Add chained offset logic for FCP (#5885)

Author: markgrahamdawson

changes.d/5885.fix.md

@@ -0,0 +1 @@
+Fixed bug in using a final cycle point with chained offsets e.g. 'final cycle point = +PT6H+PT1S'.

cylc/flow/cycling/iso8601.py

@@ -895,14 +895,23 @@ def get_dump_format():
 def get_point_relative(offset_string, base_point):
     """Create a point from offset_string applied to base_point."""
     try:
-        interval = ISO8601Interval(str(interval_parse(offset_string)))
+        operator = '+'
+        base_point_relative = base_point
+        for part in re.split(r'(\+|-)', offset_string):
+            if part == '+' or part == '-':
+                operator = part
+            elif part != '':
+                interval = interval_parse(part)
+                if operator == '-':
+                    interval *= -1
+                base_point_relative += ISO8601Interval(str(interval))
+        return base_point_relative
     except IsodatetimeError:
+        # It's a truncated time point rather than an interval
         return ISO8601Point(str(
             WorkflowSpecifics.abbrev_util.parse_timepoint(
                 offset_string, context_point=point_parse(base_point.value))
         ))
-    else:
-        return base_point + interval
 
 
 def interval_parse(interval_string):

tests/unit/test_config.py

@@ -532,6 +532,17 @@ def test_process_startcp(
             None,
             id="Relative fcp"
         ),
+        pytest.param(
+            ISO8601_CYCLING_TYPE,
+            {
+                'initial cycle point': '2017-02-11',
+                'final cycle point': '+P4D+PT3H-PT2H',
+            },
+            None,
+            '20170215T0100+0530',
+            None,
+            id="Relative fcp chained"
+        ),
         pytest.param(
             ISO8601_CYCLING_TYPE,
             {
@@ -1429,11 +1440,7 @@ def test_zero_interval(
         ('1988-02-29', '+P1M+P1Y', '1989-03-29'),
         ('1910-08-14', '+P2D-PT6H', '1910-08-15T18:00'),
         ('1850-04-10', '+P1M-P1D+PT1H', '1850-05-09T01:00'),
-        pytest.param(
-            '1066-10-14', '+PT1H+PT1M', '1066-10-14T01:01',
-            marks=pytest.mark.xfail
-            # https://github.com/cylc/cylc-flow/issues/5047
-        ),
+        ('1066-10-14', '+PT1H+PT1M', '1066-10-14T01:01'),
     ]
 )
 def test_chain_expr(