# HG changeset patch
# User Martin von Zweigbergk <martinvonz@google.com>
# Date 2018-03-28 16:36:02
# Node ID 503f936489dd43423ad8124326beb99f0f57d0a5
# Parent  df0873ab5c14ea879ef41f08b3f0ba7518c53d1b

lookup: add option to disambiguate prefix within revset

When resolving a nodeid prefix that is not unique within the repo and
the user has configured a revset that they want to disambiguate
within, we now try to look up within that revset before we fail. If
there is a unique match within the revset, we use that.

This is of course most effective at allowing a short prefix if the
revset contains few nodes. For most of our internal users at Google,
"not public()" is sufficiently small that a hex digit or two is
enough.

The implementation is currently pretty slow, but good enough for small
revsets (which is the expected use case). The scan in the revset is
linear. We may want to use a prefix tree if we want to allow users to
use a larger revset.

Credit for the idea goes to Kyle Lippincott.

Differential Revision: https://phab.mercurial-scm.org/D4037

diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -590,6 +590,9 @@ coreconfigitem('experimental', 'removeem
 coreconfigitem('experimental', 'revlogv2',
     default=None,
 )
+coreconfigitem('experimental', 'revisions.disambiguatewithin',
+    default=None,
+)
 coreconfigitem('experimental', 'single-head-per-branch',
     default=False,
 )
diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -437,9 +437,26 @@ def formatrevnode(ui, rev, node):
     return '%d:%s' % (rev, hexfunc(node))
 
 def resolvehexnodeidprefix(repo, prefix):
-    # Uses unfiltered repo because it's faster when prefix is ambiguous/
-    # This matches the shortesthexnodeidprefix() function below.
-    node = repo.unfiltered().changelog._partialmatch(prefix)
+    try:
+        # Uses unfiltered repo because it's faster when prefix is ambiguous/
+        # This matches the shortesthexnodeidprefix() function below.
+        node = repo.unfiltered().changelog._partialmatch(prefix)
+    except error.AmbiguousPrefixLookupError:
+        revset = repo.ui.config('experimental', 'revisions.disambiguatewithin')
+        if revset:
+            # Clear config to avoid infinite recursion
+            configoverrides = {('experimental',
+                                'revisions.disambiguatewithin'): None}
+            with repo.ui.configoverride(configoverrides):
+                revs = repo.anyrevs([revset], user=True)
+                matches = []
+                for rev in revs:
+                    node = repo.changelog.node(rev)
+                    if hex(node).startswith(prefix):
+                        matches.append(node)
+                if len(matches) == 1:
+                    return matches[0]
+        raise
     if node is None:
         return
     repo.changelog.rev(node)  # make sure node isn't filtered
diff --git a/tests/test-revisions.t b/tests/test-revisions.t
new file mode 100644
--- /dev/null
+++ b/tests/test-revisions.t
@@ -0,0 +1,37 @@
+  $ hg init repo
+  $ cd repo
+
+  $ echo 0 > a
+  $ hg ci -qAm 0
+  $ for i in 5 8 14 43; do
+  >   hg up -q 0
+  >   echo $i > a
+  >   hg ci -qm $i
+  > done
+  $ cat <<EOF >> .hg/hgrc
+  > [alias]
+  > l = log -T '{rev}:{shortest(node,1)}\n'
+  > EOF
+
+  $ hg l
+  4:7ba5d
+  3:7ba57
+  2:72
+  1:9
+  0:b
+  $ cat <<EOF >> .hg/hgrc
+  > [experimental]
+  > revisions.disambiguatewithin=:3
+  > EOF
+9 was unambiguous and still is
+  $ hg l -r 9
+  1:9
+7 was ambiguous and still is
+  $ hg l -r 7
+  abort: 00changelog.i@7: ambiguous identifier!
+  [255]
+7b is no longer ambiguous
+  $ hg l -r 7b
+  3:7ba57
+
+  $ cd ..