[linux] Do not hard-code fd numbers in epoll test

In case file descriptor 3 and 4 are being used already, the executable
created by epoll.c exits with exit code 1, and prints the message -

	epoll_ctl<6>: Invalid argument

causing case-20-epoll.bash to FAIL.

dialects/linux/tests/case-20-epoll.bash

@@ -8,8 +8,8 @@ if ! [ -x $TARGET ]; then
 fi
 
 $TARGET 2>> $report | {
-    read pid epfd
-    if [[ -z "$pid" || -z "$epfd" ]]; then
+    read pid epfd evfd0 evfd1
+    if [[ -z "$pid" || -z "$epfd" || -z "$evfd0" || -z "$evfd1" ]]; then
 	echo "unexpected output form target ( $TARGET )" >> $report
 	exit 1
     fi
@@ -25,7 +25,7 @@ $TARGET 2>> $report | {
 	echo done
     } >> $report
     if $lsof -p $pid -a -d $epfd |
-	    grep -q "epoll *[0-9]* *.* *${epfd}u *a_inode *[0-9]*,[0-9]* *[0-9]* *[0-9]* *\[eventpoll:5,6\]"; then
+	    grep -q "epoll *[0-9]* *.* *${epfd}u *a_inode *[0-9]*,[0-9]* *[0-9]* *[0-9]* *\[eventpoll:${evfd0},${evfd1}\]"; then
 	kill $pid
 	exit 0
     else

dialects/linux/tests/epoll.c

@@ -11,37 +11,38 @@ int main(int argc, char **argv) {
         return 1;
     }
 
-    struct epoll_event ev;
-    int fd[2];
-    if (pipe(fd) < 0)
-        if (fd < 0) {
-            perror("pipe");
-            return 1;
-        }
-    if (dup2(fd[0], 5) < 0) {
-        perror("dup2(fd[0], 5)");
+    int pipefd[2];
+    if (pipe(pipefd) < 0) {
+        perror("pipe");
         return 1;
     }
-    if (dup2(fd[1], 6) < 0) {
-        perror("dup2(fd[1], 6)");
+
+    int evfd[2];
+    if ((evfd[0] = dup(pipefd[0])) < 0) {
+        perror("dup(pipefd[0])");
+        return 1;
+    }
+    if ((evfd[1] = dup(pipefd[1])) < 0) {
+        perror("dup(pipefd[1])");
         return 1;
     }
 
+    struct epoll_event ev;
     ev.events = EPOLLOUT;
-    ev.data.fd = 6;
+    ev.data.fd = evfd[1];
     if (epoll_ctl(epfd, EPOLL_CTL_ADD, ev.data.fd, &ev) < 0) {
-        perror("epoll_ctl<6>");
+        perror("epoll_ctl<evfd[1]>");
         return 1;
     }
 
     ev.events = EPOLLIN;
-    ev.data.fd = 5;
+    ev.data.fd = evfd[0];
     if (epoll_ctl(epfd, EPOLL_CTL_ADD, ev.data.fd, &ev) < 0) {
-        perror("epoll_ctl<5>");
+        perror("epoll_ctl<evfd[0]>");
         return 1;
     }
 
-    printf("%d %d\n", getpid(), epfd);
+    printf("%d %d %d %d\n", getpid(), epfd, evfd[0], evfd[1]);
     fflush(stdout);
     pause();
     return 0;