crecord: fallback to text mode if diffs are too big for curses mode

crecord uses curses.newpad to create a region that we can then scroll around in
by moving the main 'screen' as a veiwport into the (probably larger than the
actual screen) pad.  Internally, at least in ncurses, pads are implemented using
windows, which have their dimensions limited to a certain size.  Depending on
compilation options for ncurses, this size might be pretty small: (signed)
short, or it might be larger ((signed) int).

crecord wants to have enough room to have all of the contents of the main area
of the chunkselector in the pad; this means that the full size with everything
expanded must be less than these (undocumented, afaict) limits.

It's not easy to write tests for this because the limits are platform- and
installation- dependent and undocumented / unqueryable, as far as I can tell.

Differential Revision: https://phab.mercurial-scm.org/D3577

mercurial/cmdutil.py

@@ -197,17 +197,21 b' def setupwrapcolorwrite(ui):'
     return oldwrite
 
 def filterchunks(ui, originalhunks, usecurses, testfile, operation=None):
-    if usecurses:
-        if testfile:
-            recordfn = crecordmod.testdecorator(testfile,
-                                                crecordmod.testchunkselector)
-        else:
-            recordfn = crecordmod.chunkselector
-
-        return crecordmod.filterpatch(ui, originalhunks, recordfn, operation)
-
-    else:
-        return patch.filterpatch(ui, originalhunks, operation)
+    try:
+        if usecurses:
+            if testfile:
+                recordfn = crecordmod.testdecorator(
+                    testfile, crecordmod.testchunkselector)
+            else:
+                recordfn = crecordmod.chunkselector
+
+            return crecordmod.filterpatch(ui, originalhunks, recordfn,
+                                          operation)
+    except crecordmod.fallbackerror as e:
+        ui.warn('%s\n' % e.message)
+        ui.warn(_('falling back to text mode\n'))
+
+    return patch.filterpatch(ui, originalhunks, operation)
 
 def recordfilter(ui, originalhunks, operation=None):
     """ Prompts the user to filter the originalhunks and return a list of

mercurial/crecord.py

@@ -65,6 +65,11 b' except ImportError:'
         # compiled with curses
         curses = False
 
+class fallbackerror(error.Abort):
+    """Error that indicates the client should try to fallback to text mode."""
+    # Inherits from error.Abort so that existing behavior is preserved if the
+    # calling code does not know how to fallback.
+
 def checkcurses(ui):
     """Return True if the user wants to use curses
 
@@ -529,8 +534,8 b' def chunkselector(ui, headerlist, operat'
         origsigtstp = signal.getsignal(signal.SIGTSTP)
     try:
         curses.wrapper(chunkselector.main)
-        if chunkselector.initerr is not None:
-            raise error.Abort(chunkselector.initerr)
+        if chunkselector.initexc is not None:
+            raise chunkselector.initexc
         # ncurses does not restore signal handler for SIGTSTP
     finally:
         if origsigtstp is not sentinel:
@@ -1718,7 +1723,7 b' are you sure you want to review/edit and'
         self.stdscr = stdscr
         # error during initialization, cannot be printed in the curses
         # interface, it should be printed by the calling code
-        self.initerr = None
+        self.initexc = None
         self.yscreensize, self.xscreensize = self.stdscr.getmaxyx()
 
         curses.start_color()
@@ -1751,7 +1756,8 b' are you sure you want to review/edit and'
         try:
             self.chunkpad = curses.newpad(self.numpadlines, self.xscreensize)
         except curses.error:
-            self.initerr = _('this diff is too large to be displayed')
+            self.initexc = fallbackerror(
+                _('this diff is too large to be displayed'))
             return
         # initialize selecteditemendline (initial start-line is 0)
         self.selecteditemendline = self.getnumlinesdisplayed(