commit c74311c5ee4dc722cded3cffcb748cd5fe9da86c
parent 9eb03ebdb77803f4855049bdd6254036417eb98d
Author: Jack Mordaunt <jackmordaunt.dev@gmail.com>
Date: Fri, 18 Sep 2026 16:09:39 -0400
ico: check the format against Windows itself
Windows reads a PNG icon only when it carries an alpha channel, which Go's
encoder drops from an image that is entirely opaque, so a 256 pixel icon was
invisible to every Windows decoder. Nothing short of asking Windows finds
that.
Diffstat:
2 files changed, 152 insertions(+), 1 deletion(-)
diff --git a/ico/oracle_windows_test.go b/ico/oracle_windows_test.go
@@ -0,0 +1,151 @@
+//go:build windows
+
+package ico
+
+// Windows decides what an ico file may contain, so these tests hand it files
+// this package wrote and read back a file it wrote itself. They run wherever
+// Windows PowerShell is present, which covers the Windows CI runner.
+
+import (
+ "image"
+ "image/color"
+ "os"
+ "os/exec"
+ "path/filepath"
+ "strconv"
+ "strings"
+ "testing"
+)
+
+// readFrames lists every frame the Windows Imaging Component finds, as
+// "width height r g b a". This is the decoder behind Explorer's preview.
+const readFrames = `
+param([string]$Path)
+Add-Type -AssemblyName PresentationCore
+$stream = [System.IO.File]::OpenRead($Path)
+$decoder = [System.Windows.Media.Imaging.BitmapDecoder]::Create($stream, 'None', 'OnLoad')
+foreach ($frame in $decoder.Frames) {
+ $converted = New-Object System.Windows.Media.Imaging.FormatConvertedBitmap($frame, [System.Windows.Media.PixelFormats]::Bgra32, $null, 0)
+ $pixel = New-Object byte[] 4
+ $at = New-Object System.Windows.Int32Rect ([int]($converted.PixelWidth/2)), ([int]($converted.PixelHeight/2)), 1, 1
+ $converted.CopyPixels($at, $pixel, 4, 0)
+ "{0} {1} {2} {3} {4} {5}" -f $converted.PixelWidth, $converted.PixelHeight, $pixel[2], $pixel[1], $pixel[0], $pixel[3]
+}
+$stream.Close()
+`
+
+// writeIcon has Windows write an ico file of one solid colour.
+const writeIcon = `
+param([string]$Path, [int]$Size, [int]$R, [int]$G, [int]$B)
+Add-Type -AssemblyName System.Drawing
+$bitmap = New-Object System.Drawing.Bitmap($Size, $Size, [System.Drawing.Imaging.PixelFormat]::Format32bppArgb)
+$graphics = [System.Drawing.Graphics]::FromImage($bitmap)
+$graphics.Clear([System.Drawing.Color]::FromArgb(255, $R, $G, $B))
+$graphics.Dispose()
+$icon = [System.Drawing.Icon]::FromHandle($bitmap.GetHicon())
+$stream = [System.IO.File]::Create($Path)
+$icon.Save($stream)
+$stream.Close()
+`
+
+// powershell runs a script and returns what it printed, failing the test
+// with whatever it wrote if it did not run.
+func powershell(t *testing.T, script string, args ...string) string {
+ t.Helper()
+ path := filepath.Join(t.TempDir(), "oracle.ps1")
+ if err := os.WriteFile(path, []byte(script), 0o600); err != nil {
+ t.Fatal(err)
+ }
+ args = append([]string{"-NoProfile", "-ExecutionPolicy", "Bypass", "-File", path}, args...)
+ out, err := exec.Command("powershell.exe", args...).CombinedOutput()
+ if err != nil {
+ t.Fatalf("powershell %v: %v\n%s", args, err, out)
+ }
+ return string(out)
+}
+
+// TestOracleWindowsReads checks that Windows finds every icon this package
+// writes, at the size and colour it was given.
