Update how table packing falls back to fontTools from harfbuzz.

Introduce three compilation modes:
1. Pure python: only uses the existing fonttools packing and overflow resolution.
2. Harfbuzz+python: uses harfbuzz packing and python overflow resolution. Extensions are allowed to be shared.
3. Python fallback: if harfbuzz+python runs out of resolution options, this disables extension sharing and only uses python packing. Once it succeeds control is passed back to the harfbuzz packer to produce the final packing with extension sharing enabled.

Lib/fontTools/ttLib/tables/otBase.py

@@ -52,6 +52,17 @@ class BaseTTXConverter(DefaultTable):
 		self.table = tableClass()
 		self.table.decompile(reader, font)
 
+	# Pack only with fontTools, don't allow sharing between extensions.
+	MODE_PURE_FT = 1
+
+	# Attempt to pack with harfbuzz (allowing sharing between extensions)
+	# use fontTools to attempt overflow resolution.
+	MODE_HB_FT = 2
+
+	# Fallback if HB/FT packing gets stuck. Pack only with fontTools, don't allow sharing between
+	# extensions.
+	MODE_FT_FALLBACK = 3
+
 	def compile(self, font):
 		"""Compiles the table into binary. Called automatically on save."""
 
@@ -96,16 +107,45 @@ class BaseTTXConverter(DefaultTable):
 						self.tableTag,
 					)
 
+		if (use_hb_repack in (None, True)
+				and have_uharfbuzz
+				and self.tableTag in ("GSUB", "GPOS")):
+			mode = self.MODE_HB_FT
+		else:
+			mode = self.MODE_PURE_FT
+
 		hb_first_error_logged = False
+		lastOverflowRecord = None
 		while True:
 			try:
 				writer = OTTableWriter(tableTag=self.tableTag)
 				self.table.compile(writer, font)
-				if (
+				if mode == self.MODE_HB_FT:
-					use_hb_repack in (None, True)
+					return self.tryPackingHarfbuzz(writer, hb_first_error_logged)
-					and have_uharfbuzz
+				elif mode == self.MODE_PURE_FT:
-					and self.tableTag in ("GSUB", "GPOS")
+					return self.tryPackingFontTools(writer)
-				):
+				elif mode == self.MODE_FT_FALLBACK:
+					self.tryPackingFontTools(writer)
+					log.info("Re-enabling sharing between extensions and switching back to "
+										"harfbuzz+fontTools packing.")
+					mode = self.MODE_HB_FT
+
+			except OTLOffsetOverflowError as e:
+				hb_first_error_logged = True
+				ok = self.tryResolveOverflow(font, e, lastOverflowRecord)
+				lastOverflowRecord = e.value
+
+				if ok:
+					continue
+
+				if mode is self.MODE_HB_FT:
+					log.info("Harfbuzz packing out of resolutions, disabling sharing between extensions and "
+									 "switching to fontTools only packing.")
+					mode = self.MODE_FT_FALLBACK
+				else:
+					raise
+
+	def tryPackingHarfbuzz(self, writer, hb_first_error_logged):
 		try:
 			log.debug("serializing '%s' with hb.repack", self.tableTag)
 			return writer.getAllDataUsingHarfbuzz()
@@ -119,39 +159,43 @@ class BaseTTXConverter(DefaultTable):
 				if str(e) != "":
 					error_msg += f": {e}"
 				log.warning(
-								"hb.repack failed to serialize '%s', reverting to "
+						"hb.repack failed to serialize '%s', attempting fonttools resolutions "
-								"pure-python serializer; the error message was: %s",
+						"; the error message was: %s",
 						self.tableTag,
 						error_msg,
 				)
 				hb_first_error_logged = True
 			return writer.getAllData(remove_duplicate=False)
+
+
+	def tryPackingFontTools(self, writer):
 		return writer.getAllData()
 
-			except OTLOffsetOverflowError as e:
 
-				if overflowRecord == e.value:
+	def tryResolveOverflow(self, font, e, lastOverflowRecord):
-					raise # Oh well...
+		ok = 0
+		if lastOverflowRecord == e.value:
+			# Oh well...
+			return ok
 
 		overflowRecord = e.value
 		log.info("Attempting to fix OTLOffsetOverflowError %s", e)
-				lastItem = overflowRecord
 
-				ok = 0
 		if overflowRecord.itemName is None:
 			from .otTables import fixLookupOverFlows
 			ok = fixLookupOverFlows(font, overflowRecord)
 		else:
 			from .otTables import fixSubTableOverFlows
 			ok = fixSubTableOverFlows(font, overflowRecord)
-				if not ok:
+
+		if ok:
+			return ok
+
 		# Try upgrading lookup to Extension and hope
 		# that cross-lookup sharing not happening would
 		# fix overflow...
 		from .otTables import fixLookupOverFlows
-					ok = fixLookupOverFlows(font, overflowRecord)
+		return fixLookupOverFlows(font, overflowRecord)
-					if not ok:
-						raise
 
 	def toXML(self, writer, font):
 		self.table.toXML2(writer, font)
@@ -535,7 +579,7 @@ class OTTableWriter(object):
 		internedTables = {}
 		# TODO: Restore shareExtension=True after we fix
 		# https://github.com/fonttools/fonttools/issues/2661
-		self._doneWriting(internedTables, shareExtension=False)
+		self._doneWriting(internedTables, shareExtension=True)
 		tables = []
 		obj_list = []
 		done = {}