block: compile out assert_bdrv_graph_readable() by default

reader_count() is a performance bottleneck because the global
aio_context_list_lock mutex causes thread contention. Put this debugging
assertion behind a new ./configure --enable-debug-graph-lock option and
disable it by default.

The --enable-debug-graph-lock option is also enabled by the more general
--enable-debug option.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230501173443.153062-1-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/scripts/meson-buildoptions.sh b/scripts/meson-buildoptions.sh
index 34d82de..2805d1c 100644
--- a/scripts/meson-buildoptions.sh
+++ b/scripts/meson-buildoptions.sh
@@ -23,6 +23,8 @@
   printf "%s\n" '                           QEMU'
   printf "%s\n" '  --enable-cfi             Control-Flow Integrity (CFI)'
   printf "%s\n" '  --enable-cfi-debug       Verbose errors in case of CFI violation'
+  printf "%s\n" '  --enable-debug-graph-lock'
+  printf "%s\n" '                           graph lock debugging support'
   printf "%s\n" '  --enable-debug-mutex     mutex debugging support'
   printf "%s\n" '  --enable-debug-stack-usage'
   printf "%s\n" '                           measure coroutine stack usage'
@@ -254,6 +256,8 @@
     --datadir=*) quote_sh "-Ddatadir=$2" ;;
     --enable-dbus-display) printf "%s" -Ddbus_display=enabled ;;
     --disable-dbus-display) printf "%s" -Ddbus_display=disabled ;;
+    --enable-debug-graph-lock) printf "%s" -Ddebug_graph_lock=true ;;
+    --disable-debug-graph-lock) printf "%s" -Ddebug_graph_lock=false ;;
     --enable-debug-mutex) printf "%s" -Ddebug_mutex=true ;;
     --disable-debug-mutex) printf "%s" -Ddebug_mutex=false ;;
     --enable-debug-stack-usage) printf "%s" -Ddebug_stack_usage=true ;;