Fix hang stracing forking processes but not following child

* ntdll.h (PROCESSINFOCLASS): Define ProcessDebugFlags.
        * sigproc.cc (child_info::child_info): Only propagate _CI_STRACED to
        child if strace is actually tracing child processes.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/ChangeLog

@@ -1,3 +1,9 @@
+2015-08-24  Corinna Vinschen  <corinna@vinschen.de>
+
+	* ntdll.h (PROCESSINFOCLASS): Define ProcessDebugFlags.
+	* sigproc.cc (child_info::child_info): Only propagate _CI_STRACED to
+	child if strace is actually tracing child processes.
+
 2015-08-24  Corinna Vinschen  <corinna@vinschen.de>
 
 	* include/cygwin/version.h (CYGWIN_VERSION_DLL_MINOR): Bump to 2.

winsup/cygwin/ntdll.h

@@ -532,7 +532,8 @@ typedef enum _PROCESSINFOCLASS
   ProcessTimes = 4,
   ProcessSessionInformation = 24,
   ProcessWow64Information = 26,
-  ProcessImageFileName = 27
+  ProcessImageFileName = 27,
+  ProcessDebugFlags = 31
 } PROCESSINFOCLASS;
 
 /* Checked on 64 bit. */

winsup/cygwin/release/2.2.2

@@ -0,0 +1,14 @@
+What's new:
+-----------
+
+
+What changed:
+-------------
+
+
+Bug Fixes
+---------
+
+- Fix a hang when stracing a forking or spawning process without activating
+  stracing of child processes.
+  Addresses: https://cygwin.com/ml/cygwin/2015-08/msg00390.html

winsup/cygwin/sigproc.cc

@@ -772,7 +772,21 @@ child_info::child_info (unsigned in_cb, child_info_types chtype,
   fhandler_union_cb = sizeof (fhandler_union);
   user_h = cygwin_user_h;
   if (strace.active ())
-    flag |= _CI_STRACED;
+    {
+      NTSTATUS status;
+      ULONG DebugFlags;
+
+      /* Only propagate _CI_STRACED to child if strace is actually tracing
+	 child processes of this process.  The undocumented ProcessDebugFlags
+	 returns 0 if EPROCESS->NoDebugInherit is TRUE, 1 otherwise.
+	 This avoids a hang when stracing a forking or spawning process
+	 with the -f flag set to "don't follow fork". */
+      status = NtQueryInformationProcess (GetCurrentProcess (),
+					  ProcessDebugFlags, &DebugFlags,
+					  sizeof (DebugFlags), NULL);
+      if (NT_SUCCESS (status) && DebugFlags)
+	flag |= _CI_STRACED;
+    }
   if (need_subproc_ready)
     {
       subproc_ready = CreateEvent (&sec_all, FALSE, FALSE, NULL);