From 2acaf3fdaa8ab04a7d1c86210698323038233d94 Mon Sep 17 00:00:00 2001
From: Jany Belluz <jany.belluz@daltonmaag.com>
Date: Thu, 19 Jul 2018 16:46:32 +0100
Subject: [PATCH] Fix crash when round-tripping a GSFeaturePrefix containing
 only a comment

---
 Lib/glyphsLib/builder/features.py | 19 +++++++++++--------
 tests/builder/features_test.py    | 21 +++++++++++++++++++++
 2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/Lib/glyphsLib/builder/features.py b/Lib/glyphsLib/builder/features.py
index 17cfc6d47..71d13743e 100644
--- a/Lib/glyphsLib/builder/features.py
+++ b/Lib/glyphsLib/builder/features.py
@@ -346,8 +346,8 @@ def __init__(self, list):
         self.index = 0
         self.list = list
 
-    def has_next(self):
-        return self.index < len(self.list)
+    def has_next(self, n=0):
+        return (self.index + n) < len(self.list)
 
     def next(self):
         res = self.list[self.index]
@@ -425,10 +425,11 @@ def _process_prefix(self):
                 if self.PREFIX_RE.match(st.text):
                     break
                 # ...or if it is the "automatic" comment just before a class
-                next_st = self.statements.peek(1)
-                if self.AUTOMATIC_RE.match(st.text) and isinstance(
-                        next_st, ast.GlyphClassDefinition):
-                    break
+                if self.statements.has_next(1):
+                    next_st = self.statements.peek(1)
+                    if self.AUTOMATIC_RE.match(st.text) and isinstance(
+                            next_st, ast.GlyphClassDefinition):
+                        break
             prefix_statements.append(st)
             self.statements.next()
 
@@ -576,7 +577,9 @@ def _pop_comment_block(self, statements, header_re):
         return (match, dedent(''.join(c.text[1:] + "\n" for c in comments)),
                 res)
 
+    # Strip up to the given number of newlines from the right end of the string
     def _rstrip_newlines(self, string, number=1):
-        if len(string) >= number and string[-number:] == '\n' * number:
-            string = string[:-number]
+        for i in range(number):
+            if string and string[-1] == '\n':
+                string = string[:-1]
         return string
diff --git a/tests/builder/features_test.py b/tests/builder/features_test.py
index e2f27c04f..9fdebf8af 100644
--- a/tests/builder/features_test.py
+++ b/tests/builder/features_test.py
@@ -335,3 +335,24 @@ def test_roundtrip_automatic_feature():
     assert feature_r.name == "ccmp"
     assert feature_r.code == "sub c by c.ss03;"
     assert feature_r.automatic is True
+
+
+def test_roundtrip_feature_prefix_with_only_a_comment():
+    font = to_glyphs([defcon.Font()])
+    prefix = classes.GSFeaturePrefix(name="include")
+    # Contents: just a comment
+    prefix.code = "#include(../family.fea)"
+    font.featurePrefixes.append(prefix)
+
+    ufo, = to_ufos(font)
+
+    assert ufo.features.text == dedent('''\
+        # Prefix: include
+        #include(../family.fea)
+    ''')
+
+    font_r = to_glyphs([ufo])
+    assert len(font_r.featurePrefixes) == 1
+    prefix_r = font_r.featurePrefixes[0]
+    assert prefix_r.name == 'include'
+    assert prefix_r.code == "#include(../family.fea)"