 Lock_AF_UNIX(unsigned short portNumber, char *unixSocketName)
 {
 	UNIXSOCK_PATH(sock_path, portNumber, unixSocketName);
+	if (strlen(sock_path) >= UNIXSOCK_PATH_BUFLEN)
+	{
+		ereport(LOG,
+				(errmsg("Unix-domain socket path \"%s\" is too long (maximum %d bytes)",
+						sock_path,
+						(int) (UNIXSOCK_PATH_BUFLEN - 1))));
+		return STATUS_ERROR;
+	}
 
 	/*
 	 * Grab an interlock file associated with the socket file.

src/include/libpq/pqcomm.h

@@ -73,6 +73,19 @@ typedef struct
 				DEFAULT_PGSOCKET_DIR, \
 				(port))
 
+/*
+ * The maximum workable length of a socket path is what will fit into
+ * struct sockaddr_un.  This is usually only 100 or so bytes :-(.
+ *
+ * For consistency, always pass a MAXPGPATH-sized buffer to UNIXSOCK_PATH(),
+ * then complain if the resulting string is >= UNIXSOCK_PATH_BUFLEN bytes.
+ * (Because the standard API for getaddrinfo doesn't allow it to complain in
+ * a useful way when the socket pathname is too long, we have to test for
+ * this explicitly, instead of just letting the subroutine return an error.)
+ */
+#define UNIXSOCK_PATH_BUFLEN sizeof(((struct sockaddr_un *) NULL)->sun_path)
+
+
 /*
  * These manipulate the frontend/backend protocol version number.
  *

src/interfaces/libpq/fe-connect.c

@@ -1300,7 +1300,7 @@ static int
 connectDBStart(PGconn *conn)
 {
 	int			portnum;
-	char		portstr[128];
+	char		portstr[MAXPGPATH];
 	struct addrinfo *addrs = NULL;
 	struct addrinfo hint;
 	const char *node;
@@ -1362,6 +1362,15 @@ connectDBStart(PGconn *conn)
 		node = NULL;
 		hint.ai_family = AF_UNIX;
 		UNIXSOCK_PATH(portstr, portnum, conn->pgunixsocket);
+		if (strlen(portstr) >= UNIXSOCK_PATH_BUFLEN)
+		{
+			appendPQExpBuffer(&conn->errorMessage,
+							  libpq_gettext("Unix-domain socket path \"%s\" is too long (maximum %d bytes)\n"),
+											portstr,
+											(int) (UNIXSOCK_PATH_BUFLEN - 1));
+			conn->options_valid = false;
+			goto connect_errReturn;
+		}
 #else
 		/* Without Unix sockets, default to localhost instead */
 		node = DefaultHost;