Implement basic locking

4 files changed, 115 insertions, 86 deletions

diff --git a/lock.ur b/lock.ur
new file mode 100644
index 0000000..9accf81
--- /dev/null
+++ b/lock.ur
@@ -0,0 +1,27 @@
+(* Copyright 2015 the Massachusetts Institute of Technology
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License.  You may obtain a copy of the
+License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+CONDITIONS OF ANY KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations under the License. *)
+
+table lock : { Title : string,
+               Holder : client }
+  PRIMARY KEY Title
+
+fun take page_title =
+  new_holder <- self;
+  result <- tryDml (INSERT INTO lock (Title, Holder)
+                    VALUES ({[page_title]}, {[new_holder]}));
+  return (Option.isNone result)
+
+fun release page_title =
+  requestor <- self;
+  dml (DELETE FROM lock
+       WHERE Holder = {[requestor]})
diff --git a/lock.urs b/lock.urs
new file mode 100644
index 0000000..f7282cc
--- /dev/null
+++ b/lock.urs
@@ -0,0 +1,16 @@
+(* Copyright 2015 the Massachusetts Institute of Technology
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License.  You may obtain a copy of the
+License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+CONDITIONS OF ANY KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations under the License. *)
+
+val take : string -> transaction bool
+
+val release : string -> transaction unit
diff --git a/main.ur b/main.ur
index 4110c96..6478dd1 100644
--- a/main.ur
+++ b/main.ur
@@ -20,16 +20,11 @@ type id = int
 
 table commit : { Id : id,
                  Created : time,
+                 Title : string,
                  Content : string }
   PRIMARY KEY Id
 sequence commit_id_next
 
-table article : { Title : string,
-                  Head : id }
-  PRIMARY KEY Title,
-  CONSTRAINT Head
-    FOREIGN KEY Head REFERENCES commit(Id)
-
 datatype mode = View | Edit
 val eq_mode =
   mkEq (fn x y =>
@@ -44,90 +39,79 @@ fun only_in mode_source target_mode =
           then null
           else Style.invisible)
 
-fun commit_article title text : transaction unit =
+fun current_revision title =
+  commits <- queryL1 (SELECT commit.Created, commit.Title, commit.Content
+                      FROM commit
+                      WHERE commit.Title = {[title]}
+                      ORDER BY commit.Created ASC);
+  return (case commits of
+              Nil => {Title = title, Body = "Not found."}
+            | art :: _ => {Title = art.Title,
+                           Body = show art.Content})
+
+fun create_commit title text : transaction unit =
   id <- nextval commit_id_next;
   creation_time <- now;
-  dml (INSERT INTO commit (Id, Created, Content)
-       VALUES ({[id]}, {[creation_time]}, {[text]}));
-  (* TODO(bbaren): This is ugly.  Use CAS instead? *)
-  sql_error <- tryDml (INSERT INTO article (Title, Head)
-                       VALUES ({[title]}, {[id]}));
-  case sql_error of
-      None =>
-        (* We created a new article. *)
-        return ()
-    | Some _ =>
-        (* The article already exists. *)
-        dml (UPDATE article
-             SET Head = {[id]}
-             WHERE Title = {[title]})
+  dml (INSERT INTO commit (Id, Created, Title, Content)
+       VALUES ({[id]}, {[creation_time]}, {[title]}, {[text]}))
 
