Drop the fit=cover test cases that fill memory without the fix (part of #282)

Without the fix, the 8192x64 and 64x8192 cases each made libvips ask for
about 24 GiB, so a return of the bug would take the build host's memory
instead of failing the test. The cases left fail at once without the fix.

Model: opus-5-5

TODO.md

@@ -30,6 +30,16 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 a thin source requested with `fit=cover` no longer makes libvips
+  build a huge image (closes #282): libvips scaled the whole source to cover the
+  requested size and copied that into memory before it cut out the centre, so an
+  8192x64 source at 8192x8192 made it allocate about 24 GiB, and an 8192x1
+  source answered 500. The image processor now cuts the centre of the source to
+  the requested size's proportions first and resizes that, so no image larger
+  than the source or the output is made; the crop is the same centre as before.
+  With the runtime image's libvips and JPEG output at 8192x8192, resident memory
+  peaked at about 44 MiB for the 8192x64 source, 40 MiB for the 8192x1 one, and
+  117 MiB for a 4000x3000 one, which took 373 MiB before.
 - 2026-10-15 the README says how a source URL with a query is written into an
   image URL, and the image route accepts it (closes #255): the query follows the
   source path after `%3F`. pixa now reads the image URL's path decoded once, so

internal/imageprocessor/fit_cover_internal_test.go

@@ -0,0 +1,141 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"image"
+	"image/color"
+	"image/png"
+	"testing"
+)
+
+// TestImageProcessor_FitCoverServesThinSource verifies that FitCover serves
+// a thin source at the requested size. An 8192x1 source scaled to cover
+// 8192x8192 is 8192 times as long as the output on its long side. The centre
+// of an 8192x1 source with the proportions of 1x8192 is 1/8192 of a pixel
+// wide. Each case is also turned on its side.
+func TestImageProcessor_FitCoverServesThinSource(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                    string
+		inputWidth, inputHeight int
+		size                    Size
+	}{
+		{"8192x1 at 8192x8192", 8192, 1, Size{Width: 8192, Height: 8192}},
+		{"8192x1 at 1x8192", 8192, 1, Size{Width: 1, Height: 8192}},
+		{"1x8192 at 8192x8192", 1, 8192, Size{Width: 8192, Height: 8192}},
+		{"1x8192 at 8192x1", 1, 8192, Size{Width: 8192, Height: 1}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			input := createTestJPEG(t, tt.inputWidth, tt.inputHeight)
+
+			result, err := New(Params{}).Process(t.Context(),
+				bytes.NewReader(input),
+				&Request{Size: tt.size, Format: FormatJPEG, FitMode: FitCover})
+			if err != nil {
+				t.Fatalf("Process() error = %v", err)
+			}
+
+			defer func() { _ = result.Content.Close() }()
+
+			t.Logf("Process() made %dx%d", result.Width, result.Height)
+
+			if result.Width != tt.size.Width || result.Height != tt.size.Height {
+				t.Errorf("Process() made %dx%d, want %dx%d",
+					result.Width, result.Height, tt.size.Width, tt.size.Height)
+			}
+		})
+	}
+}
+
+// createBandsPNG creates a PNG of three equal bands, red, green and blue,
+// side by side when it is wider than tall and top to bottom otherwise.
+func createBandsPNG(t *testing.T, width, height int) []byte {
+	t.Helper()
+
+	bands := []color.RGBA{
+		{R: 255, A: 255},
+		{G: 255, A: 255},
+		{B: 255, A: 255},
+	}
+
+	img := image.NewRGBA(image.Rect(0, 0, width, height))
+	for y := range height {
+		for x := range width {
+			band := 3 * y / height
+			if width > height {
+				band = 3 * x / width
+			}
+
+			img.Set(x, y, bands[band])
+		}
+	}
+
+	return encodePNG(t, img)
+}
+
+// TestImageProcessor_FitCoverCropsTheCentre verifies that FitCover keeps the
+// centre of the source: a 300x100 source of red, green and blue bands side
+// by side, or a 100x300 one with them top to bottom, comes out at 50x50 as
+// its green band.
+func TestImageProcessor_FitCoverCropsTheCentre(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                    string
+		inputWidth, inputHeight int
+	}{
+		{"bands side by side", 300, 100},
+		{"bands top to bottom", 100, 300},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			input := createBandsPNG(t, tt.inputWidth, tt.inputHeight)
+
+			result, err := New(Params{}).Process(t.Context(),
+				bytes.NewReader(input),
+				&Request{
+					Size:    Size{Width: 50, Height: 50},
+					Format:  FormatPNG,
+					FitMode: FitCover,
+				})
+			if err != nil {
+				t.Fatalf("Process() error = %v", err)
+			}
+
+			defer func() { _ = result.Content.Close() }()
+
+			output, err := png.Decode(result.Content)
+			if err != nil {
+				t.Fatalf("failed to decode output: %v", err)
+			}
+
+			bounds := output.Bounds()
+			t.Logf("Process() made %dx%d", bounds.Dx(), bounds.Dy())
+
+			if bounds.Dx() != 50 || bounds.Dy() != 50 {
+				t.Fatalf("Process() made %dx%d, want 50x50",
+					bounds.Dx(), bounds.Dy())
+			}
+
+			// Resampling may blend a little of the bands beside the
+			// green one into the output's edges.
+			for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+				for x := bounds.Min.X; x < bounds.Max.X; x++ {
+					r, g, b, _ := output.At(x, y).RGBA()
+					if r>>8 > 64 || g>>8 < 192 || b>>8 > 64 {
+						t.Fatalf("output pixel %d,%d is %d,%d,%d, want green",
+							x, y, r>>8, g>>8, b>>8)
+					}
+				}
+			}
+		})
+	}
+}

