Add Cocoapods podspec generation. (apple#568)

Motivation:

A number of users have asked for the ability to install SwiftNIO
via Cocoapods. This is an entirely reasonable request, so let's
do it.

Modifications:

- Added a script that can generate, and optionally upload,
  podspecs.
- Made a slight modification to the dependency script to provide
  target-specific dependencies.

Result:

Users can use cocoapods if they prefer.

Author: Lukasa

.gitignore

@@ -6,3 +6,4 @@ Package.pins
 *.pem
 /docs
 Package.resolved
+.podspecs

scripts/build_podspecs.sh

@@ -0,0 +1,102 @@
+#!/bin/bash
+##===----------------------------------------------------------------------===##
+##
+## 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
+##
+##===----------------------------------------------------------------------===##
+
+set -eu
+
+function usage() {
+  echo "$0 [-u] VERSION"
+  echo
+  echo "OPTIONS:"
+  echo "  -u: Additionally, upload the podspecs as they are generated"
+}
+
+OPTIND=1
+upload=false
+while getopts ":u" opt; do
+  case $opt in
+    u)
+      upload=true
+      ;;
+    \?)
+      usage
+      exit 1
+      ;;
+  esac
+done
+shift "$((OPTIND-1))"
+
+if [[ $# -eq 0 ]]; then
+  echo "Must provide target version"
+  exit 1
+fi
+
+version=$1
+newline=$'\n'
+
+here="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+tmpfile=$(mktemp -d /tmp/.build_podspecsXXXXXX)
+echo "Building podspecs in $tmpfile"
+
+# We have a name transformation here. We want the pod names to be
+# SwiftX or CX, but our target names aren't. We add SwiftX to the front
+# of all NIO targets, and then remove it in the source files declaration
+# if needed.
+targets=( $("${here}/list_topsorted_dependencies.sh" -l -r | grep -v "NIOPriorityQueue" | sed 's/^NIO/SwiftNIO/') )
+
+for target in "${targets[@]}"; do
+  echo "Building podspec for $target"
+
+  dependencies=()
+
+  while read -r raw_dependency; do
+    dependencies+=( "${newline}  s.dependency '$raw_dependency', s.version.to_s" )
+  done < <("${here}/list_topsorted_dependencies.sh" -d "${target#Swift}" | grep -v "NIOPriorityQueue" | sed 's/^NIO/SwiftNIO/')
+
+  libraries=""
+
+  if [[ "$target" == "CNIOZlib" ]]; then
+    libraries="s.libraries = 'z'"
+  fi
+
+  cat > "${tmpfile}/${target}.podspec" <<- EOF
+Pod::Spec.new do |s|
+  s.name = '$target'
+  s.version = '$version'
+  s.license = { :type => 'Apache 2.0', :file => 'LICENSE.txt' }
+  s.summary = 'Event-driven network application framework for high performance protocol servers & clients, non-blocking.'
+  s.homepage = 'https://github.com/apple/swift-nio'
+  s.author = 'Apple Inc.'
+  s.source = { :git => 'https://github.com/apple/swift-nio.git', :tag => s.version.to_s }
+  s.documentation_url = 'https://apple.github.io/swift-nio/docs/current/NIO/index.html'
+  s.module_name = '${target#Swift}'
+
+  s.swift_version = '4.1'
+  s.cocoapods_version = '>=1.1.0'
+  s.ios.deployment_target = '10.0'
+  s.osx.deployment_target = '10.10'
+  s.tvos.deployment_target = '10.0'
+
+  s.source_files = 'Sources/${target#Swift}/**/*.{swift,c,h}'
+  ${dependencies[*]-}
+  $libraries
+end
+EOF
+
+  if $upload; then
+    echo "Uploading ${tmpfile}/${target}.podspec"
+    pod trunk push "${tmpfile}/${target}.podspec"
+  fi
+
+done

scripts/list_topsorted_dependencies.sh

@@ -23,6 +23,7 @@ function usage() {
     echo "OPTIONS:"
     echo "  -l: Only dependencies of library targets"
     echo "  -r: Reverse the output"
+    echo "  -d <PACKAGE>: Prints the dependencies of the given package"
 }
 
 function tac_compat() {
@@ -33,14 +34,18 @@ tmpfile=$(mktemp /tmp/.list_topsorted_dependencies_XXXXXX)
 
 only_libs=false
 do_reversed=false
-while getopts "lr" opt; do
+package_dependency=""
+while getopts "lrd:" opt; do
     case $opt in
         l)
             only_libs=true
             ;;
         r)
             do_reversed=true
             ;;
+        d)
+            package_dependency="$OPTARG"
+            ;;
         \?)
             usage
             exit 1
@@ -53,6 +58,12 @@ if $do_reversed; then
     transform=tac_compat
 fi
 
+if [[ ! -z "$package_dependency" ]]; then
+  swift package dump-package | jq -r ".targets |
+                                      map(select(.name == \"$package_dependency\" and .type == \"regular\") | .dependencies | map(.name)) | .[] | .[]"
+  exit 0
+fi
+
 (
 cd "$here/.."
 if $only_libs; then