Dup and redirect stdout to defend against accidental print()

We currently have a regression where sourcekit-lsp stops working  in
some packages, because swiftpm started printing a warning to stdout.
While we should also fix swiftpm, it behooves us to be more robust in
sourcekit-lsp in case this happens again. Dup stdout for use by the
protocol, and redirect the stdout file descriptor to stderr, which does
not interfere with the protocol.

Author: benlangmuir

Sources/sourcekit-lsp/main.swift

@@ -107,10 +107,20 @@ do {
   exit(1)
 }
 
+// Dup stdout and redirect the fd to stderr so that a careless print()
+// will not break our connection stream.
+let realStdout = dup(STDOUT_FILENO)
+if realStdout == -1 {
+  fatalError("failed to dup stdout:  \(strerror(errno))")
+}
+if dup2(STDERR_FILENO, STDOUT_FILENO) == -1 {
+  fatalError("failed to redirect stdout -> stderr: \(strerror(errno))")
+}
+
 let clientConnection = JSONRPCConnection(
   protocol: MessageRegistry.lspProtocol,
   inFD: STDIN_FILENO,
-  outFD: STDOUT_FILENO,
+  outFD: realStdout,
   syncRequests: options.syncRequests)
 
 let installPath = AbsolutePath(Bundle.main.bundlePath)