Worker: make blender-finder find blender.exe and not blender-launcher.exe

`blender-launcher.exe` doesn't pipe Blender's stdout/stderr output, so it's
not useful for Flamenco.

internal/worker/command_blender.go

@@ -15,6 +15,7 @@ import (
 	"github.com/google/shlex"
 	"github.com/rs/zerolog"
 
+	"git.blender.org/flamenco/internal/worker/find_blender"
 	"git.blender.org/flamenco/pkg/api"
 )
 
@@ -117,7 +118,7 @@ func (ce *CommandExecutor) cmdBlenderRenderCommand(
 			// Attempt a platform-specific way to find which Blender executable to
 			// use. If Blender cannot not be found, just use the configured command
 			// and let the OS produce the errors.
-			path, err := FindBlender()
+			path, err := find_blender.FindBlender()
 			if err == nil {
 				logger.Info().Str("path", path).Msg("found Blender")
 				parameters.exe = path

internal/worker/find_blender/nonwindows.go

@@ -1,7 +1,7 @@
 //go:build !windows
 
 // SPDX-License-Identifier: GPL-3.0-or-later
-package worker
+package find_blender
 
 import "errors"
 

internal/worker/find_blender/windows.go

@@ -1,16 +1,45 @@
 //go:build windows
 
 // SPDX-License-Identifier: GPL-3.0-or-later
-package worker
+package find_blender
 
 import (
 	"fmt"
+	"os"
+	"path/filepath"
 	"syscall"
 	"unsafe"
 )
 
-// FindBlender returns the path of `blender.exe` associated with ".blend" files.
+// FindBlender returns the full path of `blender.exe` associated with ".blend" files.
 func FindBlender() (string, error) {
+	exe, err := fileAssociation(".blend")
+	if err != nil {
+		return "", err
+	}
+
+	// Often the association will be with blender-launcher.exe, which is
+	// unsuitable for use in Flamenco. Use its path to find its `blender.exe`.
+	dir, file := filepath.Split(exe)
+	if file != "blender-launcher.exe" {
+		return exe, nil
+	}
+
+	blenderPath := filepath.Join(dir, "blender.exe")
+	_, err = os.Stat(blenderPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return "", fmt.Errorf("blender-launcher found at %s but not its blender.exe", exe)
+		}
+		return "", fmt.Errorf("investigating %s: %w", blenderPath, err)
+	}
+
+	return blenderPath, nil
+}
+
+// fileAssociation finds the executable associated with the given extension.
+// The extension must be a string like ".blend".
+func fileAssociation(extension string) (string, error) {
 	// Load library.
 	libname := "shlwapi.dll"
 	libshlwapi, err := syscall.LoadLibrary(libname)
@@ -27,7 +56,7 @@ func FindBlender() (string, error) {
 	}
 
 	// https://docs.microsoft.com/en-gb/windows/win32/api/shlwapi/nf-shlwapi-assocquerystringw
-	pszAssoc, err := syscall.UTF16PtrFromString(".blend")
+	pszAssoc, err := syscall.UTF16PtrFromString(extension)
 	if err != nil {
 		return "", fmt.Errorf("converting string to UTF16: %w", err)
 	}

internal/worker/find_blender/windows_test.go

@@ -1,7 +1,7 @@
 //go:build windows
 
 // SPDX-License-Identifier: GPL-3.0-or-later
-package worker
+package find_blender
 
 import (
 	"testing"
@@ -13,9 +13,12 @@ import (
 // It would be too fragile to always require a file association to be set up with Blender.
 func TestFindBlender(t *testing.T) {
 	exe, err := FindBlender()
-	if err == nil {
-		assert.NotEmpty(t, exe)
-	} else {
+	if err != nil {
 		assert.Empty(t, exe)
+		return
 	}
+	assert.NotEmpty(t, exe)
+	assert.NotContains(t, exe,
+		"blender-launcher",
+		"FindBlender should find blender.exe, not blender-launcher.exe")
 }