perf trace: Avoid early exit due SIGCHLD from non-workload processes

The function trace__symbols_init() runs "perf-read-vdso32" and that ends up
with a SIGCHLD delivered to 'perf'. And this SIGCHLD make perf exit early.

'perf trace' should exit only if the SIGCHLD is from our workload process.
So let's use sigaction() instead of signal() to match such condition.

Committer notes:

Use memset to zero the 'struct sigaction' variable as the '= { 0 }'
method isn't accepted in many compiler versions, e.g.:

4 34.02 alpine:3.6 : FAIL clang version 4.0.0 (tags/RELEASE_400/final)
builtin-trace.c:4897:35: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
struct sigaction sigchld_act = { 0 };
^
{}
builtin-trace.c:4897:37: error: missing field 'sa_mask' initializer [-Werror,-Wmissing-field-initializers]
struct sigaction sigchld_act = { 0 };
^
2 errors generated.
6 32.60 alpine:3.8 : FAIL gcc version 6.4.0 (Alpine 6.4.0)
builtin-trace.c:4897:35: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
struct sigaction sigchld_act = { 0 };
^
{}
builtin-trace.c:4897:37: error: missing field 'sa_mask' initializer [-Werror,-Wmissing-field-initializers]
struct sigaction sigchld_act = { 0 };
^
2 errors generated.
7 34.82 alpine:3.9 : FAIL gcc version 8.3.0 (Alpine 8.3.0)
builtin-trace.c:4897:35: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
struct sigaction sigchld_act = { 0 };
^
{}
builtin-trace.c:4897:37: error: missing field 'sa_mask' initializer [-Werror,-Wmissing-field-initializers]
struct sigaction sigchld_act = { 0 };
^
2 errors generated.

Signed-off-by: Changbin Du <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-trace.c

···
 	return fprintf(fp, "         ? ");
 }
 
+static pid_t workload_pid = -1;
 static bool done = false;
 static bool interrupted = false;
 
-static void sig_handler(int sig)
+static void sighandler_interrupt(int sig __maybe_unused)
 {
-	done = true;
-	interrupted = sig == SIGINT;
+	done = interrupted = true;
+}
+
+static void sighandler_chld(int sig __maybe_unused, siginfo_t *info,
+			    void *context __maybe_unused)
+{
+	if (info->si_pid == workload_pid)
+		done = true;
 }
 
 static size_t trace__fprintf_comm_tid(struct trace *trace, struct thread *thread, FILE *fp)
···
 	bool draining = false;
 
 	trace->live = true;
-	signal(SIGCHLD, sig_handler);
 
 	if (!trace->raw_augmented_syscalls) {
 		if (trace->trace_syscalls && trace__add_syscall_newtp(trace))
···
 			fprintf(trace->output, "Couldn't run the workload!\n");
 			goto out_delete_evlist;
 		}
+		workload_pid = evlist->workload.pid;
 	}
 
 	err = evlist__open(evlist);
···
 	const char * const trace_subcommands[] = { "record", NULL };
 	int err = -1;
 	char bf[BUFSIZ];
+	struct sigaction sigchld_act;
 
 	signal(SIGSEGV, sighandler_dump_stack);
 	signal(SIGFPE, sighandler_dump_stack);
-	signal(SIGINT, sig_handler);
+	signal(SIGINT, sighandler_interrupt);
+
+	memset(&sigchld_act, 0, sizeof(sigchld_act));
+	sigchld_act.sa_flags = SA_SIGINFO;
+	sigchld_act.sa_sigaction = sighandler_chld;
+	sigaction(SIGCHLD, &sigchld_act, NULL);
 
 	trace.evlist = evlist__new();
 	trace.sctbl = syscalltbl__new();