Blender finder: fix compatibility with Windows Home

For some reason, calling `AssocQueryStringW` on Windows Home returns error
code 122, "The data area passed to a system call is too small", even when
the data area is large enough. Furthermore, the API actually describes that
in such cases `S_FALSE` is supposed to be returned, with `*pcchOut` set to
the required size. Because of this apparent violation of the documentation,
and because it just works, Flamenco now ignores this particular error and
just returns the obtained string.

internal/find_blender/find_blender.go

@@ -18,9 +18,10 @@ import (
 )
 
 var (
-	ErrNotAvailable = errors.New("not available on this platform")
-	ErrNotBlender   = errors.New("not a Blender executable")
-	ErrTimedOut     = errors.New("version check took too long")
+	ErrNotAvailable        = errors.New("not available on this platform")
+	ErrAssociationNotFound = errors.New("no program is associated with .blend files")
+	ErrNotBlender          = errors.New("not a Blender executable")
+	ErrTimedOut            = errors.New("version check took too long")
 )
 
 // blenderVersionTimeout is how long `blender --version` is allowed to take,

internal/find_blender/windows.go

@@ -71,24 +71,44 @@ func getFileAssociation(extension string) (string, error) {
 		return "", fmt.Errorf("converting string to UTF16: %w", err)
 	}
 
-	var cchOut uint32 = 65535
-	buf := make([]uint16, cchOut)
-	pszOut := unsafe.Pointer(&buf[0])
-
-	result1, _, errno := 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
+	var (
+		bufferSize uint32 = 1024
+		result1    uintptr
+		errno      syscall.Errno
 	)
-	if errno != 0 {
-		return "", fmt.Errorf("error calling AssocQueryStringW from shlwapi.dll: %w", errno)
+	buf := make([]uint16, bufferSize)
+
+	pszOut := unsafe.Pointer(&buf[0])
+	result1, _, errno = syscall.SyscallN(
+		assocQueryString,
+		uintptr(ASSOCF_NONE),                 // [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(&bufferSize)), // [in, out]       DWORD    *pcchOut
+	)
+	// Somehow Windows Home returns an error 122 "The data area passed to a system
+	// call is too small" even when the data area is big enough. So, for the sake
+	// of sanity of this software developer, let's just ignore that particular
+	// error.
+	if errno != 0 && errno != 122 {
+		exe := syscall.UTF16ToString(buf)
+		return "", fmt.Errorf("error calling %s from %s: %d; %w (with bufsize=%v and pszOut=%q)",
+			funcname, libname, errno, errno, bufferSize, exe)
 	}
-	if result1 != 0 {
-		return "", fmt.Errorf("unknown result %d calling AssocQueryStringW from shlwapi.dll: %w", result1, err)
+	switch result1 {
+	case S_OK:
+		// Continue below with the happy flow.
+	case ERROR_NO_ASSOCIATION:
+		// No association with .blend files exists, and that's fine.
+		return "", ErrAssociationNotFound
+	case S_FALSE:
+		return "", fmt.Errorf("error calling %s from %s: buffer too small, should be %v", funcname, libname, bufferSize)
+	case E_POINTER:
+		return "", fmt.Errorf("error calling %s from %s: invalid pointer", funcname, libname)
+	default:
+		return "", fmt.Errorf("unknown result %#x calling %s from %s", result1, funcname, libname)
 	}
 
 	exe := syscall.UTF16ToString(buf)
@@ -146,3 +166,12 @@ const (
 )
 
 type ASSOCSTR uint32
+
+// Source: https://docs.microsoft.com/en-us/windows/win32/seccrypto/common-hresult-values
+// and some other random Duck Duck Go searches.
+const (
+	S_OK                 uintptr = 0x00000000
+	E_POINTER            uintptr = 0x80004003
+	S_FALSE              uintptr = 0x00000001
+	ERROR_NO_ASSOCIATION uintptr = 0x80070483
+)