Database connection pooling

Reusing connections is a win with asynchronous transactions.  More
fine-tuning in the future of cache pages and such will help improve
Geary's database performance.

Also fixed a naming and namespace problem with Geary.DatabaseError
and a hole where, if a connection could not be opened (for resource
problems, most likely), the asynchronous transaction would never
complete.

src/CMakeLists.txt

@@ -45,11 +45,11 @@ engine/api/geary-special-folder-type.vala
 
 engine/common/common-message-data.vala
 
-engine/db/database-error.vala
 engine/db/db.vala
 engine/db/db-connection.vala
 engine/db/db-context.vala
 engine/db/db-database.vala
+engine/db/db-database-error.vala
 engine/db/db-result.vala
 engine/db/db-statement.vala
 engine/db/db-synchronous-mode.vala

src/engine/db/db-database-error.vala

@@ -4,7 +4,7 @@
  * (version 2.1 or later).  See the COPYING file in this distribution. 
  */
 
-public errordomain DatabaseError {
+public errordomain Geary.DatabaseError {
     GENERAL,
     OPEN_REQUIRED,
     BUSY,

src/engine/db/db-database.vala

@@ -40,6 +40,7 @@ public class Geary.Db.Database : Geary.Db.Context {
     private Connection? master_connection = null;
     private int outstanding_async_jobs = 0;
     private ThreadPool<TransactionAsyncJob>? thread_pool = null;
+    private Gee.LinkedList<Connection>? cx_pool = null;
     private unowned PrepareConnection? prepare_cb = null;
     
     public Database(File db_file) {
@@ -76,9 +77,13 @@ public class Geary.Db.Database : Geary.Db.Context {
         }
         
         if (threadsafe()) {
-            assert(thread_pool == null);
-            thread_pool = new ThreadPool<TransactionAsyncJob>.with_owned_data(on_async_job,
-                DEFAULT_MAX_CONCURRENCY, true);
+            if (thread_pool == null) {
+                thread_pool = new ThreadPool<TransactionAsyncJob>.with_owned_data(on_async_job,
+                    DEFAULT_MAX_CONCURRENCY, true);
+            }
+            
+            if (cx_pool == null)
+                cx_pool = new Gee.LinkedList<Connection>();
         } else {
             warning("SQLite not thread-safe: asynchronous queries will not be available");
         }
@@ -101,6 +106,9 @@ public class Geary.Db.Database : Geary.Db.Context {
         // drop the master connection, which holds a ref back to this object
         master_connection = null;
         
+        // As per the contract above, can't simply drop the thread and connection pools; that would
+        // be bad.
+        
         is_open = false;
     }
     
@@ -220,23 +228,37 @@ public class Geary.Db.Database : Geary.Db.Context {
     
     // This method must be thread-safe.
     private void on_async_job(owned TransactionAsyncJob job) {
-        // create connection for this thread
-        // TODO: Use connection pool -- *never* use master connection
+        // go to connection pool before creating a connection -- *never* use master connection for
+        // threaded operations
         Connection? cx = null;
-        try {
-            cx = open_connection();
-        } catch (Error err) {
-            debug("Warning: unable to open database connection to %s, cancelling AsyncJob: %s",
-                db_file.get_path(), err.message);
+        lock (cx_pool) {
+            cx = cx_pool.poll();
+        }
+        
+        Error? open_err = null;
+        if (cx == null) {
+            try {
+                cx = open_connection();
+            } catch (Error err) {
+                open_err = err;
+                debug("Warning: unable to open database connection to %s, cancelling AsyncJob: %s",
+                    db_file.get_path(), err.message);
+            }
         }
         
         if (cx != null)
             job.execute(cx);
+        else
+            job.failed(open_err);
         
         lock (outstanding_async_jobs) {
             assert(outstanding_async_jobs > 0);
             --outstanding_async_jobs;
         }
+        
+        lock (cx_pool) {
+            cx_pool.offer(cx);
+        }
     }
     
     public override Database? get_database() {

src/engine/db/db-transaction-async-job.vala

@@ -42,6 +42,18 @@ private class Geary.Db.TransactionAsyncJob : Object {
             caught_err = err;
         }
         
+        schedule_completion();
+    }
+    
+    // Called in background thread context
+    internal void failed(Error err) {
+        // store as a caught thread to report to original caller
+        caught_err = err;
+        
+        schedule_completion();
+    }
+    
+    private void schedule_completion() {
         // notify foreground thread of completion
         // because Idle doesn't hold a ref, manually keep this object alive
         ref();