revlog: simplify the try nesting in the `_writing` context

Lets use a single try, with conditional cleanup. This make is easier to add a
file handle dedicated to sidedata.

Differential Revision: https://phab.mercurial-scm.org/D10775

mercurial/revlog.py

@@ -2232,7 +2232,10 b' class revlog(object):'
         if self._writinghandles is not None:
             yield
         else:
+            ifh = dfh = None
+            try:
             r = len(self)
+                # opening the data file.
             dsize = 0
             if r:
                 dsize = self.end(r - 1)
@@ -2249,24 +2252,23 b' class revlog(object):'
                         raise
                     dfh = self._datafp(b"w+")
                 transaction.add(self._datafile, dsize)
-            try:
+
+                # opening the index file.
                 isize = r * self.index.entry_size
                 ifh = self.__index_write_fp()
                 if self._inline:
                     transaction.add(self._indexfile, dsize + isize)
                 else:
                     transaction.add(self._indexfile, isize)
-                try:
+                # exposing all file handle for writing.
                     self._writinghandles = (ifh, dfh)
-                    try:
                         yield
                         if self._docket is not None:
                             self._write_docket(transaction)
                     finally:
                         self._writinghandles = None
-                finally:
+                if ifh is not None:
                     ifh.close()
-            finally:
                 if dfh is not None:
                     dfh.close()