Make statement handle storage threadsafe

Connection objects retain a map of ongoing statements. With SQLite,
one can use a single database handle to execute queries from many
threads when it is in serialized mode (which is the default). This
works out OK with SQLite itself, however the handling of the map of
active statement handles was not threadsafe, and this could cause
segfaults.

This refactors how active statement handles are tracked to allow the
insertion of locking, and then adds that locking.

lib/DBDish/Connection.pm6

@@ -17,12 +17,16 @@ unit role DBDish::Connection does DBDish::ErrorHandling;
 =head5 do
 =end pod
 
-has %.Statements;
+has %!statements;
+has Lock $!statements-lock .= new;
 has $.last-sth-id is rw;
 has $.last-rows is rw;
 
 method dispose() {
-    $_.dispose for %!Statements.values;
+    $!statements-lock.protect: {
+        $_.dispose for %!statements.values;
+        %!statements = ();
+    }
     self._disconnect;
     ?($.parent.Connections{self.WHICH}:delete);
 }
@@ -47,7 +51,7 @@ method prepare(Str $statement, *%args) { ... }
 
 method do(Str $statement, *@params, *%args) {
     LEAVE {
-        with %!Statements{$!last-sth-id} {
+        with $!statements-lock.protect({ %!statements{$!last-sth-id} }) {
             warn "'do' should not be used for statements that return rows"
             unless .Finished;
             .dispose;
@@ -65,14 +69,31 @@ method do(Str $statement, *@params, *%args) {
 
 method rows {
     if $!last-sth-id {
-        with %!Statements{$!last-sth-id} {
+        with $!statements-lock.protect({ %!statements{$!last-sth-id} }) {
             .rows;
         } else {
             $!last-rows
         }
     }
 }
 
+method register-statement-handle($handle) {
+    $!statements-lock.protect: {
+        %!statements{$handle.WHICH} = $handle;
+    }
+}
+
+method unregister-statement-handle($handle) {
+    $!statements-lock.protect: {
+        %!statements{$handle.WHICH}:delete;
+    }
+}
+
+method Statements() {
+    # Defensive copy, since %!statements access must be done under lock
+    $!statements-lock.protect: { %!statements.clone }
+}
+
 =begin pod
 =head5 quote-identifier
 

lib/DBDish/StatementHandle.pm6

@@ -52,13 +52,13 @@ method !done-execute(Int $rows, $fields) {
 
 method new(*%args) {
     my \sth = ::?CLASS.bless(|%args);
-    %args<parent>.Statements{sth.WHICH} = sth;
+    %args<parent>.register-statement-handle(sth)
 }
 
 method dispose() {
     self.finish unless $!Finished;
     self._free;
-    with $.parent.Statements{self.WHICH}:delete {
+    with $.parent.unregister-statement-handle(self) {
         $.parent.last-rows = self.rows;
         True;
     } else { False };

lib/DBDish/mysql/Connection.pm6

@@ -39,8 +39,8 @@ method execute(Str $statement, *%args) {
 }
 
 method insert-id() {
-    with $!last-sth-id andthen %!Statements{$_} {
-    .insert-id;
+    with $!last-sth-id andthen $!statements-lock.protect({ %!statements{$_} }) {
+        .insert-id;
     }
 }