add useful dtrace scripts to debug allocations (#352)

Motivation:

dtrace is extrmely useful when debugging allocation related issues. This
adds two scripts that have helped me many times.

Modifications:

- `dev/malloc-aggregation.d` which prints an aggregation of all
  stacks that have allocated
- `dev/boxed-existentials.d` which live prints all allocations of boxed
  existentials

Result:

sharing debugging tools is great

dev/boxed-existentials.d

@@ -0,0 +1,23 @@
+#!/usr/sbin/dtrace -q -s
+/*===----------------------------------------------------------------------===*
+ *
+ *  This source file is part of the SwiftNIO open source project
+ *
+ *  Copyright (c) 2017-2018 Apple Inc. and the SwiftNIO project authors
+ *  Licensed under Apache License v2.0
+ *
+ *  See LICENSE.txt for license information
+ *  See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+ *
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *===----------------------------------------------------------------------===*/
+
+/*
+ * example invocation:
+ *   sudo dev/boxed-existentials.d -c .build/release/NIOHTTP1Server
+ */
+
+pid$target::__swift_allocate_boxed_opaque_existential*:entry {
+    ustack();
+}

dev/malloc-aggregation.d

@@ -0,0 +1,37 @@
+#!/usr/sbin/dtrace -q -s
+/*===----------------------------------------------------------------------===*
+ *
+ *  This source file is part of the SwiftNIO open source project
+ *
+ *  Copyright (c) 2017-2018 Apple Inc. and the SwiftNIO project authors
+ *  Licensed under Apache License v2.0
+ *
+ *  See LICENSE.txt for license information
+ *  See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+ *
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *===----------------------------------------------------------------------===*/
+
+/*
+ * example invocation:
+ *   sudo dev/malloc-aggregation.d -c .build/release/NIOHTTP1Server
+ */
+
+::BEGIN {
+    printf("\n\n");
+    printf("=====\n");
+    printf("This will collect stack shots of allocations and print it when ");
+    printf("you exit dtrace.\n");
+    printf("So go ahead, run your tests and then press Ctrl+C in this window ");
+    printf("to see the aggregated result\n");
+    printf("=====\n");
+}
+
+pid$target::malloc:entry {
+    @malloc_calls[ustack()] = count();
+}
+
+::END {
+    printa(@malloc_calls);
+}

scripts/sanity.sh

@@ -31,7 +31,7 @@ fi
 printf "=> Checking license headers... "
 tmp=$(mktemp /tmp/.swift-nio-sanity_XXXXXX)
 
-for language in swift-or-c bash; do
+for language in swift-or-c bash dtrace; do
   declare -a matching_files
   declare -a exceptions
   expections=( )
@@ -73,6 +73,25 @@ EOF
 ## SPDX-License-Identifier: Apache-2.0
 ##
 ##===----------------------------------------------------------------------===##
+EOF
+      ;;
+      dtrace)
+        matching_files=( -name '*.d' )
+        cat > "$tmp" <<"EOF"
+#!/usr/sbin/dtrace -q -s
+/*===----------------------------------------------------------------------===*
+ *
+ *  This source file is part of the SwiftNIO open source project
+ *
+ *  Copyright (c) 2017-2018 Apple Inc. and the SwiftNIO project authors
+ *  Licensed under Apache License v2.0
+ *
+ *  See LICENSE.txt for license information
+ *  See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+ *
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *===----------------------------------------------------------------------===*/
 EOF
       ;;
     *)