screencastService: Add fallback mechanism for gstreamer pipelines

Introduce a fallback mechanism for gstreamer pipelines that allows to
define multiple pipelines and prefer them over each other.

The way this works is that we introduce a new STARTING PipelineState.
While the Recorder is in that state, it is allowed to tear down the
current pipeline and start another one whenever an error happens, this is
used to try multiple pipelines in a fixed order until a working one is
found.

Right now there's just a single pipeline using the existing vp8 encoder, the
actual new encoders and pipelines will be added in a separate commit.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2609>

js/dbusServices/screencast/screencastService.js

@@ -24,12 +24,24 @@ const ScreenCastProxy = Gio.DBusProxy.makeProxyWrapper(ScreenCastIface);
 const ScreenCastSessionProxy = Gio.DBusProxy.makeProxyWrapper(ScreenCastSessionIface);
 const ScreenCastStreamProxy = Gio.DBusProxy.makeProxyWrapper(ScreenCastStreamIface);
 
-const DEFAULT_PIPELINE = 'videoconvert chroma-mode=GST_VIDEO_CHROMA_MODE_NONE dither=GST_VIDEO_DITHER_NONE matrix-mode=GST_VIDEO_MATRIX_MODE_OUTPUT_ONLY n-threads=%T ! queue ! vp8enc cpu-used=16 max-quantizer=17 deadline=1 keyframe-mode=disabled threads=%T static-threshold=1000 buffer-size=20000 ! queue ! webmmux';
 const DEFAULT_FRAMERATE = 30;
 const DEFAULT_DRAW_CURSOR = true;
 
+const PIPELINES = [
+    {
+        pipelineString:
+            'capsfilter caps=video/x-raw,max-framerate=%F/1 ! \
+             videoconvert chroma-mode=none dither=none matrix-mode=output-only n-threads=%T ! \
+             queue ! \
+             vp8enc cpu-used=16 max-quantizer=17 deadline=1 keyframe-mode=disabled threads=%T static-threshold=1000 buffer-size=20000 ! \
+             queue ! \
+             webmmux',
+    },
+];
+
 const PipelineState = {
     INIT: 'INIT',
+    STARTING: 'STARTING',
     PLAYING: 'PLAYING',
     FLUSHING: 'FLUSHING',
     STOPPED: 'STOPPED',
@@ -65,7 +77,6 @@ var Recorder = class {
                 throw e;
         }
 
-        this._pipelineString = DEFAULT_PIPELINE;
         this._framerate = DEFAULT_FRAMERATE;
         this._drawCursor = DEFAULT_DRAW_CURSOR;
 
@@ -176,9 +187,26 @@ var Recorder = class {
         this._sessionProxy.connectSignal('Closed', this._onSessionClosed.bind(this));
     }
 
-    _startPipeline(nodeId) {
+    _tryNextPipeline() {
-        if (!this._ensurePipeline(nodeId))
+        const {done, value: pipelineConfig} = this._pipelineConfigs.next();
+        if (done) {
+            this._teardownPipeline();
+            this._handleFatalPipelineError('All pipelines failed to start');
             return;
+        }
+
+        try {
+            this._pipeline = this._createPipeline(this._nodeId, pipelineConfig,
+                this._framerate);
+        } catch (error) {
+            this._tryNextPipeline();
+            return;
+        }
+
+        if (!this._pipeline) {
+            this._tryNextPipeline();
+            return;
+        }
 
         const bus = this._pipeline.get_bus();
         bus.add_watch(bus, this._onBusMessage.bind(this));
@@ -189,7 +217,8 @@ var Recorder = class {
             retval === Gst.StateChangeReturn.ASYNC) {
             // We'll wait for the state change message to PLAYING on the bus
         } else {
-            this._handleFatalPipelineError('Failed to start pipeline');
+            this._teardownPipeline();
+            this._tryNextPipeline();
         }
     }
 
@@ -212,7 +241,11 @@ var Recorder = class {
             this._streamProxy.connectSignal('PipeWireStreamAdded',
                 (_proxy, _sender, params) => {
                     const [nodeId] = params;
-                    this._startPipeline(nodeId);
+                    this._nodeId = nodeId;
+
+                    this._pipelineState = PipelineState.STARTING;
+                    this._pipelineConfigs = PIPELINES.values();
+                    this._tryNextPipeline();
                 });
             this._sessionProxy.StartSync();
             this._sessionState = SessionState.ACTIVE;
@@ -236,7 +269,7 @@ var Recorder = class {
         case Gst.MessageType.STATE_CHANGED: {
             const [, newState] = message.parse_state_changed();
 