+func TestOracleWindowsReads(t *testing.T) {
+ var (
+ images = map[uint]image.Image{}
+ want = map[int]color.NRGBA{}
+ )
+ for i, size := range Sizes() {
+ c := color.NRGBA{R: uint8(20 + i*30), G: uint8(200 - i*20), B: 0x40, A: 0xFF}
+ images[size] = solid(int(size), c)
+ want[int(size)] = c
+ }
+ path := filepath.Join(t.TempDir(), "icon.ico")
+ f, err := os.Create(path)
+ if err != nil {
+ t.Fatal(err)
+ }
+ if err := NewEncoder(f).EncodeSizes(images); err != nil {
+ f.Close()
+ t.Fatal(err)
+ }
+ if err := f.Close(); err != nil {
+ t.Fatal(err)
+ }
+
+ out := powershell(t, readFrames, "-Path", path)
+ seen := map[int]bool{}
+ for _, line := range strings.Split(out, "\n") {
+ fields := strings.Fields(line)
+ if len(fields) != 6 {
+ continue
+ }
+ n := make([]int, 6)
+ for i, field := range fields {
+ if n[i], err = strconv.Atoi(field); err != nil {
+ t.Fatalf("the oracle printed %q", line)
+ }
+ }
+ width, height := n[0], n[1]
+ if width != height {
+ t.Errorf("Windows read a %dx%d icon, which is not square", width, height)
+ }
+ got := color.NRGBA{R: uint8(n[2]), G: uint8(n[3]), B: uint8(n[4]), A: uint8(n[5])}
+ if got != want[width] {
+ t.Errorf("Windows read the %d pixel icon as %v, want %v", width, got, want[width])
+ }
+ seen[width] = true
+ }
+ for size := range want {
+ if !seen[size] {
+ t.Errorf("Windows did not read the %d pixel icon at all:\n%s", size, out)
+ }
+ }
+}
+
+// TestOracleWindowsWrites decodes an icon Windows wrote. It writes them
+// indexed, so this covers the colour table against a real writer.
+func TestOracleWindowsWrites(t *testing.T) {
+ const size = 48
+ // Red is in the palette Windows picks, so the colour survives exactly.
+ want := color.NRGBA{R: 0xFF, A: 0xFF}
+ path := filepath.Join(t.TempDir(), "windows.ico")
+ powershell(t, writeIcon,
+ "-Path", path,
+ "-Size", strconv.Itoa(size),
+ "-R", strconv.Itoa(int(want.R)),
+ "-G", strconv.Itoa(int(want.G)),
+ "-B", strconv.Itoa(int(want.B)),
+ )
+ f, err := os.Open(path)
+ if err != nil {
+ t.Fatal(err)
+ }
+ defer f.Close()
+ img, err := Decode(f)
+ if err != nil {
+ t.Fatal(err)
+ }
+ if got := img.Bounds().Dx(); got != size {
+ t.Errorf("decoded a %d pixel icon, want %d", got, size)
+ }
+ if got := centre(img); got != want {
+ t.Errorf("centre = %v, want %v", got, want)
+ }
+}
diff --git a/readme.md b/readme.md
@@ -200,7 +200,7 @@ go test ./... ./cmd/preview/... ./cmd/shell-extension/...
The two command modules require the library at a published tag. Inside the workspace that requirement only shapes the module graph; the code always comes from the working tree, so the tag can lag behind without affecting development.
-CI builds and tests the library on Linux, macOS and Windows, runs the encoder under the race detector, fuzzes the decoder, and reports `gofmt`, `go vet` and `govulncheck`. The shell extension is tested on Windows, where cgo can reach a C compiler, and `preview` is built on macOS and Windows, since Gio needs a long list of X11 and Wayland headers on Linux. Both command modules are also built with `GOWORK=off`, which is what `go install` sees.
+CI builds and tests the library on Linux, macOS and Windows, runs the encoder under the race detector, fuzzes the decoder, and reports `gofmt`, `go vet` and `govulncheck`. Two oracles check the formats against the systems that own them: `iconutil` on the macOS runner, and the Windows Imaging Component and `System.Drawing` on the Windows one. The shell extension is tested on Windows, where cgo can reach a C compiler, and `preview` is built on macOS and Windows, since Gio needs a long list of X11 and Wayland headers on Linux. Both command modules are also built with `GOWORK=off`, which is what `go install` sees.
Releasing: tag the root module (`vX.Y.Z`), which releases the library and `icnsify` together. When `go install .../cmd/preview@latest`, or a shell extension built outside the checkout, should pick up a newer library, bump that module's requirement and tidy it outside the workspace:
