The same file must draw the same pixels every time

determinism_test.go

@@ -0,0 +1,67 @@
+package render
+
+import (
+	"crypto/sha256"
+	"testing"
+
+	"github.com/go-pdfkit/reader"
+)
+
+// TestTheSameFileDrawsTheSamePixelsEveryTime guards the dependency, not this
+// package's own code.
+//
+// An inline image dictionary may carry both spellings of a key — /W beside
+// /Width, /CS beside /ColorSpace — and disagree with itself. reader before
+// v0.4.2 expanded such a dictionary in one pass over its map, so the winner
+// was whichever Go's randomised iteration order yielded last, and the same
+// page drew a different picture on different runs of the same binary. Eight
+// renders of safedocs' Inline_Image_Abbreviations fixture gave five different
+// answers.
+//
+// Nothing in this package's own tests could see that: every one of them draws
+// a page once. Non-determinism is invisible to a suite that never repeats
+// itself, which is why this test repeats itself.
+//
+// The image is built here rather than committed, so nobody else's PDF enters
+// the repository. It says /W 2 and /Width 1, and /H 1 and /Height 2: the two
+// readings give a 2x1 image and a 1x2 one, which do not cover the same pixels.
+func TestTheSameFileDrawsTheSamePixelsEveryTime(t *testing.T) {
+	w := reader.NewWriter("1.7")
+	pagesRef := w.Reserve()
+	content := "q 20 0 0 20 0 0 cm " +
+		"BI /W 2 /Width 1 /H 1 /Height 2 /CS /G /ColorSpace /DeviceGray /BPC 8 " +
+		"ID \x00\xff EI Q"
+	pageRef := w.Add(reader.Dict{"Type": reader.Name("Page"), "Parent": pagesRef,
+		"MediaBox": nums(0, 0, 20, 20),
+		"Contents": w.Add(&reader.Stream{Dict: reader.Dict{}, Raw: []byte(content)})})
+	w.Put(pagesRef, reader.Dict{"Type": reader.Name("Pages"),
+		"Kids": reader.Array{pageRef}, "Count": reader.Integer(1)})
+	out, err := w.Finish(reader.Dict{"Root": w.Add(reader.Dict{
+		"Type": reader.Name("Catalog"), "Pages": pagesRef})})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var first string
+	for i := 0; i < 40; i++ {
+		d, err := reader.Open(out)
+		if err != nil {
+			t.Fatal(err)
+		}
+		img, err := Page(d, 1, Options{Scale: 1})
+		if err != nil {
+			t.Fatal(err)
+		}
+		h := sha256.Sum256(img.Pix)
+		got := string(h[:])
+		if i == 0 {
+			first = got
+			continue
+		}
+		if got != first {
+			t.Fatalf("draw %d differs from the first: the same file drew "+
+				"different pixels, so something in the read path depends on "+
+				"map iteration order", i)
+		}
+	}
+}

go.mod

@@ -6,7 +6,7 @@ require (
 	github.com/go-gfx/gfx v0.10.0
 	github.com/go-opentype/fonts v0.9.0
 	github.com/go-opentype/opentype v0.10.0
-	github.com/go-pdfkit/reader v0.4.1
+	github.com/go-pdfkit/reader v0.4.2
 )
 
 require github.com/go-pdfkit/pdffont v0.3.0

go.sum

@@ -6,5 +6,5 @@ github.com/go-opentype/opentype v0.10.0 h1:cZVMZ3RVkcijXmxlmpqyVkjlNc33aSB2ugKVW
 github.com/go-opentype/opentype v0.10.0/go.mod h1:AOixevJf7XQaH7+WG+OMIOZEbYPXfMqklVk26Y6YTUU=
 github.com/go-pdfkit/pdffont v0.3.0 h1:G5DKcAmsZJ0e17QhSrcUaL7PKEXKjUx4P/iGMl7bAbI=
 github.com/go-pdfkit/pdffont v0.3.0/go.mod h1:bfmNLna1l1CljNX/Utg55YzFylovfmI7sJnvgA3bzKI=
-github.com/go-pdfkit/reader v0.4.1 h1:pRxFqRjsn7H/VsGfWb9nYWyFuDgTU2Pjmoq/f5mgVq4=
-github.com/go-pdfkit/reader v0.4.1/go.mod h1:fQFOVfCMUui1AdvD4qhimdyvvNr9KvvJ1S7IuKZjyV8=
+github.com/go-pdfkit/reader v0.4.2 h1:0/qPShLzdG/roH4i13dwDTHnS7mEBVTaCBGwCZQhOH0=
+github.com/go-pdfkit/reader v0.4.2/go.mod h1:fQFOVfCMUui1AdvD4qhimdyvvNr9KvvJ1S7IuKZjyV8=