Worker: automatically find Blender on Windows via file association

On Windows, when the Manager doesn't explicitly point at a Blender to use,
use the the application associated with `.blend` files instead.

internal/worker/command_blender.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io"
 	"os/exec"
+	"path/filepath"
 
 	"github.com/google/shlex"
 	"github.com/rs/zerolog"
@@ -109,6 +110,21 @@ func (ce *CommandExecutor) cmdBlenderRenderCommand(
 		return nil, err
 	}
 
+	if filepath.Dir(parameters.exe) == "." {
+		// No directory path given. Check that the executable can be found on the
+		// path.
+		if _, err := exec.LookPath(parameters.exe); err != nil {
+			// 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()
+			if err == nil {
+				logger.Info().Str("path", path).Msg("found Blender")
+				parameters.exe = path
+			}
+		}
+	}
+
 	cliArgs := make([]string, 0)
 	cliArgs = append(cliArgs, parameters.argsBefore...)
 	cliArgs = append(cliArgs, parameters.blendfile)

internal/worker/command_blender_nonwindows.go

@@ -0,0 +1,11 @@
+//go:build !windows
+
+// SPDX-License-Identifier: GPL-3.0-or-later
+package worker
+
+import "errors"
+
+// FindBlender returns an error, as the file association lookup is only available on Windows.
+func FindBlender() (string, error) {
+	return "", errors.New("file association lookup is only available on Windows")
+}

