From 0a6c338f3fb637c8d04a72aed92e809f61f9c2d2 Mon Sep 17 00:00:00 2001
From: Oleg Broytman <phd@phdru.name>
Date: Sun, 8 Apr 2018 21:41:46 +0300
Subject: [PATCH] Feat(web): Do not allow to run two instances of `ml-web.py`

The second instance redirects browser to the first one and exits.
---
 docs-ru/news.rst         |  4 ++++
 docs/news.rst            |  3 +++
 m_librarian/web/utils.py | 43 +++++++++++++++++++++++++++++++++++++++-
 scripts/ml-web.py        | 13 ++++++++----
 tmp/.gitignore           |  2 ++
 5 files changed, 60 insertions(+), 5 deletions(-)
 create mode 100644 tmp/.gitignore

diff --git a/docs-ru/news.rst b/docs-ru/news.rst
index 19b1ad4..3d0e16d 100644
--- a/docs-ru/news.rst
+++ b/docs-ru/news.rst
@@ -7,5 +7,9 @@ Version 0.1.0 (2018-04-08)
 * Web-интерфейс для поиска авторов, показа списка книг автора,
   загрузки одной книги.
 
+* Программа `ml-web.py` не позволяет запускать два экземпляра.
+  Второй экземпляр перенаправляет web-обозреватель на web-интерфейс
+  первого экземпляра и прекращает работу.
+
 * Опция `--download-to` в скрипте `ml-search.py` задаёт директорию
   для сохранения книг.
diff --git a/docs/news.rst b/docs/news.rst
index 80498f1..6042e69 100644
--- a/docs/news.rst
+++ b/docs/news.rst
@@ -7,6 +7,9 @@ Version 0.1.0 (2018-04-08)
 * Web-interface: Search for authors, list books by an author,
   download a book.
 
+* Do not allow to run two instances of `ml-web.py` web-interfaces.
+  The second instance redirects browser to the first one and exits.
+
 * Option `--download-to` provides the path to the download directory
   in script `ml-search.py`.
 
diff --git a/m_librarian/web/utils.py b/m_librarian/web/utils.py
index 2efe724..12be81f 100644
--- a/m_librarian/web/utils.py
+++ b/m_librarian/web/utils.py
@@ -1,7 +1,48 @@
+from fcntl import flock, LOCK_EX, LOCK_UN, LOCK_NB
+from os import path, remove
+import socket
+
+
+lock_fname = path.join(
+    path.dirname(path.dirname(path.dirname(__file__))),
+    'tmp', 'm_librarian.lock')
+
+
+def get_lock(port):
+    try:
+        lock_file = open(lock_fname, 'r')
+    except IOError:  # no lock file
+        pass
+    else:
+        try:
+            flock(lock_file, LOCK_EX | LOCK_NB)
+        except IOError:  # locked
+            port = int(lock_file.readline())
+            lock_file.close()
+            return None, port
+        else:
+            flock(lock_file, LOCK_UN)
+            lock_file.close()
+
+    lock_file = open(lock_fname, 'w')
+    lock_file.write(str(port))
+    lock_file.close()
+    lock_file = open(lock_fname, 'r')
+    flock(lock_file, LOCK_EX | LOCK_NB)
+    return lock_file, None
+
+
+def close_lock(lock_file):
+    flock(lock_file, LOCK_UN)
+    lock_file.close()
+    lock_file = open(lock_fname, 'w')
+    lock_file.write('')
+    lock_file.close()
+    remove(lock_fname)
+
 
 def get_open_port():
     # https://stackoverflow.com/a/2838309/7976758
-    import socket
     s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
     s.bind(("", 0))
     # s.listen(1)
diff --git a/scripts/ml-web.py b/scripts/ml-web.py
index bccac81..0387bc6 100755
--- a/scripts/ml-web.py
+++ b/scripts/ml-web.py
@@ -9,7 +9,7 @@ from bottle import thread  # portable import
 from m_librarian.db import open_db
 import m_librarian.web.app  # noqa: F401 imported but unused
 from m_librarian.web.server import run_server
-from m_librarian.web.utils import get_open_port
+from m_librarian.web.utils import get_lock, close_lock, get_open_port
 
 
 def start_browser(port):
@@ -27,6 +27,11 @@ if __name__ == '__main__':
     else:
         port = get_open_port()
 
-    open_db()
-    thread.start_new_thread(start_browser, (port,))
-    run_server(port=port)
+    lock_file, old_port = get_lock(port)
+    if lock_file:
+        open_db()
+        thread.start_new_thread(start_browser, (port,))
+        run_server(port=port)
+        close_lock(lock_file)
+    else:  # Another instance of the program is being run at a different port
+        start_browser(old_port)
diff --git a/tmp/.gitignore b/tmp/.gitignore
new file mode 100644
index 0000000..120f485
--- /dev/null
+++ b/tmp/.gitignore
@@ -0,0 +1,2 @@
+*
+!/.gitignore
-- 
2.39.2