fix: swap GIT binary patch sections when reversing

diff/reverse.go

@@ -59,9 +59,83 @@ func reverseExtendedHeaders(headers []string, origName, newName string) ([]strin
 	}
 	swapHeaderValues(reversed, parsed, gitModeHeaderPair)
 	swapHeaderValues(reversed, parsed, gitRenameHeaderPair)
+	reverseBinaryPatchSections(reversed)
 	return reversed, nil
 }
 
+// reverseBinaryPatchSections swaps the two encoded sections of a GIT binary
+// patch. git apply uses the first section as the forward result and the
+// second as the reverse result.
+func reverseBinaryPatchSections(headers []string) {
+	start := -1
+	for i, header := range headers {
+		if strings.TrimRight(header, "\r") == gitExtendedHeaderBinaryPatch {
+			start = i
+			break
+		}
+	}
+	if start < 0 || start+1 >= len(headers) {
+		return
+	}
+	first, rest, ok := takeBinarySection(headers[start+1:])
+	if !ok {
+		return
+	}
+	second, tail, ok := takeBinarySection(rest)
+	if !ok {
+		return
+	}
+	swapped := make([]string, 0, len(headers))
+	swapped = append(swapped, headers[:start+1]...)
+	swapped = append(swapped, second...)
+	swapped = append(swapped, first...)
+	swapped = append(swapped, tail...)
+	copy(headers, swapped)
+}
+
+func takeBinarySection(lines []string) (section, rest []string, ok bool) {
+	if len(lines) == 0 || !isBinarySectionHeader(lines[0]) {
+		return nil, lines, false
+	}
+	end := 1
+	for end < len(lines) {
+		if isBinarySectionHeader(lines[end]) {
+			break
+		}
+		end++
+		if isBlankHeader(lines[end-1]) {
+			break
+		}
+	}
+	return lines[:end], lines[end:], true
+}
+
+func isBinarySectionHeader(line string) bool {
+	line = strings.TrimRight(line, "\r")
+	var n string
+	switch {
+	case strings.HasPrefix(line, "literal "):
+		n = strings.TrimPrefix(line, "literal ")
+	case strings.HasPrefix(line, "delta "):
+		n = strings.TrimPrefix(line, "delta ")
+	default:
+		return false
+	}
+	if n == "" {
+		return false
+	}
+	for i := 0; i < len(n); i++ {
+		if n[i] < '0' || n[i] > '9' {
+			return false
+		}
+	}
+	return true
+}
+
+func isBlankHeader(line string) bool {
+	return strings.TrimRight(line, "\r") == ""
+}
+
 // reverseDiffGitHeader swaps the two path arguments while preserving their
 // original quoting. The parsed filenames are used only to reject malformed or
 // ambiguous input; names recovered from other headers can disambiguate Git's

diff/reverse_git_test.go

@@ -114,8 +114,8 @@ func TestReverseFileDiffGitSemantics(t *testing.T) {
 	}
 }
 
-// Git binary patches contain both directions. Reversing the index hashes makes
-// git apply select the inverse payload without reordering the encoded sections.
+// git apply uses the first encoded section as the forward result and the
+// second as the reverse result. Reversing a patch swaps those sections.
 func TestReverseFileDiffGitBinarySemantics(t *testing.T) {
 	tests := []struct {
 		name    string
@@ -134,10 +134,10 @@ func TestReverseFileDiffGitBinarySemantics(t *testing.T) {
 			reverse: "diff --git b/literal.bin a/literal.bin\n" +
 				"index e0d84f457d2b5c802ecd286e7e576365d94af3b9..acdc746399905e5ea0d9d4163d2b3c4d04b05123 100644\n" +
 				"GIT binary patch\n" +
-				"literal 8\n" +
-				"PcmZQzWMXDvWn%{b09*ha\n\n" +
 				"literal 64\n" +
-				"LcmZ>CqznK65PlFr\n\n",
+				"LcmZ>CqznK65PlFr\n\n" +
+				"literal 8\n" +
+				"PcmZQzWMXDvWn%{b09*ha\n\n",
 		},
 		{
 			name: "delta",
@@ -151,10 +151,27 @@ func TestReverseFileDiffGitBinarySemantics(t *testing.T) {
 			reverse: "diff --git b/delta.bin a/delta.bin\n" +
 				"index b762194ffce13918858bfd54f1c4ded144c7e3b4..df437f42c808d41dec5d543d60ce94c8cb8a044a 100644\n" +
 				"GIT binary patch\n" +
-				"delta 20\n" +
-				"bcmZorXi(U2ft@2cBQY;MHAQjb4Gj(ePVNVE\n\n" +
 				"delta 10\n" +
-				"PcmZorXi!+h!2v`75gY=g\n\n",
+				"PcmZorXi!+h!2v`75gY=g\n\n" +
+				"delta 20\n" +
+				"bcmZorXi(U2ft@2cBQY;MHAQjb4Gj(ePVNVE\n\n",
+		},
+		{
+			name: "reported lengths",
+			forward: "diff --git a/bin.dat b/bin.dat\n" +
+				"index bdc955b7b2e610ad5a72302b139a2e6cb325519a..eaf36c1daccfdf325514461cd1a2ffbc139b5464 100644\n" +
+				"GIT binary patch\n" +
+				"literal 4\n" +
+				"LcmZQzWMT#Y01f~L\n\n" +
+				"literal 2\n" +
+				"JcmZQz1ONa700IC2\n\n",
+			reverse: "diff --git b/bin.dat a/bin.dat\n" +
+				"index eaf36c1daccfdf325514461cd1a2ffbc139b5464..bdc955b7b2e610ad5a72302b139a2e6cb325519a 100644\n" +
+				"GIT binary patch\n" +
+				"literal 2\n" +
+				"JcmZQz1ONa700IC2\n\n" +
+				"literal 4\n" +
+				"LcmZQzWMT#Y01f~L\n\n",
 		},
 	}