internal/worker/command_blender_windows.go

@@ -0,0 +1,119 @@
+//go:build windows
+
+// SPDX-License-Identifier: GPL-3.0-or-later
+package worker
+
+import (
+	"fmt"
+	"syscall"
+	"unsafe"
+)
+
+// FindBlender returns the path of `blender.exe` associated with ".blend" files.
+func FindBlender() (string, error) {
+	// Load library.
+	libname := "shlwapi.dll"
+	libshlwapi, err := syscall.LoadLibrary(libname)
+	if err != nil {
+		return "", fmt.Errorf("loading %s: %w", libname, err)
+	}
+	defer func() { _ = syscall.FreeLibrary(libshlwapi) }()
+
+	// Find function.
+	funcname := "AssocQueryStringW"
+	assocQueryString, err := syscall.GetProcAddress(libshlwapi, funcname)
+	if err != nil {
+		return "", fmt.Errorf("finding function %s in %s: %w", funcname, libname, err)
+	}
+
+	// https://docs.microsoft.com/en-gb/windows/win32/api/shlwapi/nf-shlwapi-assocquerystringw
+	pszAssoc, err := syscall.UTF16PtrFromString(".blend")
+	if err != nil {
+		return "", fmt.Errorf("converting string to UTF16: %w", err)
+	}
+
+	pszExtra, err := syscall.UTF16PtrFromString("open")
+	if err != nil {
+		return "", fmt.Errorf("converting string to UTF16: %w", err)
+	}
+
+	var cchOut uint32 = 65535
+	buf := make([]uint16, cchOut)
+	pszOut := unsafe.Pointer(&buf[0])
+
+	result1, _, err := syscall.SyscallN(
+		assocQueryString,
+		uintptr(ASSOCF_INIT_DEFAULTTOSTAR), // [in]            ASSOCF   flags
+		uintptr(ASSOCSTR_EXECUTABLE),       // [in]            ASSOCSTR str
+		uintptr(unsafe.Pointer(pszAssoc)),  // [in]            LPCWSTR  pszAssoc
+		uintptr(unsafe.Pointer(pszExtra)),  // [in, optional]  LPCWSTR  pszExtra
+		uintptr(pszOut),                    // [out, optional] LPWSTR   pszOut
+		uintptr(unsafe.Pointer(&cchOut)),   // [in, out]       DWORD    *pcchOut
+	)
+	if err != nil {
+		// This can be a syscall.Errno with code 0, indicating things are actually fine.
+		if errno, ok := err.(syscall.Errno); ok && errno == 0 {
+			// So this is fine.
+		} else {
+			return "", fmt.Errorf("error calling AssocQueryStringW from shlwapi.dll: %w", err)
+		}
+	}
+	if result1 != 0 {
+		return "", fmt.Errorf("unknown result %d calling AssocQueryStringW from shlwapi.dll: %w", result1, err)
+	}
+
+	exe := syscall.UTF16ToString(buf)
+	return exe, nil
+}
+
+// Source: https://docs.microsoft.com/en-us/windows/win32/shell/assocf_str
+const (
+	ASSOCF_NONE                 = ASSOCF(0x00000000)
+	ASSOCF_INIT_NOREMAPCLSID    = ASSOCF(0x00000001)
+	ASSOCF_INIT_BYEXENAME       = ASSOCF(0x00000002)
+	ASSOCF_OPEN_BYEXENAME       = ASSOCF(0x00000002)
+	ASSOCF_INIT_DEFAULTTOSTAR   = ASSOCF(0x00000004)
+	ASSOCF_INIT_DEFAULTTOFOLDER = ASSOCF(0x00000008)
+	ASSOCF_NOUSERSETTINGS       = ASSOCF(0x00000010)
+	ASSOCF_NOTRUNCATE           = ASSOCF(0x00000020)
+	ASSOCF_VERIFY               = ASSOCF(0x00000040)
+	ASSOCF_REMAPRUNDLL          = ASSOCF(0x00000080)
+	ASSOCF_NOFIXUPS             = ASSOCF(0x00000100)
+	ASSOCF_IGNOREBASECLASS      = ASSOCF(0x00000200)
+	ASSOCF_INIT_IGNOREUNKNOWN   = ASSOCF(0x00000400)
+	ASSOCF_INIT_FIXED_PROGID    = ASSOCF(0x00000800)
+	ASSOCF_IS_PROTOCOL          = ASSOCF(0x00001000)
+	ASSOCF_INIT_FOR_FILE        = ASSOCF(0x00002000)
+)
+
+type ASSOCF uint32
+
+// Source: https://docs.microsoft.com/en-us/windows/win32/api/shlwapi/ne-shlwapi-assocstr
+const (
+	ASSOCSTR_COMMAND ASSOCSTR = iota + 1
+	ASSOCSTR_EXECUTABLE
+	ASSOCSTR_FRIENDLYDOCNAME
+	ASSOCSTR_FRIENDLYAPPNAME
+	ASSOCSTR_NOOPEN
+	ASSOCSTR_SHELLNEWVALUE
+	ASSOCSTR_DDECOMMAND
+	ASSOCSTR_DDEIFEXEC
+	ASSOCSTR_DDEAPPLICATION
+	ASSOCSTR_DDETOPIC
+	ASSOCSTR_INFOTIP
+	ASSOCSTR_QUICKTIP
+	ASSOCSTR_TILEINFO
+	ASSOCSTR_CONTENTTYPE
+	ASSOCSTR_DEFAULTICON
+	ASSOCSTR_SHELLEXTENSION
+	ASSOCSTR_DROPTARGET
+	ASSOCSTR_DELEGATEEXECUTE
+	ASSOCSTR_SUPPORTED_URI_PROTOCOLS
+	ASSOCSTR_PROGID
+	ASSOCSTR_APPID
+	ASSOCSTR_APPPUBLISHER
+	ASSOCSTR_APPICONREFERENCE
+	ASSOCSTR_MAX
+)
+
+type ASSOCSTR uint32

internal/worker/command_blender_windows_test.go

@@ -0,0 +1,21 @@
+//go:build windows
+
+// SPDX-License-Identifier: GPL-3.0-or-later
+package worker
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestFindBlender is a "weak" test, which actually accepts both happy and unhappy flows.
+// 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 {
+		assert.Empty(t, exe)
+	}
+}