show: use consistent (and possibly shorter) node lengths

`hg show` makes heavy use of shortest() to limit the length of the node
hash.

For the "stack" and "work" views, you are often looking at multiple
lines of similar output for "lines" of work. It is visually appeasing
for things to vertically align. A naive use of {shortest(node, N)}
could result in variable length nodes and for the first character of
the description to vary by a column or two.

We implement a function to determine the longest shortest prefix for
a set of revisions. The new function is used to determine the printed
node length for all `hg show` views.

.. feature::

   show: use consistent node length in views

Our previous shortest node length of 5 was arbitrarily chosen.

shortest() already does the work of ensuring that a partial node
isn't ambiguous with an integer revision, which is our primary risk
of a collision for very short nodes. It should be safe to go with the
shortest node possible.

Existing code is also optimized to handle nodes as short as 4.

So, we decrease the minimum hash length from 5 to 4.

We also add a test demonstrating that prefix collisions increase the
node length.

.. feature::

   show: decrease minimum displayed hash length from 5 to 4

Differential Revision: https://phab.mercurial-scm.org/D558

hgext/show.py

@@ -161,9 +161,10 b' def showbookmarks(ui, repo, fm):'
             ui.write(_('(no bookmarks set)\n'))
         return
 
+    revs = [repo[node].rev() for node in marks.values()]
     active = repo._activebookmark
     longestname = max(len(b) for b in marks)
-    # TODO consider exposing longest shortest(node).
+    nodelen = longestshortest(repo, revs)
 
     for bm, node in sorted(marks.items()):
         fm.startitem()
@@ -172,7 +173,7 b' def showbookmarks(ui, repo, fm):'
         fm.write('node', fm.hexfunc(node), fm.hexfunc(node))
         fm.data(active=bm == active,
                 longestbookmarklen=longestname,
-                nodelen=5)
+                nodelen=nodelen)
 
 @showview('stack', csettopic='stack')
 def showstack(ui, repo, displayer):
@@ -236,6 +237,9 b' def showstack(ui, repo, displayer):'
     else:
         newheads = set()
 
+    allrevs = set(stackrevs) | newheads | set([baserev])
+    nodelen = longestshortest(repo, allrevs)
+
     try:
         cmdutil.findcmd('rebase', commands.table)
         haverebase = True
@@ -247,7 +251,7 b' def showstack(ui, repo, displayer):'
     # our simplicity and the customizations required.
     # TODO use proper graph symbols from graphmod
 
-    shortesttmpl = formatter.maketemplater(ui, '{shortest(node, 5)}')
+    shortesttmpl = formatter.maketemplater(ui, '{shortest(node, %d)}' % nodelen)
     def shortest(ctx):
         return shortesttmpl.render({'ctx': ctx, 'node': ctx.hex()})
 
@@ -278,7 +282,7 b' def showstack(ui, repo, displayer):'
                 ui.write('  ')
 
             ui.write(('o  '))
-            displayer.show(ctx, nodelen=5)
+            displayer.show(ctx, nodelen=nodelen)
             displayer.flush(ctx)
             ui.write('\n')
 
@@ -318,7 +322,7 b' def showstack(ui, repo, displayer):'
             ui.write('  ')
 
         ui.write(symbol, '  ')
-        displayer.show(ctx, nodelen=5)
+        displayer.show(ctx, nodelen=nodelen)
         displayer.flush(ctx)
         ui.write('\n')
 
@@ -335,7 +339,7 b' def showstack(ui, repo, displayer):'
         ui.write(_('(stack base)'), '\n', label='stack.label')
         ui.write(('o  '))
 
-        displayer.show(basectx, nodelen=5)
+        displayer.show(basectx, nodelen=nodelen)
         displayer.flush(basectx)
         ui.write('\n')
 
