works.diff

diff --git a/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb b/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb
index f03e020..73e99d7 100644
--- a/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb
+++ b/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb
@@ -516,6 +516,7 @@ struct _ParentProcess {
   internal var _childStdin: _FDOutputStream = _FDOutputStream(fd: -1)
   internal var _childStdout: _FDInputStream = _FDInputStream(fd: -1)
   internal var _childStderr: _FDInputStream = _FDInputStream(fd: -1)
+  internal var _childToParent: _FDInputStream = _FDInputStream(fd: -1)
 
   internal var _runTestsInProcess: Bool
   internal var _filter: String?
@@ -529,22 +530,30 @@ struct _ParentProcess {
 
   mutating func _spawnChild() {
     let params = [ "--stdlib-unittest-run-child" ] + _args
-    let (pid, childStdinFD, childStdoutFD, childStderrFD) = spawnChild(params)
+    let (
+      pid,
+      childStdinFD,
+      childStdoutFD,
+      childStderrFD,
+      childToParentFD) = spawnChild(params)
     _pid = pid
     _childStdin = _FDOutputStream(fd: childStdinFD)
     _childStdout = _FDInputStream(fd: childStdoutFD)
     _childStderr = _FDInputStream(fd: childStderrFD)
+    _childToParent = _FDInputStream(fd: childToParentFD)
   }
 
   mutating func _waitForChild() -> ProcessTerminationStatus {
-    let status = posixWaitpid(_pid)
+    let status = posixWaitpid(_pid, childToParentFD: _childToParent.fd)
     _pid = -1
     _childStdin.close()
     _childStdout.close()
     _childStderr.close()
+    _childToParent.close()
     _childStdin = _FDOutputStream(fd: -1)
     _childStdout = _FDInputStream(fd: -1)
     _childStderr = _FDInputStream(fd: -1)
+    _childToParent = _FDInputStream(fd: -1)
     return status
   }
 
diff --git a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
index 2857a93..84419bb 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
@@ -74,19 +74,36 @@ func posixPipe() -> (readFD: CInt, writeFD: CInt) {
 /// Start the same executable as a child process, redirecting its stdout and
 /// stderr.
 public func spawnChild(_ args: [String])
-  -> (pid: pid_t, stdinFD: CInt, stdoutFD: CInt, stderrFD: CInt) {
+  -> (
+    pid: pid_t,
+    stdinFD: CInt,
+    stdoutFD: CInt,
+    stderrFD: CInt,
+    childToParentFD: CInt) {
   // The stdout, stdin, and stderr from the child process will be redirected
   // to these pipes.
   let childStdout = posixPipe()
   let childStdin = posixPipe()
   let childStderr = posixPipe()
 
-#if os(Android)
+  var childArgs = args
+  childArgs.insert(Process.arguments[0], at: 0)
+  let interpreter = getenv("SWIFT_INTERPRETER")
+  if interpreter != nil {
+    if let invocation = String(validatingUTF8: interpreter) {
+      childArgs.insert(invocation, at: 0)
+    }
+  }
+
   // posix_spawn isn't available on Android. Instead, we fork and exec.
   // To correctly communicate the exit status of the child process to this
   // (parent) process, we'll use this pipe.
   let childToParentPipe = posixPipe()
+  if close(childToParentPipe.readFD) != 0 {
+    preconditionFailure("close() failed")
+  }
 
+#if os(Android)
   let pid = fork()
   precondition(pid >= 0, "fork() failed")
   if pid == 0 {
@@ -109,7 +126,7 @@ public func spawnChild(_ args: [String])
     // Start the executable. If execve() does not encounter an error, the
     // code after this block will never be executed, and the parent write pipe
     // will be closed.
-    withArrayOfCStrings([Process.arguments[0]] + args) {
+    withArrayOfCStrings(childArgs) {
       execve(Process.arguments[0], $0, _getEnviron())
     }
 
@@ -138,6 +155,12 @@ public func spawnChild(_ args: [String])
     close(childToParentPipe.writeFD)
   }
 #else
+  // Close the write end of the child-to-parent pipe, as we don't use it on
+  // non-Android platforms.
+  if close(childStderr.writeFD) != 0 {
+    preconditionFailure("close() failed")
+  }
+
   var fileActions = _make_posix_spawn_file_actions_t()
   if swift_posix_spawn_file_actions_init(&fileActions) != 0 {
     preconditionFailure("swift_posix_spawn_file_actions_init() failed")
@@ -180,14 +203,6 @@ public func spawnChild(_ args: [String])
   }
 
   var pid: pid_t = -1
-  var childArgs = args
-  childArgs.insert(Process.arguments[0], at: 0)
-  let interpreter = getenv("SWIFT_INTERPRETER")
-  if interpreter != nil {
-    if let invocation = String(validatingUTF8: interpreter) {
-      childArgs.insert(invocation, at: 0)
-    }
-  }
   let spawnResult = withArrayOfCStrings(childArgs) {
     swift_posix_spawn(
       &pid, childArgs[0], &fileActions, nil, $0, _getEnviron())
@@ -217,7 +232,12 @@ public func spawnChild(_ args: [String])
     preconditionFailure("close() failed")
   }
 
-  return (pid, childStdin.writeFD, childStdout.readFD, childStderr.readFD)
+  return (
+    pid,
+    childStdin.writeFD,
+    childStdout.readFD,
+    childStderr.readFD,
+    childToParentPipe.writeFD)
 }
 
 #if !os(Android)
@@ -284,11 +304,29 @@ public enum ProcessTerminationStatus : CustomStringConvertible {
   }
 }
 
-public func posixWaitpid(_ pid: pid_t) -> ProcessTerminationStatus {
+public func posixWaitpid(_ pid: pid_t, childToParentFD: CInt) -> ProcessTerminationStatus {
   var status: CInt = 0
   if waitpid(pid, &status, 0) < 0 {
     preconditionFailure("waitpid() failed")
   }
+#if os(Android)
+  // Read the results from our child-to-parent pipe.
+  var result: Int = 0
+  let bufferSize = 256
+  var buffer = [Int8](repeating: 0, count: bufferSize)
+  buffer.withUnsafeMutableBufferPointer {
+    result = read(childToParentFD, $0.baseAddress, bufferSize)
+  }
+  precondition(result == 0, "Could not read from child-to-parent pipe.")
+
+  withUnsafePointer(&buffer[0]) {
+    if let outputString = String(validatingUTF8: $0) {
+      if let output = CInt(outputString) {
+        status = output
+      }
+    }
+  }
+#endif
   if (WIFEXITED(status)) {
     return .exit(Int(WEXITSTATUS(status)))
   }
@@ -300,7 +338,7 @@ public func posixWaitpid(_ pid: pid_t) -> ProcessTerminationStatus {
 
 public func runChild(_ args: [String])
   -> (stdout: String, stderr: String, status: ProcessTerminationStatus) {
-  let (pid, _, stdoutFD, stderrFD) = spawnChild(args)
+  let (pid, _, stdoutFD, stderrFD, childToParentFD) = spawnChild(args)
 
   // FIXME: reading stdout and stderr sequentially can block.  Should use
   // select().  This is not so simple to implement because of:
@@ -314,7 +352,10 @@ public func runChild(_ args: [String])
   if close(stderrFD) != 0 {
     preconditionFailure("close() failed")
   }
-  let status = posixWaitpid(pid)
+  let status = posixWaitpid(pid, childToParentFD: childToParentFD)
+  if close(childToParentFD) != 0 {
+    preconditionFailure("close() failed")
+  }
   return (stdout, stderr, status)
 }