allow creation of a Subprocess from an existing child pid

Summary:
Add a static method to Subprocess that allows creating a `Subprocess` object
from an existing child process ID.  This allows creating `Subprocess` objects
for children processes that were originally spawned through other APIs than
Subprocess itself.

This allows using the Subprocess `wait()` and `poll()` APIs for these child
processes, and allows passing them to existing parts of the code that expect
to work with `Subprocess` objects.

Reviewed By: yfeldblum

Differential Revision: D19588079

fbshipit-source-id: e29b56be4def659cedfd007ddb3a85e826c0a44c

folly/Subprocess.cpp

@@ -214,6 +214,13 @@ Subprocess::Subprocess(
   spawn(cloneStrings(argv), argv[0].c_str(), options, env);
 }
 
+Subprocess Subprocess::fromExistingProcess(pid_t pid) {
+  Subprocess sp;
+  sp.pid_ = pid;
+  sp.returnCode_ = ProcessReturnCode::makeRunning();
+  return sp;
+}
+
 Subprocess::~Subprocess() {
   CHECK_NE(returnCode_.state(), ProcessReturnCode::RUNNING)
       << "Subprocess destroyed without reaping child";

folly/Subprocess.h

@@ -536,6 +536,15 @@ class Subprocess {
       const std::vector<std::string>* env = nullptr);
   ~Subprocess();
 
+  /**
+   * Create a Subprocess object for an existing child process ID.
+   *
+   * The process ID must refer to an immediate child process of the current
+   * process.  This allows using the poll() and wait() APIs on a process ID
+   * that was not originally spawned by Subprocess.
+   */
+  static Subprocess fromExistingProcess(pid_t pid);
+
   /**
    * Create a subprocess run as a shell command (as shell -c 'command')
    *

folly/test/SubprocessTest.cpp

@@ -338,6 +338,25 @@ TEST(SimpleSubprocessTest, DetachExecFails) {
       "/no/such/file");
 }
 
+TEST(SimpleSubprocessTest, FromExistingProcess) {
+  // Manually fork a child process using fork() without exec(), and test waiting
+  // for it using the Subprocess API in the parent process.
+  static int constexpr kReturnCode = 123;
+
+  auto pid = fork();
+  ASSERT_NE(pid, -1) << "fork failed";
+  if (pid == 0) {
+    // child process
+    _exit(kReturnCode);
+  }
+
+  auto child = Subprocess::fromExistingProcess(pid);
+  EXPECT_TRUE(child.returnCode().running());
+  auto retCode = child.wait();
+  EXPECT_TRUE(retCode.exited());
+  EXPECT_EQ(kReturnCode, retCode.exitStatus());
+}
+
 TEST(ParentDeathSubprocessTest, ParentDeathSignal) {
   // Find out where we are.
   const auto basename = "subprocess_test_parent_death_helper";