diff -r 86c9590fb015 app/controllers/wiki_controller.rb
--- a/app/controllers/wiki_controller.rb	Sun Jun 28 14:12:27 2009 +0200
+++ b/app/controllers/wiki_controller.rb	Tue Jun 30 20:21:43 2009 +0200
@@ -89,7 +89,13 @@
     end
   rescue ActiveRecord::StaleObjectError
     # Optimistic locking exception
-    flash[:error] = l(:notice_locking_conflict)
+    # Try to merge
+    if @content.merge
+      flash[:notice] = l(:notice_merged)
+      redirect_to :action => 'index', :id => @project, :page => @page.title
+    else
+      flash[:error] = l(:notice_locking_conflict)
+    end    
   end
   
   # rename a page
diff -r 86c9590fb015 app/models/wiki_content.rb
--- a/app/models/wiki_content.rb	Sun Jun 28 14:12:27 2009 +0200
+++ b/app/models/wiki_content.rb	Tue Jun 30 20:21:43 2009 +0200
@@ -92,4 +92,24 @@
                                               :conditions => ["wiki_content_id = ? AND version < ?", wiki_content_id, version])
     end
   end
+
+  def merge
+    transaction do 
+      parent_version = versions.find_by_version(version)
+      last_version = versions.last
+      begin
+        conflict, merged_text = Merger.merge(text, parent_version.text, last_version.text,
+                                             l(:label_merge_my), 
+                                             l(:label_merge_old), 
+                                             l(:label_merge_other,
+                                               :author_name => last_version.author.name))
+        self.text = merged_text
+        self.version = last_version.version
+        return (!conflict and save)
+      rescue Merger::DiffBinaryNotAvailable
+        return false
+      end
+    end
+  end
+
 end
diff -r 86c9590fb015 config/locales/en.yml
--- a/config/locales/en.yml	Sun Jun 28 14:12:27 2009 +0200
+++ b/config/locales/en.yml	Tue Jun 30 20:21:43 2009 +0200
@@ -121,6 +121,7 @@
   notice_successful_connection: Successful connection.
   notice_file_not_found: The page you were trying to access doesn't exist or has been removed.
   notice_locking_conflict: Data has been updated by another user.
+  notice_merged: "Another user edited this page in the meantime. An automatic merge occured."
   notice_not_authorized: You are not authorized to access this page.
   notice_email_sent: "An email was sent to {{value}}"
   notice_email_error: "An error occurred while sending mail ({{value}})"
@@ -678,6 +679,9 @@
   label_date_from_to: From {{start}} to {{end}}
   label_wiki_content_added: Wiki page added
   label_wiki_content_updated: Wiki page updated
+  label_merge_my: "My version"
+  label_merge_old: "Previous version"
+  label_merge_other: "{{author_name}}'s version"
   
   button_login: Login
   button_submit: Submit
diff -r 86c9590fb015 config/locales/fr.yml
--- a/config/locales/fr.yml	Sun Jun 28 14:12:27 2009 +0200
+++ b/config/locales/fr.yml	Tue Jun 30 20:21:43 2009 +0200
@@ -153,6 +153,7 @@
   notice_successful_connection: Connection réussie.
   notice_file_not_found: "La page à laquelle vous souhaitez accéder n'existe pas ou a été supprimée."
   notice_locking_conflict: Les données ont été mises à jour par un autre utilisateur. Mise à jour impossible.
+  notice_merged: "Un autre utilisateur a modifié la page en même temps que vous. Un assemblage automatique a donc eu lieu."
   notice_not_authorized: "Vous n'êtes pas autorisés à accéder à cette page."
   notice_email_sent: "Un email a été envoyé à {{value}}"
   notice_email_error: "Erreur lors de l'envoi de l'email ({{value}})"
@@ -708,6 +709,9 @@
   label_date_from_to: Du {{start}} au {{end}}
   label_wiki_content_added: Page wiki ajoutée
   label_wiki_content_updated: Page wiki mise à jour
+  label_merge_my: "Ma version"
+  label_merge_old: "Version précédente"
+  label_merge_other: "Version de {{author_name}}"
   
   button_login: Connexion
   button_submit: Soumettre
diff -r 86c9590fb015 lib/merger.rb
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/merger.rb	Tue Jun 30 20:21:43 2009 +0200
@@ -0,0 +1,63 @@
+module Merger
+  
+  class DiffBinaryNotAvailable < Exception
+  end
+
+
+  class << self    
+    private
+    
+    def shell_quote(str)
+      if RUBY_PLATFORM =~ /mswin/
+        '"' + str.gsub(/"/, '\\"') + '"'
+      else
+        "'" + str.gsub(/'/, "'\"'\"'") + "'"
+      end
+    end
+    
+    def diff3_command(*opts)
+      command = format("diff3 %s",
+                       opts.join(" "))
+      output = nil
+      IO.popen(command) do |io|
+        output = io.read  
+      end
+      raise "diff3 exited with an error" if $?.exitstatus > 1
+      return output
+    end
+    
+    def tempfile(content, &block)
+      Tempfile.open("redmine_wiki_merge") do |t_file|
+        t_file << content
+        t_file.open
+        yield t_file
+      end
+    end
+  end
+  
+  begin
+    diff3_command('-v').blank?
+    def self.merge(mine, old, other, mine_label, old_label, other_label)
+      tempfile(mine) do |my_file|
+        tempfile(old) do |old_file|
+          tempfile(other) do |other_file|
+            conflict = diff3_command("-a", "-x", shell_quote(my_file.path),
+                                     shell_quote(old_file.path),
+                                     shell_quote(other_file.path))
+            return !conflict.blank?, diff3_command("-a", "-m", "-E", 
+                                                   "-L #{shell_quote(mine_label)}",
+                                                   "-L #{shell_quote(old_label)}",
+                                                   "-L #{shell_quote(other_label)}",
+                                                   shell_quote(my_file.path),
+                                                   shell_quote(old_file.path),
+                                                   shell_quote(other_file.path))
+          end
+        end
+      end
+    end
+  rescue
+    def self.merge(*args)
+      raise "diff3 binary not found."
+    end
+  end
+end
diff -r 86c9590fb015 test/unit/merger_test.rb
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/unit/merger_test.rb	Tue Jun 30 20:21:43 2009 +0200
@@ -0,0 +1,10 @@
+require File.dirname(__FILE__) + '/../test_helper'
+
+class MergerTest < Test::Unit::TestCase
+
+  def test_simple_merge
+    assert_equal [false, "aa\n\nbb\n"], Merger.merge("a\n\nbb\n", "a\n\nb\n", "aa\n\nb\n",
+                                                     "my", "old", "yours")
+  end
+
+end