{v,h}mtx: allow to compile/decompile even without {v,h}hhea table

when no horizontal/vertical header table is present, we assume numberOfHMetrics == maxp.numGlyphs.
This allows to compile/decompile incomplete fonts, only containing head, maxp, hmtx, loca and glyf (or CFF),
that is the bare minimum to temporarily store glyph outline and metrics data for use with sparse/intermediate
master TTFs in varLib.
See googlefonts/ufo2ft#308

Lib/fontTools/ttLib/tables/_h_m_t_x.py

@@ -23,7 +23,11 @@ class table__h_m_t_x(DefaultTable.DefaultTable):
 
 	def decompile(self, data, ttFont):
 		numGlyphs = ttFont['maxp'].numGlyphs
-		numberOfMetrics = int(getattr(ttFont[self.headerTag], self.numberOfMetricsName))
+		headerTable = ttFont.get(self.headerTag)
+		if headerTable is not None:
+			numberOfMetrics = int(getattr(headerTable, self.numberOfMetricsName))
+		else:
+			numberOfMetrics = numGlyphs
 		if numberOfMetrics > numGlyphs:
 			log.warning("The %s.%s exceeds the maxp.numGlyphs" % (
 				self.headerTag, self.numberOfMetricsName))
@@ -69,19 +73,26 @@ def compile(self, ttFont):
 					glyphName, self.advanceName))
 				hasNegativeAdvances = True
 			metrics.append([advanceWidth, sideBearing])
-		lastAdvance = metrics[-1][0]
-		lastIndex = len(metrics)
-		while metrics[lastIndex-2][0] == lastAdvance:
-			lastIndex -= 1
-			if lastIndex <= 1:
-				# all advances are equal
-				lastIndex = 1
-				break
-		additionalMetrics = metrics[lastIndex:]
-		additionalMetrics = [otRound(sb) for _, sb in additionalMetrics]
-		metrics = metrics[:lastIndex]
-		numberOfMetrics = len(metrics)
-		setattr(ttFont[self.headerTag], self.numberOfMetricsName, numberOfMetrics)
+
+		headerTable = ttFont.get(self.headerTag)
+		if headerTable is not None:
+			lastAdvance = metrics[-1][0]
+			lastIndex = len(metrics)
+			while metrics[lastIndex-2][0] == lastAdvance:
+				lastIndex -= 1
+				if lastIndex <= 1:
+					# all advances are equal
+					lastIndex = 1
+					break
+			additionalMetrics = metrics[lastIndex:]
+			additionalMetrics = [otRound(sb) for _, sb in additionalMetrics]
+			metrics = metrics[:lastIndex]
+			numberOfMetrics = len(metrics)
+			setattr(headerTable, self.numberOfMetricsName, numberOfMetrics)
+		else:
+			# no hhea/vhea, can't store numberOfMetrics; assume == numGlyphs
+			numberOfMetrics = ttFont["maxp"].numGlyphs
+			additionalMetrics = []
 
 		allMetrics = []
 		for advance, sb in metrics:

Tests/ttLib/tables/_h_m_t_x_test.py

@@ -107,6 +107,27 @@ def test_decompile_possibly_negative_advance(self):
             len([r for r in captor.records
                 if "has a huge advance" in r.msg]) == 1)
 
+    def test_decompile_no_header_table(self):
+        font = TTFont()
+        maxp = font['maxp'] = newTable('maxp')
+        maxp.numGlyphs = 3
+        font.glyphOrder = ["A", "B", "C"]
+
+        self.assertNotIn(self.tableClass.headerTag, font)
+
+        data = deHexStr("0190 001E 0190 0028 0190 0032")
+        mtxTable = newTable(self.tag)
+        mtxTable.decompile(data, font)
+
+        self.assertEqual(
+            mtxTable.metrics,
+            {
+                "A": (400, 30),
+                "B": (400, 40),
+                "C": (400, 50),
+            }
+        )
+
     def test_compile(self):
         # we set the wrong 'numberOfMetrics' to check it gets adjusted
         font = self.makeFont(numGlyphs=3, numberOfMetrics=4)
@@ -173,6 +194,24 @@ def test_compile_round_float_values(self):
 
         self.assertEqual(data, deHexStr("0001 0001 0000 0001 0000"))
 
+    def test_compile_no_header_table(self):
+        font = TTFont()
+        maxp = font['maxp'] = newTable('maxp')
+        maxp.numGlyphs = 3
+        font.glyphOrder = [chr(i) for i in range(65, 68)]
+        mtxTable = font[self.tag] = newTable(self.tag)
+        mtxTable.metrics = {
+            "A": (400, 30),
+            "B": (400, 40),
+            "C": (400, 50),
+        }
+
+        self.assertNotIn(self.tableClass.headerTag, font)
+
+        data = mtxTable.compile(font)
+
+        self.assertEqual(data, deHexStr("0190 001E 0190 0028 0190 0032"))
+
     def test_toXML(self):
         font = self.makeFont(numGlyphs=2, numberOfMetrics=2)
         mtxTable = font[self.tag] = newTable(self.tag)