@@ -394,12 +398,13 b' def showwork(ui, repo, displayer):'
     """changesets that aren't finished"""
     # TODO support date-based limiting when calling revset.
     revs = repo.revs('sort(_underway(), topo)')
+    nodelen = longestshortest(repo, revs)
 
     revdag = graphmod.dagwalker(repo, revs)
 
     ui.setconfig('experimental', 'graphshorten', True)
     cmdutil.displaygraph(ui, repo, revdag, displayer, graphmod.asciiedges,
-                         props={'nodelen': 5})
+                         props={'nodelen': nodelen})
 
 def extsetup(ui):
     # Alias `hg <prefix><view>` to `hg show <view>`.
@@ -420,6 +425,27 b' def extsetup(ui):'
 
             ui.setconfig('alias', name, 'show %s' % view, source='show')
 
+def longestshortest(repo, revs, minlen=4):
+    """Return the length of the longest shortest node to identify revisions.
+
+    The result of this function can be used with the ``shortest()`` template
+    function to ensure that a value is unique and unambiguous for a given
+    set of nodes.
+
+    The number of revisions in the repo is taken into account to prevent
+    a numeric node prefix from conflicting with an integer revision number.
+    If we fail to do this, a value of e.g. ``10023`` could mean either
+    revision 10023 or node ``10023abc...``.
+    """
+    tmpl = formatter.maketemplater(repo.ui, '{shortest(node, %d)}' % minlen)
+    lens = [minlen]
+    for rev in revs:
+        ctx = repo[rev]
+        shortest = tmpl.render({'ctx': ctx, 'node': ctx.hex()})
+        lens.append(len(shortest))
+
+    return max(lens)
+
 # Adjust the docstring of the show command so it shows all registered views.
 # This is a bit hacky because it runs at the end of module load. When moved
 # into core or when another extension wants to provide a view, we'll need

tests/test-show-stack.t

@@ -17,7 +17,7 b' Stack displays single draft changeset as'
   $ echo 0 > foo
   $ hg -q commit -A -m 'commit 0'
   $ hg show stack
-    @  9f171 commit 0
+    @  9f17 commit 0
 
 Stack displays multiple draft changesets
 
@@ -30,48 +30,48 b' Stack displays multiple draft changesets'
   $ echo 4 > foo
   $ hg commit -m 'commit 4'
   $ hg show stack
-    @  2737b commit 4
-    o  d1a69 commit 3
-    o  128c8 commit 2
-    o  181cc commit 1
-    o  9f171 commit 0
+    @  2737 commit 4
+    o  d1a6 commit 3
+    o  128c commit 2
+    o  181c commit 1
+    o  9f17 commit 0
 
 Public parent of draft base is displayed, separated from stack
 
   $ hg phase --public -r 0
   $ hg show stack
-    @  2737b commit 4
-    o  d1a69 commit 3
-    o  128c8 commit 2
-    o  181cc commit 1
+    @  2737 commit 4
+    o  d1a6 commit 3
+    o  128c commit 2
+    o  181c commit 1
    /   (stack base)
-  o  9f171 commit 0
+  o  9f17 commit 0
 
   $ hg phase --public -r 1
   $ hg show stack
-    @  2737b commit 4
-    o  d1a69 commit 3
-    o  128c8 commit 2
+    @  2737 commit 4
+    o  d1a6 commit 3
+    o  128c commit 2
    /   (stack base)
-  o  181cc commit 1
+  o  181c commit 1
 
 Draft descendants are shown
 
   $ hg -q up 2
   $ hg show stack
-    o  2737b commit 4
-    o  d1a69 commit 3
-    @  128c8 commit 2
+    o  2737 commit 4
+    o  d1a6 commit 3
+    @  128c commit 2
    /   (stack base)
-  o  181cc commit 1
+  o  181c commit 1
 
   $ hg -q up 3
   $ hg show stack
-    o  2737b commit 4
-    @  d1a69 commit 3
-    o  128c8 commit 2
+    o  2737 commit 4
+    @  d1a6 commit 3
+    o  128c commit 2
    /   (stack base)
-  o  181cc commit 1
+  o  181c commit 1
 
 working dir on public changeset should display special message
 
@@ -89,10 +89,10 b' Branch point in descendants displayed at'
   $ hg show stack
    \ /  (multiple children)
     |
-    o  d1a69 commit 3
-    @  128c8 commit 2
+    o  d1a6 commit 3
+    @  128c commit 2
    /   (stack base)
-  o  181cc commit 1
+  o  181c commit 1
 
   $ cd ..
 
