Boot - set env vars for pipewire (matching pw-jack's behaviour) (#3349)

pw-jack sets the PIPEWIRE_QUANTUM and LD_LIBRARY_PATH env vars before execing a binary. To use this we'd need to run sonic pi from pw-jack (so the exec would do its work correctly). Instead of that we set the env vars within the call to popen2e when starting scsynth to match this behaviour.

It is now also possible to use new change pipewire's buffsize and samplerate with two new audio-settings.toml config options:

```
linux_pipewire_buffsize = 1024
linux_pipewire_samplerate = 4800
```

app/config/user-examples/audio-settings.toml

@@ -121,6 +121,17 @@
 # num_random_seeds = 64
 
 
+## ===============================
+## Linux Audio - Pipewire settings
+## ===============================
+#
+# Sonic Pi uses pipewire for Audio on Linux
+# You can modify the pipewire parameters here:
+
+# linux_pipewire_buffsize = 1024
+# linux_pipewire_samplerate = 48000
+
+
 
 ## ============
 ## Escape hatch

app/server/ruby/bin/daemon.rb

@@ -744,7 +744,8 @@ def idempotent_exit_cleanup
 
     class ProcessBooter
       attr_reader :pid, :args, :cmd, :log
-      def initialize(cmd, args, log_path, record_log=false)
+      def initialize(cmd, args, log_path, record_log=false, env=nil)
+        @env = env
         @pid = nil
         @log_file = nil
         @args = args.map {|el| el.to_s}
@@ -786,7 +787,11 @@ def disable_internal_log_recording!
       def boot
         Util.log "Process Booter - booting #{@cmd} with args #{@args.inspect}"
         Util.log "#{@cmd} #{@args.join(' ')}"
-        @stdin, @stdout_and_err, @wait_thr = Open3.popen2e @cmd, *@args
+        if @env
+          @stdin, @stdout_and_err, @wait_thr = Open3.popen2e @env, @cmd, *@args
+        else
+          @stdin, @stdout_and_err, @wait_thr = Open3.popen2e @cmd, *@args
+        end
         @pid = @wait_thr.pid
         if @log_file
           @io_thr = Thread.new do
@@ -1030,7 +1035,6 @@ def kill
       end
     end
 
-
     class JackBooter < ProcessBooter
       def initialize
         cmd = "jackd"
@@ -1134,6 +1138,10 @@ def initialize(ports, no_scsynth_inputs=false)
           toml_opts_hash = {}
         end
 
+        # freeze toml_opts_hash in case any nasty mutation happens below
+        # (oh for immutable data structures by default!)
+        toml_opts_hash.freeze
+
         Util.log "Got Audio Settings toml hash: #{toml_opts_hash.inspect}"
         opts = unify_toml_opts_hash(toml_opts_hash)
         Util.log "Unified Audio Settings toml hash: #{opts.inspect}"
@@ -1145,9 +1153,40 @@ def initialize(ports, no_scsynth_inputs=false)
         @num_outputs = opts["-o"].to_i
         args = opts.to_a.flatten
         cmd = Paths.scsynth_path
+
+        case Util.os
+        when :linux, :raspberry
+          toml_pw_buffsize = toml_opts_hash[:linux_pipewire_buffsize].to_i
+          toml_pw_samplerate = toml_opts_hash[:linux_pipewire_samplerate].to_i
+          pw_buffsize = 1024
+          pw_samplerate = 48000
+
+          if (toml_opts_hash.has_key?(:linux_pipewire_buffsize) && (toml_pw_buffsize > 0))
+            Util.log "Setting pipewire buffsize to: #{toml_pw_buffsize}"
+            pw_buffsize = toml_pw_buffsize
+          else
+            Util.log "Using default pipewire buffsize of: 1024"
+          end
+
+          if (toml_opts_hash.has_key?(:linux_pipewire_samplerate) && (toml_pw_samplerate > 0))
+            Util.log "Setting pipewire samplerate to: #{toml_pw_samplerate}"
+            pw_samplerate = toml_pw_samplerate
+          else
+            Util.log "Using default pipewire samplerate of: 48000"
+          end
+
+          ld_library_path = `pw-jack /bin/sh -c 'echo $LD_LIBRARY_PATH'`.strip
+          pw_quantum ="#{pw_buffsize}/#{pw_samplerate}"
+
+          Util.log "Starting scsynth with LD_LIBRARY_PATH set to #{ld_library_path.inspect} so it uses pipewire's jack"
+          env = { "PIPEWIRE_QUANTUM" => pw_quantum ,  "LD_LIBRARY_PATH" => ld_library_path }
+        else
+          env = nil
+        end
+
         @success = Promise.new
         run_pre_start_commands
-        super(cmd, args, Paths.scsynth_log_path, true)
+        super(cmd, args, Paths.scsynth_log_path, true, env)
         run_post_start_commands
         success = wait_for_boot
         disable_internal_log_recording!
@@ -1234,7 +1273,7 @@ def run_pre_start_commands
           end
         end
       end
-            
+
       def run_post_start_commands
         case Util.os
         when :raspberry, :linux