Exit trace loop if got ECHILD, not if nprocs == 0.

Comment gives a testcase which wasn't handled correctly
by the old code.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>

diff --git a/strace.c b/strace.c
index a489db43e52c3dcc370ce39b514e83a3a7ad89cc..abaa463b21434dd8529f134f23f949f27084b7fb 100644 (file)
--- a/strace.c
+++ b/strace.c
@@ -1964,7 +1964,17 @@ trace(void)
 {
        struct rusage ru;
 
-       while (nprocs != 0) {
+       /* Used to be "while (nprocs != 0)", but in this testcase:
+        *  int main() { _exit(!!fork()); }
+        * under strace -f, parent sometimes (rarely) manages
+        * to exit before we see the first stop of the child,
+        * and we are losing track of it:
+        *  19923 clone(...) = 19924
+        *  19923 exit_group(1)     = ?
+        *  19923 +++ exited with 1 +++
+        * Waiting for ECHILD works better.
+        */
+       while (1) {
                int pid;
                int wait_errno;
                int status, sig;
@@ -1985,9 +1995,11 @@ trace(void)
                if (pid < 0) {
                        if (wait_errno == EINTR)
                                continue;
-                       if (wait_errno == ECHILD)
-                               /* Should not happen since nprocs > 0 */
+                       if (nprocs == 0 && wait_errno == ECHILD)
                                return;
+                       /* If nprocs > 0, ECHILD is not expected,
+                        * treat it as any other error here:
+                        */
                        errno = wait_errno;
                        perror_msg_and_die("wait4(__WALL)");
                }