debuggerd_handler: don't use waitpid(..., __WCLONE).

waitpid(..., __WCLONE) fails with ECHILD when passed an explicit PID to
wait for. __WALL and __WCLONE don't seem to be necessary when waiting
for a specific pid, so just pass 0 in the flags instead.

Bug: http://b/35327712
Test: /data/nativetest/debuggerd_test/debuggerd_test32 --gtest_filter="*zombie*"
Change-Id: I3dd7a1bdf7ff35fdfbf631429c089ef4e3172855

debuggerd/debuggerd_test.cpp

@@ -407,3 +407,35 @@ TEST_F(CrasherTest, PR_SET_DUMPABLE_0_raise) {
   });
   AssertDeath(SIGUSR1);
 }
+
+TEST(crash_dump, zombie) {
+  pid_t forkpid = fork();
+
+  int pipefd[2];
+  ASSERT_EQ(0, pipe2(pipefd, O_CLOEXEC));
+
+  pid_t rc;
+  int status;
+
+  if (forkpid == 0) {
+    errno = 0;
+    rc = waitpid(-1, &status, WNOHANG | __WALL | __WNOTHREAD);
+    if (rc != -1 || errno != ECHILD) {
+      errx(2, "first waitpid returned %d (%s), expected failure with ECHILD", rc, strerror(errno));
+    }
+
+    raise(DEBUGGER_SIGNAL);
+
+    errno = 0;
+    rc = waitpid(-1, &status, __WALL | __WNOTHREAD);
+    if (rc != -1 || errno != ECHILD) {
+      errx(2, "second waitpid returned %d (%s), expected failure with ECHILD", rc, strerror(errno));
+    }
+    _exit(0);
+  } else {
+    rc = waitpid(forkpid, &status, 0);
+    ASSERT_EQ(forkpid, rc);
+    ASSERT_TRUE(WIFEXITED(status));
+    ASSERT_EQ(0, WEXITSTATUS(status));
+  }
+}

debuggerd/handler/debuggerd_handler.cpp

@@ -249,7 +249,7 @@ static int debuggerd_dispatch_pseudothread(void* arg) {
 
     // Don't leave a zombie child.
     int status;
-    if (TEMP_FAILURE_RETRY(waitpid(forkpid, &status, __WCLONE)) == -1 && errno != ECHILD) {
+    if (TEMP_FAILURE_RETRY(waitpid(forkpid, &status, 0)) == -1) {
       __libc_format_log(ANDROID_LOG_FATAL, "libc", "failed to wait for crash_dump helper: %s",
                         strerror(errno));
     } else if (WIFSTOPPED(status) || WIFSIGNALED(status)) {