-            if (this._pipelineState === PipelineState.INIT &&
+            if (this._pipelineState === PipelineState.STARTING &&
                 message.src === this._pipeline &&
                 newState === Gst.State.PLAYING) {
                 this._pipelineState = PipelineState.PLAYING;
@@ -250,21 +283,22 @@ var Recorder = class {
 
         case Gst.MessageType.EOS:
             switch (this._pipelineState) {
+            case PipelineState.INIT:
             case PipelineState.STOPPED:
             case PipelineState.ERROR:
                 // In these cases there should be no pipeline, so should never happen
                 break;
 
+            case PipelineState.STARTING:
+                // This is something we can handle, try to switch to the next pipeline
+                this._tryNextPipeline();
+                break;
+
             case PipelineState.PLAYING:
                 this._addRecentItem();
                 this._handleFatalPipelineError('Unexpected EOS message');
                 break;
 
-            case PipelineState.INIT:
-                this._handleFatalPipelineError(
-                    'Unexpected EOS message while in state INIT');
-                break;
-
             case PipelineState.FLUSHING:
                 this._addRecentItem();
 
@@ -283,12 +317,17 @@ var Recorder = class {
 
         case Gst.MessageType.ERROR:
             switch (this._pipelineState) {
+            case PipelineState.INIT:
             case PipelineState.STOPPED:
             case PipelineState.ERROR:
                 // In these cases there should be no pipeline, so should never happen
                 break;
 
-            case PipelineState.INIT:
+            case PipelineState.STARTING:
+                // This is something we can handle, try to switch to the next pipeline
+                this._tryNextPipeline();
+                break;
+
             case PipelineState.PLAYING:
             case PipelineState.FLUSHING:
                 // Everything else we can't handle, so error out
@@ -308,45 +347,53 @@ var Recorder = class {
         return true;
     }
 
-    _substituteThreadCount(pipelineDescr) {
+    _substituteVariables(pipelineDescr, framerate) {
         const numProcessors = GLib.get_num_processors();
         const numThreads = Math.min(Math.max(1, numProcessors), 64);
-        return pipelineDescr.replaceAll('%T', numThreads);
+        return pipelineDescr.replaceAll('%T', numThreads).replaceAll('%F', framerate);
     }
 
-    _ensurePipeline(nodeId) {
+    _createPipeline(nodeId, pipelineConfig, framerate) {
-        const framerate = this._framerate;
+        const {pipelineString} = pipelineConfig;
+        const finalPipelineString = this._substituteVariables(pipelineString, framerate);
 
-        let fullPipeline = `
+        const fullPipeline = `
             pipewiresrc path=${nodeId}
                         do-timestamp=true
                         keepalive-time=1000
                         resend-last=true !
-            video/x-raw,max-framerate=${framerate}/1 !
+            ${finalPipelineString} !
-            ${this._pipelineString} !
             filesink location="${this._filePath}"`;
-        fullPipeline = this._substituteThreadCount(fullPipeline);
 
-        try {
+        return Gst.parse_launch_full(fullPipeline, null,
-            this._pipeline = Gst.parse_launch_full(fullPipeline,
+            Gst.ParseFlags.FATAL_ERRORS);
-                null,
-                Gst.ParseFlags.FATAL_ERRORS);
-        } catch (e) {
-            this._handleFatalPipelineError(`Failed to create pipeline: ${e.message}`);
-        }
-        return !!this._pipeline;
     }
 };
 
 var ScreencastService = class extends ServiceImplementation {
     static canScreencast() {
-        const elements = [
+        if (!Gst.init_check(null))
+            return false;
+
+        let elements = [
             'pipewiresrc',
             'filesink',
-            ...DEFAULT_PIPELINE.split('!').map(e => e.trim().split(' ').at(0)),
         ];
-        return Gst.init_check(null) &&
+
-            elements.every(e => Gst.ElementFactory.find(e) != null);
+        if (elements.some(e => Gst.ElementFactory.find(e) === null))
+            return false;
+
+        // The fallback pipeline must be available, the other ones are not
+        // guaranteed to work because they depend on hw encoders.
+        const fallbackPipeline = PIPELINES.at(-1);
+
+        elements = fallbackPipeline.pipelineString.split('!').map(
+            e => e.trim().split(' ').at(0));
+
+        if (elements.every(e => Gst.ElementFactory.find(e) !== null))
+            return true;
+
+        return false;
     }
 
     constructor() {