Position-size should NEVER be over available_capital

Part of #4353

freqtrade/edge/edge_positioning.py

@@ -159,7 +159,8 @@ class Edge:
         available_capital = (total_capital + capital_in_trade) * self._capital_ratio
         allowed_capital_at_risk = available_capital * self._allowed_risk
         max_position_size = abs(allowed_capital_at_risk / stoploss)
-        position_size = min(max_position_size, free_capital)
+        # Position size must be below available capital.
+        position_size = min(min(max_position_size, free_capital), available_capital)
         if pair in self._cached_pairs:
             logger.info(
                 'winrate: %s, expectancy: %s, position size: %s, pair: %s,'

tests/edge/test_edge.py

@@ -209,7 +209,7 @@ def test_nonexisting_stoploss(mocker, edge_conf):
     assert edge.stoploss('N/O') == -0.1
 
 
-def test_stake_amount(mocker, edge_conf):
+def test_edge_stake_amount(mocker, edge_conf):
     freqtrade = get_patched_freqtradebot(mocker, edge_conf)
     edge = Edge(edge_conf, freqtrade.exchange, freqtrade.strategy)
     mocker.patch('freqtrade.edge.Edge._cached_pairs', mocker.PropertyMock(
@@ -217,20 +217,33 @@ def test_stake_amount(mocker, edge_conf):
             'E/F': PairInfo(-0.02, 0.66, 3.71, 0.50, 1.71, 10, 60),
         }
     ))
-    free = 100
+    assert edge._capital_ratio == 0.5
-    total = 100
+    assert edge.stake_amount('E/F', free_capital=100, total_capital=100,
-    in_trade = 25
+                             capital_in_trade=25) == 31.25
-    assert edge.stake_amount('E/F', free, total, in_trade) == 31.25
 
-    free = 20
+    assert edge.stake_amount('E/F', free_capital=20, total_capital=100,
-    total = 100
+                             capital_in_trade=25) == 20
-    in_trade = 25
-    assert edge.stake_amount('E/F', free, total, in_trade) == 20
 
-    free = 0
+    assert edge.stake_amount('E/F', free_capital=0, total_capital=100,
-    total = 100
+                             capital_in_trade=25) == 0
-    in_trade = 25
+
-    assert edge.stake_amount('E/F', free, total, in_trade) == 0
+    # Test with increased allowed_risk
+    # Result should be no more than allowed capital
+    edge._allowed_risk = 0.4
+    edge._capital_ratio = 0.5
+    assert edge.stake_amount('E/F', free_capital=100, total_capital=100,
+                             capital_in_trade=25) == 62.5
+
+    assert edge.stake_amount('E/F', free_capital=100, total_capital=100,
+                             capital_in_trade=0) == 50
+
+    edge._capital_ratio = 1
+    # Full capital is available
+    assert edge.stake_amount('E/F', free_capital=100, total_capital=100,
+                             capital_in_trade=0) == 100
+    # Full capital is available
+    assert edge.stake_amount('E/F', free_capital=0, total_capital=100,
+                             capital_in_trade=0) == 0
 
 
 def test_nonexisting_stake_amount(mocker, edge_conf):