Subprocess library, modeled after python's subprocess module

Summary:
Surprised we don't have one.  The API is modeled after Python's
subprocess module, http://docs.python.org/2/library/subprocess.html

Inspired by
https://www.facebook.com/groups/fbcode/permalink/445399858830192/, plus
I needed this functionality now.

Test Plan: test added

Reviewed By: chip@fb.com

FB internal diff: D614056

folly/test/SubprocessTest.cpp

+/*
+ * Copyright 2012 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "folly/Subprocess.h"
+
+#include <glog/logging.h>
+#include <gtest/gtest.h>
+
+#include "folly/Format.h"
+#include "folly/experimental/io/Stream.h"
+
+using namespace folly;
+
+TEST(SimpleSubprocessTest, ExitsSuccessfully) {
+  Subprocess proc(std::vector<std::string>{ "/bin/true" });
+  EXPECT_EQ(0, proc.wait().exitStatus());
+}
+
+TEST(SimpleSubprocessTest, ExitsSuccessfullyChecked) {
+  Subprocess proc(std::vector<std::string>{ "/bin/true" });
+  proc.waitChecked();
+}
+
+TEST(SimpleSubprocessTest, ExitsWithError) {
+  Subprocess proc(std::vector<std::string>{ "/bin/false" });
+  EXPECT_EQ(1, proc.wait().exitStatus());
+}
+
+TEST(SimpleSubprocessTest, ExitsWithErrorChecked) {
+  Subprocess proc(std::vector<std::string>{ "/bin/false" });
+  EXPECT_THROW(proc.waitChecked(), CalledProcessError);
+}
+
+TEST(SimpleSubprocessTest, ShellExitsSuccesssfully) {
+  Subprocess proc("true");
+  EXPECT_EQ(0, proc.wait().exitStatus());
+}
+
+TEST(SimpleSubprocessTest, ShellExitsWithError) {
+  Subprocess proc("false");
+  EXPECT_EQ(1, proc.wait().exitStatus());
+}
+
+TEST(PopenSubprocessTest, PopenRead) {
+  Subprocess proc("ls /", Subprocess::Options().stdout(Subprocess::PIPE));
+  int found = 0;
+  for (auto bline : byLine(proc.stdout())) {
+    StringPiece line(bline);
+    if (line == "etc" || line == "bin" || line == "usr") {
+      ++found;
+    }
+  }
+  EXPECT_EQ(3, found);
+  proc.waitChecked();
+}
+
+TEST(CommunicateSubprocessTest, SimpleRead) {
+  Subprocess proc(std::vector<std::string>{ "/bin/echo", "-n", "foo", "bar"},
+                  Subprocess::Options().stdout(Subprocess::PIPE));
+  auto p = proc.communicate();
+  EXPECT_EQ("foo bar", p.first);
+  proc.waitChecked();
+}
+
+TEST(CommunicateSubprocessTest, BigWrite) {
+  const int numLines = 1 << 20;
+  std::string line("hello\n");
+  std::string data;
+  data.reserve(numLines * line.size());
+  for (int i = 0; i < numLines; ++i) {
+    data.append(line);
+  }
+
+  Subprocess::Options options;
+  options.stdin(Subprocess::PIPE).stdout(Subprocess::PIPE);
+
+  Subprocess proc("wc -l", options);
+  auto p = proc.communicate(Subprocess::WRITE_STDIN | Subprocess::READ_STDOUT,
+                            data);
+  EXPECT_EQ(folly::format("{}\n", numLines).str(), p.first);
+  proc.waitChecked();
+}
+
+TEST(CommunicateSubprocessTest, Duplex) {
+  // Take 10MB of data and pass them through a filter.
+  // One line, as tr is line-buffered
+  const int bytes = 10 << 20;
+  std::string line(bytes, 'x');
+
+  Subprocess::Options options;
+  options.stdin(Subprocess::PIPE).stdout(Subprocess::PIPE);
+
+  Subprocess proc("tr a-z A-Z", options);
+  auto p = proc.communicate(Subprocess::WRITE_STDIN | Subprocess::READ_STDOUT,
+                            line);
+  EXPECT_EQ(bytes, p.first.size());
+  EXPECT_EQ(std::string::npos, p.first.find_first_not_of('X'));
+  proc.waitChecked();
+}
+