@@ -117,9 +117,9 b' Base is stopped at merges'
 TODO doesn't yet handle case where wdir is a draft merge
 
   $ hg show stack
-    @  8ee90 merge heads
+    @  8ee9 merge heads
    /   (stack base)
-  o  59478 head 1
+  o  5947 head 1
 
   $ echo d1 > foo
   $ hg commit -m 'draft 1'
@@ -127,10 +127,10 b" TODO doesn't yet handle case where wdir "
   $ hg commit -m 'draft 2'
 
   $ hg show stack
-    @  430d5 draft 2
-    o  787b1 draft 1
+    @  430d draft 2
+    o  787b draft 1
    /   (stack base)
-  o  8ee90 merge heads
+  o  8ee9 merge heads
 
   $ cd ..
 
@@ -156,36 +156,36 b' Now move on to stacks when there are mor'
 Newer draft heads don't impact output
 
   $ hg show stack
-    @  eaffc draft 2
-    o  2b218 draft 1
+    @  eaff draft 2
+    o  2b21 draft 1
    /   (stack base)
-  o  b66bb base
+  o  b66b base
 
 Newer public heads are rendered
 
   $ hg phase --public -r '::tip'
 
   $ hg show stack
-    o  baa4b new 2
+    o  baa4 new 2
    /    (2 commits ahead)
   :
   :    (stack head)
-  : @  eaffc draft 2
-  : o  2b218 draft 1
+  : @  eaff draft 2
+  : o  2b21 draft 1
   :/   (stack base)
-  o  b66bb base
+  o  b66b base
 
 If rebase is available, we show a hint how to rebase to that head
 
   $ hg --config extensions.rebase= show stack
-    o  baa4b new 2
-   /    (2 commits ahead; hg rebase --source 2b218 --dest baa4b)
+    o  baa4 new 2
+   /    (2 commits ahead; hg rebase --source 2b21 --dest baa4)
   :
   :    (stack head)
-  : @  eaffc draft 2
-  : o  2b218 draft 1
+  : @  eaff draft 2
+  : o  2b21 draft 1
   :/   (stack base)
-  o  b66bb base
+  o  b66b base
 
 Similar tests but for multiple heads
 
@@ -196,25 +196,25 b' Similar tests but for multiple heads'
   $ hg -q up 2
 
   $ hg show stack
-    o  baa4b new 2
+    o  baa4 new 2
    /    (2 commits ahead)
-  : o  9a848 new head 2
+  : o  9a84 new head 2
   :/    (1 commits ahead)
   :
   :    (stack head)
-  : @  eaffc draft 2
-  : o  2b218 draft 1
+  : @  eaff draft 2
+  : o  2b21 draft 1
   :/   (stack base)
-  o  b66bb base
+  o  b66b base
 
   $ hg --config extensions.rebase= show stack
-    o  baa4b new 2
-   /    (2 commits ahead; hg rebase --source 2b218 --dest baa4b)
-  : o  9a848 new head 2
-  :/    (1 commits ahead; hg rebase --source 2b218 --dest 9a848)
+    o  baa4 new 2
+   /    (2 commits ahead; hg rebase --source 2b21 --dest baa4)
+  : o  9a84 new head 2
+  :/    (1 commits ahead; hg rebase --source 2b21 --dest 9a84)
   :
   :    (stack head)
-  : @  eaffc draft 2
-  : o  2b218 draft 1
+  : @  eaff draft 2
+  : o  2b21 draft 1
   :/   (stack base)
-  o  b66bb base
+  o  b66b base

tests/test-show-work.t

@@ -16,20 +16,20 b' Single draft changeset shown'
   $ hg -q commit -A -m 'commit 0'
 
