Fix bug in writing an empty INDEX table. This is supposed to contain only the two-bye count field. Code was also writing the final sentinel offset in the offset list, even though there were no items. This is harmless, but adds a couple useless bytes. Also fixed the calculation of the offset size in the CFF header. Again harmless, since this value isn't used anywhere, but makes it harder to compare CFF's built by different tools.

Lib/fontTools/cffLib.py

@@ -155,6 +155,10 @@ class CFFWriter(object):
 		log.log(DEBUG, "CFFWriter.toFile() writing to file.")
 		begin = file.tell()
 		posList = [0]
+		cffOffSize = calcOffSize(lastPosList[-1])
+		headerBytes = self.data[0]
+		headerBytes = headerBytes[:-1] + chr(cffOffSize)
+		self.data[0] = headerBytes
 		for item in self.data:
 			if hasattr(item, "toFile"):
 				item.toFile(file)
@@ -186,43 +190,53 @@ class IndexCompiler(object):
 		return items
 
 	def getOffsets(self):
-		pos = 1
-		offsets = [pos]
-		for item in self.items:
-			if hasattr(item, "getDataLength"):
-				pos = pos + item.getDataLength()
-			else:
-				pos = pos + len(item)
-			offsets.append(pos)
+		# An empty INDEX contains only the count field.
+		if self.items:
+			pos = 1
+			offsets = [pos]
+			for item in self.items:
+				if hasattr(item, "getDataLength"):
+					pos = pos + item.getDataLength()
+				else:
+					pos = pos + len(item)
+				offsets.append(pos)
+		else:
+			offsets = []
 		return offsets
 
 	def getDataLength(self):
-		lastOffset = self.getOffsets()[-1]
-		offSize = calcOffSize(lastOffset)
-		dataLength = (
-			2 +                                # count
-			1 +                                # offSize
-			(len(self.items) + 1) * offSize +  # the offsets
-			lastOffset - 1                     # size of object data
-		)
+		if self.items:
+			lastOffset = self.getOffsets()[-1]
+			offSize = calcOffSize(lastOffset)
+			dataLength = (
+				2 +                                # count
+				1 +                                # offSize
+				(len(self.items) + 1) * offSize +  # the offsets
+				lastOffset - 1                     # size of object data
+			)
+		else:
+			dataLength = 2  # count. For empty INDEX tables, this is the only entry.
+
 		return dataLength
 
 	def toFile(self, file):
 		offsets = self.getOffsets()
 		writeCard16(file, len(self.items))
-		offSize = calcOffSize(offsets[-1])
-		writeCard8(file, offSize)
-		offSize = -offSize
-		pack = struct.pack
-		for offset in offsets:
-			binOffset = pack(">l", offset)[offSize:]
-			assert len(binOffset) == -offSize
-			file.write(binOffset)
-		for item in self.items:
-			if hasattr(item, "toFile"):
-				item.toFile(file)
-			else:
-				file.write(tobytes(item, encoding="latin1"))
+		# An empty INDEX contains only the count field.
+		if self.items:
+			offSize = calcOffSize(offsets[-1])
+			writeCard8(file, offSize)
+			offSize = -offSize
+			pack = struct.pack
+			for offset in offsets:
+				binOffset = pack(">l", offset)[offSize:]
+				assert len(binOffset) == -offSize
+				file.write(binOffset)
+			for item in self.items:
+				if hasattr(item, "toFile"):
+					item.toFile(file)
+				else:
+					file.write(tobytes(item, encoding="latin1"))
 
 
 class IndexedStringsCompiler(IndexCompiler):