internal/imageprocessor/imageprocessor.go

@@ -475,8 +475,8 @@ func (p *ImageProcessor) resize(
 ) error {
 	switch fit {
 	case FitCover, "":
-		// Resize and crop to fill exact dimensions (default)
-		return img.Thumbnail(width, height, vips.InterestingCentre)
+		// Crop the centre and resize to fill exact dimensions (default)
+		return cropCentreAndResize(img, width, height)
 
 	case FitContain:
 		// Resize to fit within dimensions, maintaining aspect ratio
@@ -530,6 +530,37 @@ func (p *ImageProcessor) resize(
 	}
 }
 
+// cropCentreAndResize cuts img to its centre, the largest area it holds with
+// the proportions of width x height, and resizes that to width x height.
+// This is the crop libvips' thumbnail makes with InterestingCentre, but
+// libvips scales the whole image to cover the size and copies the result
+// into memory before it crops, and for a thin image that is far larger than
+// the image or the output: 8192x64 at 8192x8192 is 1048576x8192, about 24
+// GiB. Cropping first makes no image larger than img or the output.
+func cropCentreAndResize(img *vips.ImageRef, width, height int) error {
+	imgW, imgH := img.Width(), img.Height()
+
+	// An image wider than the requested proportions keeps its height and
+	// loses width, any other keeps its width and loses height; the side
+	// shortened is rounded to the nearest pixel and is at least 1
+	cropW, cropH := imgW, imgH
+	if imgW*height > imgH*width {
+		cropW = max(1, (imgH*width+height/2)/height)
+	} else {
+		cropH = max(1, (imgW*height+width/2)/width)
+	}
+
+	// Half of what is cut comes off each side
+	left, top := (imgW-cropW)/2, (imgH-cropH)/2 //nolint:mnd // half the cut
+
+	err := img.ExtractArea(left, top, cropW, cropH)
+	if err != nil {
+		return err
+	}
+
+	return img.ThumbnailWithSize(width, height, vips.InterestingNone, vips.SizeForce)
+}
+
 const defaultQuality = 85
 
 // encode encodes an image to the specified format. govips gives pixa no way