Implement module-always-source

This is basically a copy of module-always-sink but doing the same for
sources. Whenever no source is available, a module-null-source is loaded
and whenever a new source is available again, module-null-source is
unloaded.

By this, anything using a source will automatically be switched to the
null source when the actual source disappears, and back to the actual
source if it appears again.

po/POTFILES.in

@@ -24,6 +24,7 @@ src/modules/jack/module-jack-source.c
 src/modules/macosx/module-coreaudio-device.c
 src/modules/module-allow-passthrough.c
 src/modules/module-always-sink.c
+src/modules/module-always-source.c
 src/modules/module-cli.c
 src/modules/module-combine.c
 src/modules/module-console-kit.c

src/Makefile.am

@@ -1193,6 +1193,7 @@ modlibexec_LTLIBRARIES += \
 		module-card-restore.la \
 		module-default-device-restore.la \
 		module-always-sink.la \
+		module-always-source.la \
 		module-rescue-streams.la \
 		module-intended-roles.la \
 		module-suspend-on-idle.la \
@@ -1534,6 +1535,7 @@ SYMDEF_FILES = \
 		module-card-restore-symdef.h \
 		module-default-device-restore-symdef.h \
 		module-always-sink-symdef.h \
+		module-always-source-symdef.h \
 		module-rescue-streams-symdef.h \
 		module-intended-roles-symdef.h \
 		module-suspend-on-idle-symdef.h \
@@ -2021,6 +2023,12 @@ module_always_sink_la_LDFLAGS = $(MODULE_LDFLAGS)
 module_always_sink_la_LIBADD = $(MODULE_LIBADD)
 module_always_sink_la_CFLAGS = $(AM_CFLAGS)
 
+# Always Source module
+module_always_source_la_SOURCES = modules/module-always-source.c
+module_always_source_la_LDFLAGS = $(MODULE_LDFLAGS)
+module_always_source_la_LIBADD = $(MODULE_LIBADD)
+module_always_source_la_CFLAGS = $(AM_CFLAGS)
+
 # Rescue streams module
 module_rescue_streams_la_SOURCES = modules/module-rescue-streams.c
 module_rescue_streams_la_LDFLAGS = $(MODULE_LDFLAGS)

