Use newer mysql_exec_statement API whenever possible

There are a very small number of statements which fail under this API, such as LOCK TABLE. In nearly every other case it provides an improved interface for correctness. For consistency in things like NULL handling, which are tricky to fully test otherwise, always try this API and fallback if it fails with the specific 'This command is not supported in the prepared statement protocol yet' message.

lib/DBDish/mysql/Connection.pm6

@@ -29,12 +29,12 @@ method prepare(Str $statement, *%args) {
     self.protect-connection: {
         with $!mysql-client.mysql_stmt_init -> $stmt {
             with self!handle-errors(
-            $stmt.mysql_stmt_prepare($statement, $statement.encode.bytes)
-            ) {
+                    $stmt.mysql_stmt_prepare($statement, $statement.encode.bytes)
+                    ) {
                 DBDish::mysql::StatementHandle.new(
-                    :$!mysql-client, :parent(self), :$stmt
-                    :$statement, :$!RaiseError, |%args
-                );
+                        :$!mysql-client, :parent(self), :$stmt
+                        :$statement, :$!RaiseError, |%args
+                        );
             } else { .fail }
         } else {
             self!set-err(-1, "Can't allocate memory");
@@ -44,15 +44,25 @@ method prepare(Str $statement, *%args) {
 
 # Override DBIish::Connection.execute as statements such as LOCK TABLE cannot
 # be prepared in MySQL.
-# Avoid looking into the query string by using a simple parameter count
-# and skipping the prepare step for queries without parameters.
 method execute(Str $statement, **@params, *%args) {
-    if @params.elems == 0 {
-        return DBDish::mysql::StatementHandle.new(
-                :$!mysql-client, :parent(self), :$statement, :$!RaiseError, |%args).execute;
-    } else {
+    try {
         # Copied from the DBIish::Connection.execute
         return self.prepare($statement, |%args).execute(|@params);
+
+        # A small number of statements cannot be executed using prepare/execute. Handle these
+        # by calling the older protocol. We try very hard to use the newer protocol first as
+        # it has several bug and minor behaviour fixes such as the handling of NULLs.
+        CATCH {
+            when X::DBDish::DBError {
+                when $_.code == 1 and $_.native-message eq 'This command is not supported in the prepared statement protocol yet' {
+                    return DBDish::mysql::StatementHandle.new(
+                            :$!mysql-client, :parent(self), :$statement, :$!RaiseError, |%args).execute;
+                }
+                default {
+                    .rethrow;
+                }
+            }
+        }
     }
 }
 

lib/DBDish/mysql/StatementHandle.pm6

@@ -141,7 +141,7 @@ method execute(*@params --> DBDish::StatementHandle) {
         }
         without self!handle-errors { .fail }
     }
-    else { # Unprepared path
+    else { # Unprepared path, seldome used.
         $!parent.protect-connection: {
             my $status = $!mysql-client.mysql_query($!statement)
                     and self!set-err($status, $!mysql-client.mysql_error).fail;