From 334d3cb564c72bd430a17c6e6f01aeb488fb191c Mon Sep 17 00:00:00 2001
From: Carl Hetherington <cth@carlh.net>
Date: Fri, 7 Aug 2020 00:10:59 +0200
Subject: Check that we have a connection to the disk writer before starting to
 use it in earnest.

---
 src/lib/disk_writer_messages.h     | 10 ++++++++++
 src/tools/dcpomatic_disk.cc        | 25 +++++++++++++++++++++++++
 src/tools/dcpomatic_disk_writer.cc |  2 ++
 3 files changed, 37 insertions(+)

diff --git a/src/lib/disk_writer_messages.h b/src/lib/disk_writer_messages.h
index d31a9cb0a..406afc61d 100644
--- a/src/lib/disk_writer_messages.h
+++ b/src/lib/disk_writer_messages.h
@@ -23,6 +23,16 @@
  * by \n.
  */
 
+/* PING */
+
+// Front-end sends:
+
+#define DISK_WRITER_PING "P"
+
+// Back-end responds
+
+#define DISK_WRITER_PONG "O"
+
 /* REQUEST TO WRITE DCP */
 
 // Front-end sends:
diff --git a/src/tools/dcpomatic_disk.cc b/src/tools/dcpomatic_disk.cc
index b94d4bf94..7bea4d8e9 100644
--- a/src/tools/dcpomatic_disk.cc
+++ b/src/tools/dcpomatic_disk.cc
@@ -210,6 +210,31 @@ private:
 		DCPOMATIC_ASSERT (_drive->GetSelection() != wxNOT_FOUND);
 		DCPOMATIC_ASSERT (static_cast<bool>(_dcp_path));
 
+		bool have_writer = true;
+		if (!_nanomsg.send(DISK_WRITER_PING "\n", 2000)) {
+			have_writer = false;
+		} else {
+			optional<string> reply = _nanomsg.receive (2000);
+			if (!reply || *reply != DISK_WRITER_PONG) {
+				have_writer = false;
+			}
+		}
+
+		if (!have_writer) {
+#ifdef DCPOMATIC_WINDOWS
+			MessageDialog* m = new MessageDialog (
+				this,
+				_("DCP-o-matic Disk Writer"),
+				_("Do you see a 'User Account Control' dialogue asking about dcpomatic2_disk_writer.exe?  If so, click 'Yes', then try again.")
+				);
+			m->ShowModal ();
+			m->Destroy ();
+			return;
+#else
+			throw CommunicationFailedError ();
+#endif
+		}
+
 		Drive const& drive = _drives[_drive->GetSelection()];
 		if (drive.mounted()) {
 			TryUnmountDialog* d = new TryUnmountDialog(this, drive.description());
diff --git a/src/tools/dcpomatic_disk_writer.cc b/src/tools/dcpomatic_disk_writer.cc
index 2c3753e57..a2788e18b 100644
--- a/src/tools/dcpomatic_disk_writer.cc
+++ b/src/tools/dcpomatic_disk_writer.cc
@@ -460,6 +460,8 @@ try
 
 	if (*s == DISK_WRITER_QUIT) {
 		exit (EXIT_SUCCESS);
+	} else if (*s == DISK_WRITER_PING) {
+		nanomsg->send(DISK_WRITER_PONG "\n", LONG_TIMEOUT);
 	} else if (*s == DISK_WRITER_UNMOUNT) {
 		/* XXX: should do Linux polkit stuff here */
 		optional<string> xml_head = nanomsg->receive (LONG_TIMEOUT);
-- 
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