debuggerd: call setsid in our children.

There appears to be a kernel bug that causes SIGHUP and SIGCONT to be
sent to the parent process group we spawn from if the process group
contains stopped jobs (e.g. the parent itself, because of wait_for_gdb).

Call setsid in all of our children to prevent this from happening.

Bug: http://b/31124563
Test: adb shell 'setprop debug.debuggerd.wait_for_gdb 1; killall -ABRT surfaceflinger'
Change-Id: I1a48d70886880a5bfbe2deb80d48deece55faf09
(cherry picked from commit 18cb681247)

debuggerd/crash_dump.cpp

@@ -363,6 +363,12 @@ int main(int argc, char** argv) {
   DefuseSignalHandlers();
   InstallSigPipeHandler();
 
+  // There appears to be a bug in the kernel where our death causes SIGHUP to
+  // be sent to our process group if we exit while it has stopped jobs (e.g.
+  // because of wait_for_gdb). Use setsid to create a new process group to
+  // avoid hitting this.
+  setsid();
+
   atrace_begin(ATRACE_TAG, "before reparent");
   pid_t target_process = getppid();
 

debuggerd/handler/debuggerd_handler.cpp

@@ -268,8 +268,15 @@ static void create_vm_process() {
       _exit(errno);
     }
 
-    // Exit immediately on both sides of the fork.
-    // crash_dump is ptracing us, so it'll get to do whatever it wants in between.
+    // crash_dump is ptracing both sides of the fork; it'll let the parent exit,
+    // but keep the orphan stopped to peek at its memory.
+
+    // There appears to be a bug in the kernel where our death causes SIGHUP to
+    // be sent to our process group if we exit while it has stopped jobs (e.g.
+    // because of wait_for_gdb). Use setsid to create a new process group to
+    // avoid hitting this.
+    setsid();
+
     _exit(0);
   }