Support pre-build-action hook in blueprint

This allows for a bazel-invocation hook in mixed builds, which allows
for mixed builds to take place in only a single pass, greatly improving
its performance and complexity.

Test: Conjunction with build/soong CL
Change-Id: If89fb56830b4eb06d3263d6ca6da7b285e7ba315

bootstrap/command.go

@@ -113,6 +113,13 @@ func RunBlueprint(args Args, stopBefore StopBefore, ctx *blueprint.Context, conf
 		return ninjaDeps
 	}
 
+	if ctx.BeforePrepareBuildActionsHook != nil {
+		err := ctx.BeforePrepareBuildActionsHook()
+		if err != nil {
+			fatalErrors([]error{err})
+		}
+	}
+
 	extraDeps, errs = ctx.PrepareBuildActions(config)
 	if len(errs) > 0 {
 		fatalErrors(errs)

context.go

@@ -75,6 +75,8 @@ type Context struct {
 	// Used for metrics-related event logging.
 	EventHandler *metrics.EventHandler
 
+	BeforePrepareBuildActionsHook func() error
+
 	moduleFactories     map[string]ModuleFactory
 	nameInterface       NameInterface
 	moduleGroups        []*moduleGroup
@@ -4095,6 +4097,10 @@ func (c *Context) EndEvent(name string) {
 	c.EventHandler.End(name)
 }
 
+func (c *Context) SetBeforePrepareBuildActionsHook(hookFn func() error) {
+	c.BeforePrepareBuildActionsHook = hookFn
+}
+
 func (c *Context) writeLocalBuildActions(nw *ninjaWriter,
 	defs *localBuildActions) error {