 fun wiki requested_article_title =
   (* Look up the article. *)
-  extant_articles <-
-    queryL (SELECT article.Title, commit.Content
-             FROM article LEFT JOIN commit ON article.Head = commit.Id
-             WHERE article.Title = {[requested_article_title]});
-  let
-    val article =
-      case extant_articles of
-          Nil => {Title = requested_article_title, Body = "Not found."}
-        | art :: Nil => {Title = art.Article.Title,
-                         Body = show art.Commit.Content}
-        | _ :: _ :: _ => error
-                           <xml>
-                             Multiple articles with title
-                             ‘{[requested_article_title]}’
-                           </xml>
-  in
-    (* Stuff the article text in a source so we can live-update it as the user
-    edits. *)
-    article_body_source <- source article.Body;
-    (* Initially, we're in View mode, and we can switch to Edit mode on user
-    request. *)
-    page_mode <- source View;
-    return
-      <xml>
-        <head>
-          <title>
-            {[if article.Title = Configuration.main_page_name
-              then Configuration.wiki_title
-              else article.Title ^ " – " ^ Configuration.wiki_title]}
-          </title>
-          <link rel="stylesheet" href="/urwiki.css" />
-        </head>
-        <body>
-          (* Page headings *)
-          <div>
-            <h1>{[Configuration.wiki_title]}</h1>
-            <ul>
-              <li>
-                <a link={wiki Configuration.main_page_name}>
-                  {[Configuration.main_page_name]}
-                </a>
-              </li>
-            </ul>
+  article <- current_revision requested_article_title;
+  (* Stuff the article text in a source so we can live-update it as the user
+  edits. *)
+  article_body_source <- source article.Body;
+  (* Initially, we're in View mode, and we can switch to Edit mode on user
+  request. *)
+  page_mode <- source View;
+  return
+    <xml>
+      <head>
+        <title>
+          {[if article.Title = Configuration.main_page_name
+            then Configuration.wiki_title
+            else article.Title ^ " – " ^ Configuration.wiki_title]}
+        </title>
+        <link rel="stylesheet" href="/urwiki.css" />
+      </head>
+      <body>
+        (* Page headings *)
+        <div>
+          <h1>{[Configuration.wiki_title]}</h1>
+          <ul>
+            <li>
+              <a link={wiki Configuration.main_page_name}>
+                {[Configuration.main_page_name]}
+              </a>
+            </li>
+          </ul>
+        </div>
+        (* Article *)
+        <dyn signal={text <- signal article_body_source;
+                     return <xml>{[text]}</xml>} /><br />
+        (* Editing panel *)
+        <div>
+          (* Controls for View mode *)
+          <div dynClass={only_in page_mode View}>
+            <button
+               value="Edit"
+               onclick={fn _ =>
+                          lock_result <- rpc (Lock.take article.Title);
+                          if lock_result
+                            then set page_mode Edit
+                            else return ()} />
           </div>
-          (* Article *)
-          <dyn signal={text <- signal article_body_source;
-                       return <xml>{[text]}</xml>} /><br />
-          (* Editing panel *)
-          <div>
-            (* Controls for View mode *)
-            <div dynClass={only_in page_mode View}>
-              <button value="Edit" onclick={fn _ => set page_mode Edit} />
-            </div>
-            (* Controls for Edit mode *)
-            <div dynClass={only_in page_mode Edit}>
-              <ctextarea source={article_body_source} /><br />
-              <button
-                 value="Commit"
-                 onclick={fn _ =>
-                            text <- get article_body_source;
-                            rpc (commit_article article.Title text);
-                            set page_mode View} />
-            </div>
+          (* Controls for Edit mode *)
+          <div dynClass={only_in page_mode Edit}>
+            <ctextarea source={article_body_source} /><br />
+            <button
+               value="Commit"
+               onclick={fn _ =>
+                          text <- get article_body_source;
+                          rpc (create_commit article.Title text);
+                          rpc (Lock.release article.Title);
+                          set page_mode View} />
           </div>
-        </body>
-      </xml>
-  end
+        </div>
+      </body>
+    </xml>
diff --git a/urwiki.urp b/urwiki.urp
index 48ffebb..066918c 100644
--- a/urwiki.urp
+++ b/urwiki.urp
@@ -1,5 +1,7 @@
 allow url /urwiki.css
 file /urwiki.css urwiki.css
 
+$/option
 style
+lock
 main