add draft ALSA input plugin

I've created an elementary input plugin that plays sound from the
soundcard, so you can use MPD to listen to anything connected to the
line-in jack, or to Video4Linux FM radio cards that send audio through
the soundcard.  There has been a small number of posts here in the
past requesting line-in input, so here is a first, simplistic stab at
it.

The patch adds a new sheme, alsa://, which causes mpd to play data
read directly from a souncdard.  It defaults to hw:0,0, but you can
pass any ALSA device name in the URI.  So, using mpc for example:

 mpc add alsa://
 mpc play

will play from device hw:0,0.

To use a diffferent device:

 mpc add alsa://hw:1,0

Makefile.am

@@ -802,6 +802,14 @@ INPUT_LIBS = \
 	$(DESPOTIFY_LIBS) \
 	$(MMS_LIBS)
 
+if HAVE_ALSA
+libinput_a_SOURCES += \
+	src/input/AlsaInputPlugin.cxx \
+	src/input/AlsaInputPlugin.hxx
+INPUT_LIBS += $(ALSA_LIBS)
+endif
+
+
 if ENABLE_CURL
 libinput_a_SOURCES += \
 	src/input/CurlInputPlugin.cxx src/input/CurlInputPlugin.hxx \

NEWS

@@ -1,6 +1,8 @@
 ver 0.19 (not yet released)
 * protocol
   - new commands "addtagid", "cleartagid"
+* input
+  - alsa: new input plugin
 * new resampler option using libsoxr
 
 ver 0.18.6 (not yet released)

src/InputRegistry.cxx

@@ -22,6 +22,10 @@
 #include "util/Macros.hxx"
 #include "input/FileInputPlugin.hxx"
 
+#ifdef HAVE_ALSA
+#include "input/AlsaInputPlugin.hxx"
+#endif
+
 #ifdef ENABLE_ARCHIVE
 #include "input/ArchiveInputPlugin.hxx"
 #endif
@@ -48,6 +52,9 @@
 
 const InputPlugin *const input_plugins[] = {
 	&input_plugin_file,
+#ifdef HAVE_ALSA
+	&input_plugin_alsa,
+#endif
 #ifdef ENABLE_ARCHIVE
 	&input_plugin_archive,
 #endif