src/modules/module-always-source.c

@@ -0,0 +1,189 @@
+/***
+    This file is part of PulseAudio.
+
+    Copyright 2008 Colin Guthrie
+    Copyright 2017 Sebastian Dröge <sebastian@centricular.com>
+
+    PulseAudio is free software; you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License as published
+    by the Free Software Foundation; either version 2.1 of the License,
+    or (at your option) any later version.
+
+    PulseAudio 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 Lesser General Public License
+    along with PulseAudio; if not, see <http://www.gnu.org/licenses/>.
+***/
+
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <pulse/xmalloc.h>
+
+#include <pulsecore/core.h>
+#include <pulsecore/core-util.h>
+#include <pulsecore/i18n.h>
+#include <pulsecore/source.h>
+#include <pulsecore/modargs.h>
+#include <pulsecore/log.h>
+
+#include "module-always-source-symdef.h"
+
+PA_MODULE_AUTHOR("Sebastian Dröge");
+PA_MODULE_DESCRIPTION(_("Always keeps at least one source loaded even if it's a null one"));
+PA_MODULE_VERSION(PACKAGE_VERSION);
+PA_MODULE_LOAD_ONCE(true);
+PA_MODULE_USAGE(
+        "source_name=<name of source>");
+
+#define DEFAULT_SOURCE_NAME "auto_null"
+
+static const char* const valid_modargs[] = {
+    "source_name",
+    NULL,
+};
+
+struct userdata {
+    uint32_t null_module;
+    bool ignore;
+    char *source_name;
+};
+
+static void load_null_source_if_needed(pa_core *c, pa_source *source, struct userdata* u) {
+    pa_source *target;
+    uint32_t idx;
+    char *t;
+    pa_module *m;
+
+    pa_assert(c);
+    pa_assert(u);
+
+    if (u->null_module != PA_INVALID_INDEX)
+        return; /* We've already got a null-source loaded */
+
+    /* Loop through all sources and check to see if we have *any*
+     * sources. Ignore the source passed in (if it's not null), and
+     * don't count filter or monitor sources. */
+    PA_IDXSET_FOREACH(target, c->sources, idx)
+        if (!source || ((target != source) && !pa_source_is_filter(target) && target->monitor_of == NULL))
+            break;
+
+    if (target)
+        return;
+
+    pa_log_debug("Autoloading null-source as no other sources detected.");
+
+    u->ignore = true;
+
+    t = pa_sprintf_malloc("source_name=%s", u->source_name);
+    m = pa_module_load(c, "module-null-source", t);
+    u->null_module = m ? m->index : PA_INVALID_INDEX;
+    pa_xfree(t);
+
+    u->ignore = false;
+
+    if (!m)
+        pa_log_warn("Unable to load module-null-source");
+}
+
+static pa_hook_result_t put_hook_callback(pa_core *c, pa_source *source, void* userdata) {
+    struct userdata *u = userdata;
+
+    pa_assert(c);
+    pa_assert(source);
+    pa_assert(u);
+
+    /* This is us detecting ourselves on load... just ignore this. */
+    if (u->ignore)
+        return PA_HOOK_OK;
+
+    /* There's no point in doing anything if the core is shut down anyway */
+    if (c->state == PA_CORE_SHUTDOWN)
+        return PA_HOOK_OK;
+
+    /* Auto-loaded null-source not active, so ignoring newly detected source. */
+    if (u->null_module == PA_INVALID_INDEX)
+        return PA_HOOK_OK;
+
+    /* This is us detecting ourselves on load in a different way... just ignore this too. */
+    if (source->module && source->module->index == u->null_module)
+        return PA_HOOK_OK;
+
+    /* We don't count filter or monitor sources since they need a real source */
+    if (pa_source_is_filter(source) || source->monitor_of != NULL)
+        return PA_HOOK_OK;
+
+    pa_log_info("A new source has been discovered. Unloading null-source.");
+
+    pa_module_unload_request_by_index(c, u->null_module, true);
+    u->null_module = PA_INVALID_INDEX;
+
+    return PA_HOOK_OK;
+}
+
+static pa_hook_result_t unlink_hook_callback(pa_core *c, pa_source *source, void* userdata) {
+    struct userdata *u = userdata;
+
+    pa_assert(c);
+    pa_assert(source);
+    pa_assert(u);
+
+    /* First check to see if it's our own null-source that's been removed... */
+    if (u->null_module != PA_INVALID_INDEX && source->module && source->module->index == u->null_module) {
+        pa_log_debug("Autoloaded null-source removed");
+        u->null_module = PA_INVALID_INDEX;
+        return PA_HOOK_OK;
+    }
+
+    /* There's no point in doing anything if the core is shut down anyway */
+    if (c->state == PA_CORE_SHUTDOWN)
+        return PA_HOOK_OK;
+
+    load_null_source_if_needed(c, source, u);
+
+    return PA_HOOK_OK;
+}
+
+int pa__init(pa_module*m) {
+    pa_modargs *ma = NULL;
+    struct userdata *u;
+
+    pa_assert(m);
+
+    if (!(ma = pa_modargs_new(m->argument, valid_modargs))) {
+        pa_log("Failed to parse module arguments");
+        return -1;
+    }
+
+    m->userdata = u = pa_xnew(struct userdata, 1);
+    u->source_name = pa_xstrdup(pa_modargs_get_value(ma, "source_name", DEFAULT_SOURCE_NAME));
+    pa_module_hook_connect(m, &m->core->hooks[PA_CORE_HOOK_SOURCE_PUT], PA_HOOK_LATE, (pa_hook_cb_t) put_hook_callback, u);
+    pa_module_hook_connect(m, &m->core->hooks[PA_CORE_HOOK_SOURCE_UNLINK], PA_HOOK_EARLY, (pa_hook_cb_t) unlink_hook_callback, u);
+    u->null_module = PA_INVALID_INDEX;
+    u->ignore = false;
+
+    pa_modargs_free(ma);
+
+    load_null_source_if_needed(m->core, NULL, u);
+
+    return 0;
+}
+
+void pa__done(pa_module*m) {
+    struct userdata *u;
+
+    pa_assert(m);
+
+    if (!(u = m->userdata))
+        return;
+
+    if (u->null_module != PA_INVALID_INDEX && m->core->state != PA_CORE_SHUTDOWN)
+        pa_module_unload_request_by_index(m->core, u->null_module, true);
+
+    pa_xfree(u->source_name);
+    pa_xfree(u);
+}