fix an occasional stack overflow error due to infinite recursion in Posting/Transaction equality tests

This was happening with balance command on certain timelog entries, since 0.8.
Now, when testing two postings for equality, their parent transaction's identity is ignored.

hledger-lib/Hledger/Data/Types.hs

@@ -81,7 +81,12 @@ data Posting = Posting {
       ptype :: PostingType,
       ptransaction :: Maybe Transaction  -- ^ this posting's parent transaction (co-recursive types).
                                         -- Tying this knot gets tedious, Maybe makes it easier/optional.
-    } deriving (Eq)
+    }
+
+-- The equality test for postings ignores the parent transaction's
+-- identity, to avoid infinite loops.
+instance Eq Posting where
+    (==) (Posting a1 b1 c1 d1 e1 _) (Posting a2 b2 c2 d2 e2 _) =  a1==a2 && b1==b2 && c1==c2 && d1==d2 && e1==e2
 
 data Transaction = Transaction {
       tdate :: Day,
@@ -90,7 +95,7 @@ data Transaction = Transaction {
       tcode :: String,
       tdescription :: String,
       tcomment :: String,
-      tpostings :: [Posting],
+      tpostings :: [Posting],            -- ^ this transaction's postings (co-recursive types).
       tpreceding_comment_lines :: String
     } deriving (Eq)
 

tests/timelog-stack-overflow.test

@@ -0,0 +1,13 @@
+# this gave a stack space overflow error with 0.8-0.9 due to infinite
+# recursion in Posting and Transaction's equality tests:
+bin/hledger -f - balance
+<<<
+i 2010/1/1 09:00:00 a:b
+o 2010/1/1 09:03:00
+>>>=0
+# incidentally this didn't trigger it.. go figure
+bin/hledger -f - balance
+<<<
+i 2010/1/1 09:00:00 a:b
+o 2010/1/1 09:02:00
+>>>=0