   $ hg show work
-  @  9f171 commit 0
+  @  9f17 commit 0
 
 Even when it isn't the wdir
 
   $ hg -q up null
 
   $ hg show work
-  o  9f171 commit 0
+  o  9f17 commit 0
 
 Single changeset is still there when public because it is a head
 
   $ hg phase --public -r 0
   $ hg show work
-  o  9f171 commit 0
+  o  9f17 commit 0
 
 A draft child will show both it and public parent
 
@@ -38,8 +38,8 b' A draft child will show both it and publ'
   $ hg commit -m 'commit 1'
 
   $ hg show work
-  @  181cc commit 1
-  o  9f171 commit 0
+  @  181c commit 1
+  o  9f17 commit 0
 
 Multiple draft children will be shown
 
@@ -47,16 +47,16 b' Multiple draft children will be shown'
   $ hg commit -m 'commit 2'
 
   $ hg show work
-  @  128c8 commit 2
-  o  181cc commit 1
-  o  9f171 commit 0
+  @  128c commit 2
+  o  181c commit 1
+  o  9f17 commit 0
 
 Bumping first draft changeset to public will hide its parent
 
   $ hg phase --public -r 1
   $ hg show work
-  @  128c8 commit 2
-  o  181cc commit 1
+  @  128c commit 2
+  o  181c commit 1
   |
   ~
 
@@ -68,10 +68,10 b' Multiple DAG heads will be shown'
   created new head
 
   $ hg show work
-  @  f0abc commit 3
-  | o  128c8 commit 2
+  @  f0ab commit 3
+  | o  128c commit 2
   |/
-  o  181cc commit 1
+  o  181c commit 1
   |
   ~
 
@@ -80,10 +80,10 b' Even when wdir is something else'
   $ hg -q up null
 
   $ hg show work
-  o  f0abc commit 3
-  | o  128c8 commit 2
+  o  f0ab commit 3
+  | o  128c commit 2
   |/
-  o  181cc commit 1
+  o  181c commit 1
   |
   ~
 
@@ -95,13 +95,13 b' Draft child shows public head (multiple '
   created new head
 
   $ hg show work
-  @  668ca commit 4
-  | o  f0abc commit 3
-  | | o  128c8 commit 2
+  @  668c commit 4
+  | o  f0ab commit 3
+  | | o  128c commit 2
   | |/
-  | o  181cc commit 1
+  | o  181c commit 1
   |/
-  o  9f171 commit 0
+  o  9f17 commit 0
 
   $ cd ..
 
@@ -126,11 +126,11 b' Branch name appears in output'
   $ hg commit -m 'commit 4'
 
   $ hg show work
-  @  f8dd3 (mybranch) commit 4
-  o  90cfc (mybranch) commit 3
-  | o  128c8 commit 2
+  @  f8dd (mybranch) commit 4
+  o  90cf (mybranch) commit 3
+  | o  128c commit 2
   |/
-  o  181cc commit 1
+  o  181c commit 1
   |
   ~
 
@@ -157,11 +157,11 b' Bookmark name appears in output'
   $ hg bookmark mybook
 
   $ hg show work
-  @  cac82 (mybook) commit 4
-  o  f0abc commit 3
-  | o  128c8 (@) commit 2
+  @  cac8 (mybook) commit 4
+  o  f0ab commit 3
+  | o  128c (@) commit 2
   |/
-  o  181cc commit 1
+  o  181c commit 1
   |
   ~
 
@@ -182,9 +182,9 b' Tags are rendered'
   $ hg tag 0.2
 
   $ hg show work
-  @  37582 Added tag 0.2 for changeset 6379c25b76f1
-  o  6379c (0.2) commit 3
-  o  a2ad9 Added tag 0.1 for changeset 6a75536ea0b1
+  @  3758 Added tag 0.2 for changeset 6379c25b76f1
+  o  6379 (0.2) commit 3
+  o  a2ad Added tag 0.1 for changeset 6a75536ea0b1
   |
   ~
 
