From 7d579e7400069d116628da38ff3026670fe53611 Mon Sep 17 00:00:00 2001
From: Max Kellermann <max@musicpd.org>
Date: Tue, 14 Nov 2017 21:06:17 +0100
Subject: [PATCH] lib/alsa/NonBlock: throw exception on error

Avoid another potential deadlock: if no file descriptors are
registered, our non-blocking ALSA code cannot ever work.
---
 src/lib/alsa/NonBlock.cxx               | 19 ++++++++++++++-----
 src/lib/alsa/NonBlock.hxx               |  4 +++-
 src/output/plugins/AlsaOutputPlugin.cxx |  8 +++++++-
 3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/src/lib/alsa/NonBlock.cxx b/src/lib/alsa/NonBlock.cxx
index 2a079a7d5..c9850186c 100644
--- a/src/lib/alsa/NonBlock.cxx
+++ b/src/lib/alsa/NonBlock.cxx
@@ -20,22 +20,31 @@
 #include "config.h"
 #include "NonBlock.hxx"
 #include "event/MultiSocketMonitor.hxx"
+#include "util/RuntimeError.hxx"
 
 std::chrono::steady_clock::duration
 PrepareAlsaPcmSockets(MultiSocketMonitor &m, snd_pcm_t *pcm,
-		      ReusableArray<pollfd> &pfd_buffer) noexcept
+		      ReusableArray<pollfd> &pfd_buffer)
 {
 	int count = snd_pcm_poll_descriptors_count(pcm);
 	if (count <= 0) {
-		m.ClearSocketList();
-		return std::chrono::steady_clock::duration(-1);
+		if (count == 0)
+			throw std::runtime_error("snd_pcm_poll_descriptors_count() failed");
+		else
+			throw FormatRuntimeError("snd_pcm_poll_descriptors_count() failed: %s",
+						 snd_strerror(-count));
 	}
 
 	struct pollfd *pfds = pfd_buffer.Get(count);
 
 	count = snd_pcm_poll_descriptors(pcm, pfds, count);
-	if (count < 0)
-		count = 0;
+	if (count <= 0) {
+		if (count == 0)
+			throw std::runtime_error("snd_pcm_poll_descriptors() failed");
+		else
+			throw FormatRuntimeError("snd_pcm_poll_descriptors() failed: %s",
+						 snd_strerror(-count));
+	}
 
 	m.ReplaceSocketList(pfds, count);
 	return std::chrono::steady_clock::duration(-1);
diff --git a/src/lib/alsa/NonBlock.hxx b/src/lib/alsa/NonBlock.hxx
index d6fff8e16..4c3767d79 100644
--- a/src/lib/alsa/NonBlock.hxx
+++ b/src/lib/alsa/NonBlock.hxx
@@ -33,10 +33,12 @@ class MultiSocketMonitor;
  * Update #MultiSocketMonitor's socket list from
  * snd_pcm_poll_descriptors().  To be called from
  * MultiSocketMonitor::PrepareSockets().
+ *
+ * Throws exception on error.
  */
 std::chrono::steady_clock::duration
 PrepareAlsaPcmSockets(MultiSocketMonitor &m, snd_pcm_t *pcm,
-		      ReusableArray<pollfd> &pfd_buffer) noexcept;
+		      ReusableArray<pollfd> &pfd_buffer);
 
 /**
  * Update #MultiSocketMonitor's socket list from
diff --git a/src/output/plugins/AlsaOutputPlugin.cxx b/src/output/plugins/AlsaOutputPlugin.cxx
index c7dda9e9a..8a7acee95 100644
--- a/src/output/plugins/AlsaOutputPlugin.cxx
+++ b/src/output/plugins/AlsaOutputPlugin.cxx
@@ -815,7 +815,13 @@ AlsaOutput::PrepareSockets() noexcept
 		return std::chrono::steady_clock::duration(-1);
 	}
 
-	return PrepareAlsaPcmSockets(*this, pcm, pfd_buffer);
+	try {
+		return PrepareAlsaPcmSockets(*this, pcm, pfd_buffer);
+	} catch (...) {
+		ClearSocketList();
+		LockCaughtError();
+		return std::chrono::steady_clock::duration(-1);
+	}
 }
 
 void