src/input/AlsaInputPlugin.cxx

@@ -0,0 +1,206 @@
+/*
+ * Copyright (C) 2003-2013 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+/*
+ * ALSA code based on an example by Paul Davis released under GPL here:
+ * http://equalarea.com/paul/alsa-audio.html
+ * and one by Matthias Nagorni, also GPL, here:
+ * http://alsamodular.sourceforge.net/alsa_programming_howto.html
+ */
+
+#include "config.h"
+#include "AlsaInputPlugin.hxx"
+#include "InputPlugin.hxx"
+#include "InputStream.hxx"
+#include "util/Domain.hxx"
+#include "util/Error.hxx"
+#include "util/StringUtil.hxx"
+#include "Log.hxx"
+
+#include <alsa/asoundlib.h>
+
+static constexpr Domain alsa_input_domain("alsa");
+
+static constexpr const char *default_device = "hw:0,0";
+
+// this value chosen to balance between limiting latency and avoiding stutter
+static constexpr int max_frames_to_buffer = 64;
+
+// the following defaults are because the PcmDecoderPlugin forces CD format
+static constexpr snd_pcm_format_t default_format = SND_PCM_FORMAT_S16;
+static constexpr int default_channels = 2; // stereo
+static constexpr unsigned int default_rate = 44100; // cd quality
+
+struct AlsaInputStream {
+	InputStream base;
+	snd_pcm_t *capture_handle;
+	size_t frame_size;
+	size_t max_bytes_to_read;
+
+	AlsaInputStream(const char *uri, Mutex &mutex, Cond &cond,
+			snd_pcm_t *handle)
+		:base(input_plugin_alsa, uri, mutex, cond),
+		 capture_handle(handle) {
+		frame_size = snd_pcm_format_width(default_format) / 8 * default_channels;
+		max_bytes_to_read = max_frames_to_buffer * frame_size;
+		base.mime = strdup("audio/x-mpd-cdda-pcm");
+		base.seekable = false;
+		base.size = -1;
+		base.ready = true;
+	}
+
+	~AlsaInputStream() {
+		snd_pcm_close(capture_handle);
+	}
+};
+
+static InputStream *
+alsa_input_open(const char *uri, Mutex &mutex, Cond &cond, Error &error)
+{
+	int err;
+
+	// check uri is appropriate for alsa input
+	if (!StringStartsWith(uri, "alsa://"))
+		return nullptr;
+
+	const char *device = uri + 7;
+	if (device[0] == '\0')
+		device = default_device;
+
+	snd_pcm_t *capture_handle;
+	if ((err = snd_pcm_open(&capture_handle, device,
+				SND_PCM_STREAM_CAPTURE, 0)) < 0) {
+		error.Format(alsa_input_domain, "Failed to open device: %s (%s)", device, snd_strerror(err));
+		return nullptr;
+	}
+
+	snd_pcm_hw_params_t *hw_params;
+	if ((err = snd_pcm_hw_params_malloc(&hw_params)) < 0) {
+		error.Format(alsa_input_domain, "Cannot allocate hardware parameter structure (%s)", snd_strerror(err));
+		snd_pcm_close(capture_handle);
+		return nullptr;
+	}
+
+	if ((err = snd_pcm_hw_params_any(capture_handle, hw_params)) < 0) {
+		error.Format(alsa_input_domain, "Cannot initialize hardware parameter structure (%s)", snd_strerror(err));
+		snd_pcm_hw_params_free(hw_params);
+		snd_pcm_close(capture_handle);
+		return nullptr;
+	}
+
+	if ((err = snd_pcm_hw_params_set_access(capture_handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {
+		error.Format(alsa_input_domain, "Cannot set access type (%s)", snd_strerror (err));
+		snd_pcm_hw_params_free (hw_params);
+		snd_pcm_close(capture_handle);
+		return nullptr;
+	}
+
+	if ((err = snd_pcm_hw_params_set_format(capture_handle, hw_params, default_format)) < 0) {
+		snd_pcm_hw_params_free(hw_params);
+		snd_pcm_close(capture_handle);
+		error.Format(alsa_input_domain, "Cannot set sample format (%s)", snd_strerror (err));
+		return nullptr;
+	}
+
+	if ((err = snd_pcm_hw_params_set_channels(capture_handle, hw_params, default_channels)) < 0) {
+		snd_pcm_hw_params_free(hw_params);
+		snd_pcm_close(capture_handle);
+		error.Format(alsa_input_domain, "Cannot set channels (%s)", snd_strerror (err));
+		return nullptr;
+	}
+
+	if ((err = snd_pcm_hw_params_set_rate(capture_handle, hw_params, default_rate, 0)) < 0) {
+		snd_pcm_hw_params_free(hw_params);
+		snd_pcm_close(capture_handle);
+		error.Format(alsa_input_domain, "Cannot set sample rate (%s)", snd_strerror (err));
+		return nullptr;
+	}
+
+	if ((err = snd_pcm_hw_params(capture_handle, hw_params)) < 0) {
+		error.Format(alsa_input_domain, "Cannot set parameters (%s)", snd_strerror (err));
+		snd_pcm_hw_params_free(hw_params);
+		snd_pcm_close(capture_handle);
+		return nullptr;
+	}
+
+	snd_pcm_hw_params_free (hw_params);
+
+	// clear any data already in the PCM buffer
+	if ((err = snd_pcm_drop(capture_handle)) < 0) {
+		error.Format(alsa_input_domain, "Cannot clear PCM buffer (%s)", snd_strerror (err));
+		snd_pcm_hw_params_free(hw_params);
+		snd_pcm_close(capture_handle);
+		return nullptr;
+	}
+
+	AlsaInputStream *ais = new AlsaInputStream(uri, mutex, cond, capture_handle);
+	return &ais->base;
+}
+
+static void
+alsa_input_close(InputStream *is)
+{
+	AlsaInputStream *ais = (AlsaInputStream*) is;
+	delete ais;
+}
+
+static size_t
+alsa_input_read(InputStream *is, void *ptr, size_t size,
+		gcc_unused Error &error)
+{
+	AlsaInputStream *ais = (AlsaInputStream*) is;
+	int num_frames = max_frames_to_buffer;
+	if (size < ais->max_bytes_to_read)
+		// calculate number of whole frames that will fit in size bytes
+		num_frames = size / ais->frame_size;
+
+	int ret;
+	while ((ret = snd_pcm_readi(ais->capture_handle, ptr,
+				    num_frames)) < 0) {
+		snd_pcm_prepare(ais->capture_handle);
+		LogDebug(alsa_input_domain, "Buffer Overrun");
+	}
+
+	size_t nbytes = ret == max_frames_to_buffer
+		? ais->max_bytes_to_read
+		: ret * ais->frame_size;
+	is->offset += nbytes;
+	return nbytes;
+}
+
+static bool
+alsa_input_eof(gcc_unused InputStream *is)
+{
+	return false;
+};
+
+const struct InputPlugin input_plugin_alsa = {
+	.name = "alsa",
+	.init = nullptr,
+	.finish = nullptr,
+	.open = alsa_input_open,
+	.close = alsa_input_close,
+	.check = nullptr,
+	.update = nullptr,
+	.tag = nullptr,
+	.available = nullptr,
+	.read = alsa_input_read,
+	.eof = alsa_input_eof,
+	.seek = nullptr
+};

src/input/AlsaInputPlugin.hxx

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2003-2013 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_ALSA_INPUT_PLUGIN_HXX
+#define MPD_ALSA_INPUT_PLUGIN_HXX
+
+#include "InputPlugin.hxx"
+
+extern const struct InputPlugin input_plugin_alsa;
+
+
+#endif

src/ls.cxx

@@ -54,6 +54,9 @@ static const char *remoteUrlPrefixes[] = {
 #endif
 #ifdef ENABLE_DESPOTIFY
 	"spt://",
+#endif
+#ifdef HAVE_ALSA
+	"alsa://",
 #endif
 	NULL
 };