Fix handling of chomp with paragraph separator

Try to mirror IO behavior, where chomp takes out the entire paragraph
separators between entries, but does not chomp a single line separator
at the end of the string.

Partially Fixes [Bug #18768]

ext/stringio/stringio.c

@@ -1204,6 +1204,7 @@ strio_getline(struct getline_arg *arg, struct StringIO *ptr)
 	str = strio_substr(ptr, ptr->pos, e - s - w, enc);
     }
     else if ((n = RSTRING_LEN(str)) == 0) {
+        const char *paragraph_end = NULL;
 	p = s;
 	while (p[(p + 1 < e) && (*p == '\r') && 0] == '\n') {
 	    p += *p == '\r';
@@ -1213,19 +1214,21 @@ strio_getline(struct getline_arg *arg, struct StringIO *ptr)
 	}
 	s = p;
 	while ((p = memchr(p, '\n', e - p)) && (p != e)) {
-	    if (*++p == '\n') {
-		e = p + 1;
-		w = (arg->chomp ? 1 : 0);
-		break;
-	    }
-	    else if (*p == '\r' && p < e && p[1] == '\n') {
-		e = p + 2;
-		w = (arg->chomp ? 2 : 0);
-		break;
-	    }
+            p++;
+            if (!((p < e && *p == '\n') ||
+                  (p + 1 < e && *p == '\r' && *(p+1) == '\n'))) {
+                continue;
+            }
+            paragraph_end = p - ((*(p-2) == '\r') ? 2 : 1);
+            while ((p < e && *p == '\n') ||
+                   (p + 1 < e && *p == '\r' && *(p+1) == '\n')) {
+                p += (*p == '\r') ? 2 : 1;
+            }
+            e = p;
+            break;
 	}
-	if (!w && arg->chomp) {
-	    w = chomp_newline_width(s, e);
+	if (arg->chomp && paragraph_end) {
+	    w = e - paragraph_end;
 	}
 	str = strio_substr(ptr, s - RSTRING_PTR(ptr->string), e - s - w, enc);
     }

test/stringio/test_stringio.rb

@@ -93,10 +93,10 @@ def test_gets_chomp
     assert_equal("a", StringIO.new("a\nb").gets(chomp: true))
     assert_equal("abc", StringIO.new("abc\n\ndef\n").gets(chomp: true))
     assert_equal("abc\n\ndef\n", StringIO.new("abc\n\ndef\n").gets(nil, chomp: true))
-    assert_equal("abc\n", StringIO.new("abc\n\ndef\n").gets("", chomp: true))
+    assert_equal("abc", StringIO.new("abc\n\ndef\n").gets("", chomp: true))
     stringio = StringIO.new("abc\n\ndef\n")
-    assert_equal("abc\n", stringio.gets("", chomp: true))
-    assert_equal("def", stringio.gets("", chomp: true))
+    assert_equal("abc", stringio.gets("", chomp: true))
+    assert_equal("def\n", stringio.gets("", chomp: true))
 
     assert_string("", Encoding::UTF_8, StringIO.new("\n").gets(chomp: true))
   end
@@ -110,10 +110,10 @@ def test_gets_chomp_eol
     assert_equal("a", StringIO.new("a\r\nb").gets(chomp: true))
     assert_equal("abc", StringIO.new("abc\r\n\r\ndef\r\n").gets(chomp: true))
     assert_equal("abc\r\n\r\ndef\r\n", StringIO.new("abc\r\n\r\ndef\r\n").gets(nil, chomp: true))
-    assert_equal("abc\r\n", StringIO.new("abc\r\n\r\ndef\r\n").gets("", chomp: true))
+    assert_equal("abc", StringIO.new("abc\r\n\r\ndef\r\n").gets("", chomp: true))
     stringio = StringIO.new("abc\r\n\r\ndef\r\n")
-    assert_equal("abc\r\n", stringio.gets("", chomp: true))
-    assert_equal("def", stringio.gets("", chomp: true))
+    assert_equal("abc", stringio.gets("", chomp: true))
+    assert_equal("def\r\n", stringio.gets("", chomp: true))
   end
 
   def test_readlines
@@ -596,15 +596,15 @@ def test_each
     assert_equal(["foo\n", "bar\n", "baz\n"], f.each.to_a)
     f.rewind
     assert_equal(["foo", "bar", "baz"], f.each(chomp: true).to_a)
-    f = StringIO.new("foo\nbar\n\nbaz\n")
-    assert_equal(["foo\nbar\n\n", "baz\n"], f.each("").to_a)
+    f = StringIO.new("foo\nbar\n\n\nbaz\n")
+    assert_equal(["foo\nbar\n\n\n", "baz\n"], f.each("").to_a)
     f.rewind
-    assert_equal(["foo\nbar\n", "baz"], f.each("", chomp: true).to_a)
+    assert_equal(["foo\nbar", "baz\n"], f.each("", chomp: true).to_a)
 
-    f = StringIO.new("foo\r\nbar\r\n\r\nbaz\r\n")
-    assert_equal(["foo\r\nbar\r\n\r\n", "baz\r\n"], f.each("").to_a)
+    f = StringIO.new("foo\r\nbar\r\n\r\n\r\nbaz\r\n")
+    assert_equal(["foo\r\nbar\r\n\r\n\r\n", "baz\r\n"], f.each("").to_a)
     f.rewind
-    assert_equal(["foo\r\nbar\r\n", "baz"], f.each("", chomp: true).to_a)
+    assert_equal(["foo\r\nbar", "baz\r\n"], f.each("", chomp: true).to_a)
 
     f = StringIO.new("abc\n\ndef\n")
     assert_equal(["ab", "c\n", "\nd", "ef", "\n"], f.each(nil, 2, chomp: true).to_a)