revset: add expectsize to check the size of a set

`expectsize(<set>, <int>)` revset fails if `<set>` is not exactly `<int>`
elements. `expectsize(<set>, <min>:<max>)` revset fails if `<set>` is not
exactly between `<min>` and `<max>` inclusive.

This then allows an alias for `hg next` to be `update -r one(children(.))`
with sane failure behavior, and also makes some other scripting tasks
a little less difficult.

(Summary from WeShouldDoThat)

Differential Revision: https://phab.mercurial-scm.org/D5813

mercurial/revset.py

@@ -861,6 +861,37 b' def contentdivergent(repo, subset, x):'
     contentdivergent = obsmod.getrevs(repo, 'contentdivergent')
     return subset & contentdivergent
 
+@predicate('expectsize(set[, size])', safe=True, takeorder=True)
+def expectsize(repo, subset, x, order):
+    """Abort if the revset doesn't expect given size"""
+    args = getargsdict(x, 'expectsize', 'set size')
+    minsize = 0
+    maxsize = len(repo) + 1
+    err = ''
+    if 'size' not in args or 'set' not in args:
+        raise error.ParseError(_('invalid set of arguments'))
+    minsize, maxsize = getintrange(args['size'],
+                                   _('expectsize requires a size range'
+                                     ' or a positive integer'),
+                                   _('size range bounds must be integers'),
+                                   minsize, maxsize)
+    if minsize < 0 or maxsize < 0:
+        raise error.ParseError(_('negative size'))
+    rev = getset(repo, fullreposet(repo), args['set'], order=order)
+    if minsize != maxsize and (len(rev) < minsize or len(rev) > maxsize):
+        err = _('revset size mismatch.'
+                ' expected between %d and %d, got %d') % (minsize, maxsize,
+                                                          len(rev))
+    elif minsize == maxsize and len(rev) != minsize:
+        err = _('revset size mismatch.'
+                ' expected %d, got %d') % (minsize, len(rev))
+    if err:
+        raise error.RepoLookupError(err)
+    if order == followorder:
+        return subset & rev
+    else:
+        return rev & subset
+
 @predicate('extdata(source)', safe=False, weight=100)
 def extdata(repo, subset, x):
     """Changesets in the specified extdata source. (EXPERIMENTAL)"""

tests/test-revset.t

@@ -2978,3 +2978,63 b' test multiline revset with errors'
   * set:
   <baseset+ [0]>
   0
+
+abort if the revset doesn't expect given size
+  $ log 'expectsize()'
+  hg: parse error: invalid set of arguments
+  [255]
+  $ log 'expectsize(0:2, a)'
+  hg: parse error: expectsize requires a size range or a positive integer
+  [255]
+  $ log 'expectsize(0:2, 3)'
+  0
+  1
+  2
+
+  $ log 'expectsize(2:0, 3)'
+  2
+  1
+  0
+  $ log 'expectsize(0:1, 1)'
+  abort: revset size mismatch. expected 1, got 2!
+  [255]
+  $ log 'expectsize(0:4, -1)'
+  hg: parse error: negative size
+  [255]
+  $ log 'expectsize(0:2, 2:4)'
+  0
+  1
+  2
+  $ log 'expectsize(0:1, 3:5)'
+  abort: revset size mismatch. expected between 3 and 5, got 2!
+  [255]
+  $ log 'expectsize(0:1, -1:2)'
+  hg: parse error: negative size
+  [255]
+  $ log 'expectsize(0:1, 1:-2)'
+  hg: parse error: negative size
+  [255]
+  $ log 'expectsize(0:2, a:4)'
+  hg: parse error: size range bounds must be integers
+  [255]
+  $ log 'expectsize(0:2, 2:b)'
+  hg: parse error: size range bounds must be integers
+  [255]
+  $ log 'expectsize(0:2, 2:)'
+  0
+  1
+  2
+  $ log 'expectsize(0:2, :5)'
+  0
+  1
+  2
+  $ log 'expectsize(0:2, :)'
+  0
+  1
+  2
+  $ log 'expectsize(0:2, 4:)'
+  abort: revset size mismatch. expected between 4 and 11, got 3!
+  [255]
+  $ log 'expectsize(0:2, :2)'
+  abort: revset size mismatch. expected between 0 and 2, got 3!
+  [255]