@@ -205,15 +205,15 b' Multiple names on same changeset render '
   $ hg commit -m 'commit 2'
 
   $ hg show work
-  @  34834 (mybook) (mybranch) commit 2
-  o  97fcc commit 1
+  @  3483 (mybook) (mybranch) commit 2
+  o  97fc commit 1
 
 Multiple bookmarks on same changeset render properly
 
   $ hg book mybook2
   $ hg show work
-  @  34834 (mybook mybook2) (mybranch) commit 2
-  o  97fcc commit 1
+  @  3483 (mybook mybook2) (mybranch) commit 2
+  o  97fc commit 1
 
   $ cd ..
 
@@ -230,8 +230,38 b' Extra namespaces are rendered'
   $ hg commit -m 'commit 3'
 
   $ hg --config extensions.revnames=$TESTDIR/revnamesext.py show work
-  @  32f3e (r2) commit 3
-  o  6a755 (r1) commit 2
-  o  97fcc (r0) commit 1
+  @  32f3 (r2) commit 3
+  o  6a75 (r1) commit 2
+  o  97fc (r0) commit 1
 
   $ cd ..
+
+Prefix collision on hashes increases shortest node length
+
+  $ hg init hashcollision
+  $ cd hashcollision
+  $ echo 0 > a
+  $ hg -q commit -Am 0
+  $ for i in 17 1057 2857 4025; do
+  >   hg -q up 0
+  >   echo $i > a
+  >   hg -q commit -m $i
+  >   echo 0 > a
+  >   hg commit -m "$i commit 2"
+  > done
+
+  $ hg show work
+  @  cfd04 4025 commit 2
+  o  c562d 4025
+  | o  08048 2857 commit 2
+  | o  c5623 2857
+  |/
+  | o  6a6b6 1057 commit 2
+  | o  c5625 1057
+  |/
+  | o  96b4e 17 commit 2
+  | o  11424 17
+  |/
+  o  b4e73 0
+
+  $ cd ..

tests/test-show.t

@@ -95,8 +95,8 b' bookmarks view shows bookmarks in an ali'
   $ hg bookmark a-longer-bookmark
 
   $ hg show bookmarks
-  * a-longer-bookmark    7b570
-    book1                b757f
+  * a-longer-bookmark    7b57
+    book1                b757
 
 A custom bookmarks template works
 
@@ -113,14 +113,14 b' bookmarks JSON works'
     "bookmark": "a-longer-bookmark",
     "longestbookmarklen": 17,
     "node": "7b5709ab64cbc34da9b4367b64afff47f2c4ee83",
-    "nodelen": 5
+    "nodelen": 4
    },
    {
     "active": false,
     "bookmark": "book1",
     "longestbookmarklen": 17,
     "node": "b757f780b8ffd71267c6ccb32e0882d9d32a8cc0",
-    "nodelen": 5
+    "nodelen": 4
    }
   ]
 
@@ -138,19 +138,19 b' commands.show.aliasprefix aliases values'
   (no bookmarks set)
 
   $ hg --config commands.show.aliasprefix=sh shwork
-  @  7b570 commit for book2
-  o  b757f commit for book1
-  o  ba592 initial
+  @  7b57 commit for book2
+  o  b757 commit for book1
+  o  ba59 initial
 
   $ hg --config commands.show.aliasprefix='s sh' swork
-  @  7b570 commit for book2
-  o  b757f commit for book1
-  o  ba592 initial
+  @  7b57 commit for book2
+  o  b757 commit for book1
+  o  ba59 initial
 
   $ hg --config commands.show.aliasprefix='s sh' shwork
-  @  7b570 commit for book2
-  o  b757f commit for book1
-  o  ba592 initial
+  @  7b57 commit for book2
+  o  b757 commit for book1
+  o  ba59 initial
 
 The aliases don't appear in `hg config`