modes_kml: fix case where KML generation thread dies

SQLite does not deal well with concurrency.  Avoid database locked
errors by synchronizing database access using a threading.Lock().

python/modes_kml.py

@@ -93,14 +93,14 @@ class modes_kml(threading.Thread, modes_output_sql):
         #read the database and add KML
         q = "select distinct icao from positions where seen > datetime('now', '-5 minute')"
         c = self._db.cursor()
-        c.execute(q)
+        self.locked_execute(c, q)
         icaolist = c.fetchall()
         #now we have a list icaolist of all ICAOs seen in the last 5 minutes
 
         for icao in icaolist:
             #print "ICAO: %x" % icao
             q = "select * from positions where icao=%i and seen > datetime('now', '-2 hour') ORDER BY seen DESC" % icao
-            c.execute(q)
+            self.locked_execute(c, q)
             track = c.fetchall()
             #print "Track length: %i" % len(track)
             if len(track) != 0:
@@ -128,7 +128,7 @@ class modes_kml(threading.Thread, modes_output_sql):
 
             #now get metadata
             q = "select ident from ident where icao=%i" % icao
-            c.execute(q)
+            self.locked_execute(c, q)
             r = c.fetchall()
             if len(r) != 0:
                 ident = r[0][0]
@@ -136,7 +136,7 @@ class modes_kml(threading.Thread, modes_output_sql):
             #if ident is None: ident = ""
             #get most recent speed/heading/vertical
             q = "select seen, speed, heading, vertical from vectors where icao=%i order by seen desc limit 1" % icao
-            c.execute(q)
+            self.locked_execute(c, q)
             r = c.fetchall()
             if len(r) != 0:
                 seen = r[0][0]

python/modes_sql.py

@@ -19,7 +19,7 @@
 # Boston, MA 02110-1301, USA.
 # 
 
-import time, os, sys
+import time, os, sys, threading
 from string import split, join
 import modes_parse
 import sqlite3
@@ -28,6 +28,8 @@ from modes_exceptions import *
 class modes_output_sql(modes_parse.modes_parse):
   def __init__(self, mypos, filename):
     modes_parse.modes_parse.__init__(self, mypos)
+
+    self._lock = threading.Lock()
     #create the database
     self.db = sqlite3.connect(filename)
     #now execute a schema to create the tables you need
@@ -39,7 +41,7 @@ class modes_output_sql(modes_parse.modes_parse):
                 "lat"  REAL,
                 "lon"  REAL
             );"""
-    c.execute(query)
+    self.locked_execute(c, query)
     query = """CREATE TABLE IF NOT EXISTS "vectors" (
                 "icao"     INTEGER KEY NOT NULL,
                 "seen"     TEXT NOT NULL,
@@ -47,26 +49,31 @@ class modes_output_sql(modes_parse.modes_parse):
                 "heading"  REAL,
                 "vertical" REAL
             );"""
-    c.execute(query)
+    self.locked_execute(c, query)
     query = """CREATE TABLE IF NOT EXISTS "ident" (
                 "icao"     INTEGER PRIMARY KEY NOT NULL,
                 "ident"    TEXT NOT NULL
             );"""
-    c.execute(query)
+    self.locked_execute(c, query)
     c.close()
     self.db.commit()
 
   def __del__(self):
     self.db.close()
 
+  def locked_execute(self, c, query):
+    with self._lock:
+      c.execute(query)
+
   def output(self, message):
     try:
       query = self.make_insert_query(message)
       if query is not None:
-        c = self.db.cursor()
+        with self._lock:
-        c.execute(query)
+          c = self.db.cursor()
-        c.close()
+          c.execute(query)
-        self.db.commit()
+          c.close()
+          self.db.commit()
     except ADSBError:
       pass