ClipEffects: Build render Nodes at render time so plugins get the real sample rate

[drowaudio]

Summary

A PluginEffect clip effect prepared its hosted plugin at the wrong sample rate, so anything rate-dependent inside the plugin (delay lines, filters, oversamplers) was silently wrong in the rendered file.

AudioNodeRenderJob::initialise took an already-built tracktion::graph::Node, so every effect built its graph inside createRenderJob - before the render sample rate was known. Node building is now deferred to createAndPrepareRenderContext, which knows the rate.

Root cause

PluginNode's constructor calls initialisePlugin (sampleRateToUse, ...) → plugin->baseClassInitialise (...), so the plugin is initialised at construction time. The rates the clip effects were passing in were all wrong:

• PluginEffect and PitchShiftEffect passed job->processState.sampleRate, which is ProcessState's placeholder default of 44100.0 - the render's real rate is only set later, from the writer.
• VolumeEffect passed sourceFile.getInfo().sampleRate, which is 0 when the source is an earlier effect stage's output file that hasn't been rendered yet.

Plugin::baseClassInitialise only re-runs initialise() when the rate or block size changed, so the constructor's wrong rate could not be corrected by preparing the plugin again later. PluginNode::prepareToPlay then hits jassert (sampleRate == info.sampleRate) and carries on regardless.

The same "read the source file too early" problem affected the channel count: createWaveNodeForFile reads file.getInfo().numChannels, which is 0 (clamped to mono) for a not-yet-rendered chained source.

The change

• AudioNodeRenderJob::initialise now takes a NodeBuilder - std::function<std::unique_ptr<Node> (AudioNodeRenderJob&, double sampleRate)> - instead of a Node. The old Node-taking overload is gone, so the mistake can't recur.
• createAndPrepareRenderContext reads source.getInfo().sampleRate (valid by then - preceding jobs in the chain have run) and passes it to the builder.
• VolumeEffect, FadeInOutEffect, StepVolumeEffect, PitchShiftEffect and PluginEffect all build their Nodes from the builder. FadeInOutEffect and StepVolumeEffect capture their CachedValue properties by value up-front rather than capturing this.
• PluginNode::initialisePlugin gained jassert (sampleRateToUse > 0.0) / jassert (blockSizeToUse > 0) so a bad rate is caught where the contract is broken, not three layers down.

Regression test

ClipEffects: PluginEffect is prepared at the render sample rate in tracktion_ClipEffects.test.cpp. It registers a test-only SampleRateProbePlugin that records the PluginInitialisationInfo it was initialised with, renders a clip effect chain over a 96 kHz source (deliberately not 44100, so the ProcessState default can't accidentally pass), and checks:

• the plugin was initialised at 96000 Hz with a 512 block size
• the rendered proxy keeps the source's rate and channel count

Two subcases: the plugin effect on its own, and chained after a VolumeEffect (the case where the source file doesn't exist yet when the jobs are created).

Before the fix both subcases fail with 44100 == Approx( 96000 ); after, they pass.

Testing

TestRunner --no-juce-tests: 370/370 test cases, 21280 assertions, all passing in Release. ClipEffects, Plugins, ExternalPlugin, RackNode, RackInstance and EditNodeBuilder suites also run clean in Debug, so the new assertions don't fire anywhere else.

Fixes #417

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.75758% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.42%. Comparing base (aa6301a) to head (d3d89f0).

Files with missing lines	Patch %	Lines
..._engine/model/clips/tracktion_ClipEffects.test.cpp	94.89%	5 Missing ⚠️
...ktion_engine/model/clips/tracktion_ClipEffects.cpp	96.92%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #432      +/-   ##
===========================================
+ Coverage    59.81%   60.42%   +0.61%     
===========================================
  Files          568      568              
  Lines        79964    80082     +118     
  Branches     12381    12381              
===========================================
+ Hits         47828    48390     +562     
+ Misses       32136    31692     -444     

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