Consider palette size when converting and in getpalette()

Tests/test_image_putpalette.py

@@ -74,4 +74,5 @@ def test_putpalette_with_alpha_values():
 def test_rgba_palette(mode, palette):
     im = Image.new("P", (1, 1))
     im.putpalette(palette, mode)
+    assert im.getpalette() == [1, 2, 3]
     assert im.palette.colors == {(1, 2, 3, 4): 0}

Tests/test_image_quantize.py

@@ -108,3 +108,18 @@ def test_palette(method, color):
     converted = im.quantize(method=method)
     converted_px = converted.load()
     assert converted_px[0, 0] == converted.palette.colors[color]
+
+
+def test_small_palette():
+    # Arrange
+    im = hopper()
+
+    colors = (255, 0, 0, 0, 0, 255)
+    p = Image.new("P", (1, 1))
+    p.putpalette(colors)
+
+    # Act
+    im = im.quantize(palette=p)
+
+    # Assert
+    assert len(im.getcolors()) == 2

src/PIL/Image.py

@@ -821,7 +821,7 @@ def load(self):
         if self.im and self.palette and self.palette.dirty:
             # realize palette
             mode, arr = self.palette.getdata()
-            palette_length = self.im.putpalette(mode, arr)
+            self.im.putpalette(mode, arr)
             self.palette.dirty = 0
             self.palette.rawmode = None
             if "transparency" in self.info and mode in ("LA", "PA"):
@@ -833,9 +833,7 @@ def load(self):
             else:
                 palette_mode = "RGBA" if mode.startswith("RGBA") else "RGB"
                 self.palette.mode = palette_mode
-                self.palette.palette = self.im.getpalette(palette_mode, palette_mode)[
-                    : palette_length * len(palette_mode)
-                ]
+                self.palette.palette = self.im.getpalette(palette_mode, palette_mode)
 
         if self.im:
             if cffi and USE_CFFI_ACCESS:

src/_imaging.c

@@ -1063,7 +1063,7 @@ _gaussian_blur(ImagingObject *self, PyObject *args) {
 static PyObject *
 _getpalette(ImagingObject *self, PyObject *args) {
     PyObject *palette;
-    int palettesize = 256;
+    int palettesize;
     int bits;
     ImagingShuffler pack;
 
@@ -1084,6 +1084,7 @@ _getpalette(ImagingObject *self, PyObject *args) {
         return NULL;
     }
 
+    palettesize = self->image->palette->size;
     palette = PyBytes_FromStringAndSize(NULL, palettesize * bits / 8);
     if (!palette) {
         return NULL;
@@ -1672,9 +1673,11 @@ _putpalette(ImagingObject *self, PyObject *args) {
 
     self->image->palette = ImagingPaletteNew(palette_mode);
 
-    unpack(self->image->palette->palette, palette, palettesize * 8 / bits);
+    self->image->palette->size = palettesize * 8 / bits;
+    unpack(self->image->palette->palette, palette, self->image->palette->size);
 
-    return PyLong_FromLong(palettesize * 8 / bits);
+    Py_INCREF(Py_None);
+    return Py_None;
 }
 
 static PyObject *

src/libImaging/Imaging.h

@@ -143,6 +143,7 @@ struct ImagingPaletteInstance {
     char mode[IMAGING_MODE_LENGTH]; /* Band names */
 
     /* Data */
+    int size;
     UINT8 palette[1024]; /* Palette data (same format as image data) */
 
     INT16 *cache;   /* Palette cache (used for predefined palettes) */

src/libImaging/Palette.c

@@ -40,6 +40,7 @@ ImagingPaletteNew(const char *mode) {
     palette->mode[IMAGING_MODE_LENGTH - 1] = 0;
 
     /* Initialize to ramp */
+    palette->size = 256;
     for (i = 0; i < 256; i++) {
         palette->palette[i * 4 + 0] = palette->palette[i * 4 + 1] =
             palette->palette[i * 4 + 2] = (UINT8)i;
@@ -193,7 +194,7 @@ ImagingPaletteCacheUpdate(ImagingPalette palette, int r, int g, int b) {
 
     dmax = (unsigned int)~0;
 
-    for (i = 0; i < 256; i++) {
+    for (i = 0; i < palette->size; i++) {
         int r, g, b;
         unsigned int tmin, tmax;
 
@@ -226,7 +227,7 @@ ImagingPaletteCacheUpdate(ImagingPalette palette, int r, int g, int b) {
         d[i] = (unsigned int)~0;
     }
 
-    for (i = 0; i < 256; i++) {
+    for (i = 0; i < palette->size; i++) {
         if (dmin[i] <= dmax) {
             int rd, gd, bd;
             int ri, gi, bi;