revset: add a 'descend' argument to followlines to return descendants

This is useful to follow changes in a block of lines forward in the history
(for instance, when one wants to find out how a function evolved from a point
in history).

We added a 'descend' parameter to followlines(), which defaults to False. If
True, followlines() returns descendants of startrev.

Because context.blockdescendants() does not follow renames, these are not
followed by the revset either, so history will end when a rename occurs (as
can be seen in tests).

mercurial/revset.py

@@ -901,17 +901,22 b' def _followfirst(repo, subset, x):'
     # of every revisions or files revisions.
     return _follow(repo, subset, x, '_followfirst', followfirst=True)
 
-@predicate('followlines(file, fromline:toline[, startrev=.])', safe=True)
+@predicate('followlines(file, fromline:toline[, startrev=., descend=False])',
+           safe=True)
 def followlines(repo, subset, x):
     """Changesets modifying `file` in line range ('fromline', 'toline').
 
     Line range corresponds to 'file' content at 'startrev' and should hence be
     consistent with file size. If startrev is not specified, working directory's
     parent is used.
+
+    By default, ancestors of 'startrev' are returned. If 'descend' is True,
+    descendants of 'startrev' are returned though renames are (currently) not
+    followed in this direction.
     """
     from . import context  # avoid circular import issues
 
-    args = getargsdict(x, 'followlines', 'file *lines startrev')
+    args = getargsdict(x, 'followlines', 'file *lines startrev descend')
     if len(args['lines']) != 1:
         raise error.ParseError(_("followlines requires a line range"))
 
@@ -939,9 +944,17 b' def followlines(repo, subset, x):'
     fromline, toline = util.processlinerange(fromline, toline)
 
     fctx = repo[rev].filectx(fname)
-    revs = (c.rev() for c, _linerange
-            in context.blockancestors(fctx, fromline, toline))
-    return subset & generatorset(revs, iterasc=False)
+    if args.get('descend', False):
+        rs = generatorset(
+            (c.rev() for c, _linerange
+             in context.blockdescendants(fctx, fromline, toline)),
+            iterasc=True)
+    else:
+        rs = generatorset(
+            (c.rev() for c, _linerange
+             in context.blockancestors(fctx, fromline, toline)),
+            iterasc=False)
+    return subset & rs
 
 @predicate('all()', safe=True)
 def getall(repo, subset, x):

tests/test-annotate.t

@@ -484,7 +484,9 b' annotate removed file'
   $ hg id -n
   20
 
-Test followlines() revset
+Test followlines() revset; we usually check both followlines(pat, range) and
+followlines(pat, range, descend=True) to make sure both give the same result
+when they should.
 
   $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 3:5)'
   16: baz:0
@@ -494,9 +496,11 b' Test followlines() revset'
   16: baz:0
   19: baz:3
   20: baz:4
-  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 3:5, startrev=.^)'
+  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 3:5, startrev=19)'
   16: baz:0
   19: baz:3
+  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 3:5, startrev=19, descend=True)'
+  20: baz:4
   $ printf "0\n0\n" | cat - baz > baz1
   $ mv baz1 baz
   $ hg ci -m 'added two lines with 0'
@@ -504,12 +508,16 b' Test followlines() revset'
   16: baz:0
   19: baz:3
   20: baz:4
+  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 3:5, descend=True, startrev=19)'
+  20: baz:4
   $ echo 6 >> baz
   $ hg ci -m 'added line 8'
   $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 5:7)'
   16: baz:0
   19: baz:3
   20: baz:4
+  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 3:5, startrev=19, descend=True)'
+  20: baz:4
   $ sed 's/3/3+/' baz > baz.new
   $ mv baz.new baz
   $ hg ci -m 'baz:3->3+'
@@ -518,6 +526,9 b' Test followlines() revset'
   19: baz:3
   20: baz:4
   23: baz:3->3+
+  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 3:5, startrev=19, descend=True)'
+  20: baz:4
+  23: baz:3->3+
   $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 1:2)'
   21: added two lines with 0
 
@@ -536,9 +547,13 b' renames are followed'
   20: baz:4
   23: baz:3->3+
   24: qux:4->4+
-  $ hg up 23 --quiet
+
+but are missed when following children
+  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 5:7, startrev=22, descend=True)'
+  23: baz:3->3+
 
 merge
+  $ hg up 23 --quiet
   $ echo 7 >> baz
   $ hg ci -m 'one more line, out of line range'
   created new head
@@ -581,6 +596,10 b' merge'
   28: merge from other side
   $ hg up 23 --quiet
 
+we are missing the branch with rename when following children
+  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 5:7, startrev=25, descend=True)'
+  26: baz:3+->3-
+
 check error cases
   $ hg log -r 'followlines()'
   hg: parse error: followlines takes at least 1 positional arguments