Cygwin: cygtls: fix context alignment

A hang was encountered, apparently triggered by commit 63b503916d,
changing tls_pathbufs from malloc'ed to HeapAlloc'ed memory.  After
lengthy debugging it transpired that adding the heap handle to the
tls_pathbuf struct added 8 bytes to the cygtls area, thus moving
the "context" member by 8 bytes, too, so it was suddently unaligned.

Fix this for now by changing the alignment.

Fix this once and for all, by adding code to the gentls_offsets script
to check if the alignment of the "context" member is 16 bytes.  If not,
print a matching error message, remove the just generated file, and exit
with error.

FIXME: It would be really nice to find a way to auomate the correct
alignment of the "context" member, but I don't see any way to use
alignment attributes to get what we need here.

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

winsup/cygwin/local_includes/cygtls.h

@@ -178,11 +178,14 @@ public: /* Do NOT remove this public: line, it's a marker for gentls_offsets. */
   siginfo_t *sigwait_info;
   HANDLE signal_arrived;
   bool will_wait_for_signal;
+#if 0
   long __align;			/* Needed to align context to 16 byte. */
+#endif
   /* context MUST be aligned to 16 byte, otherwise RtlCaptureContext fails.
      If you prepend cygtls members here, make sure context stays 16 byte
-     aligned. */
+     aligned. The gentls_offsets script checks for that now and fails
-  ucontext_t context;
+     if the alignment is wrong. */
+  ucontext_t __attribute__((__aligned__(__alignof__(long double)))) context;
   DWORD thread_id;
   siginfo_t infodata;
   struct pthread *tid;

winsup/cygwin/scripts/gentls_offsets

@@ -98,3 +98,12 @@ gawk -v start_offset="$start_offset" '\
     }
   }
 ' ${tmp_file} > "${output_file}"
+# Check if the `context' member is 16 bytes aligned.  Delete output_file
+# and bail out with error if not.
+MOD=$(awk '/_cygtls.context_p/{ print $3 % 16; }' "${output_file}")
+if [ $MOD -ne 0 ]
+then
+  echo "Error: _cygtls.context member is not 16 bytes aligned!"
+  rm "${output_file}"
+  exit 1
+fi