se_compat_cil: Prepend generated files with a header

to ensure the file size is greater than 0, as secilc cannot handle
zero-sized cil files.

Fixes: 185256986
Bug: 183362912
Test: Forrest re-run broken test
Change-Id: Ief3039d38728fbeff67c6e39d6b15bddb006e5f8
diff --git a/build/soong/compat_cil.go b/build/soong/compat_cil.go
index 230fdc3..5cc73f9 100644
--- a/build/soong/compat_cil.go
+++ b/build/soong/compat_cil.go
@@ -87,6 +87,15 @@
 
 	srcPaths := c.expandSeSources(ctx)
 	out := android.PathForModuleGen(ctx, c.Name())
+
+	// TODO(b/183362912): Patch secilc to handle empty cil files.
+	// Put a header so that the generated cil mustn't be empty.
+	header := android.PathForModuleGen(ctx, c.Name()+"_header")
+	rule := android.NewRuleBuilder(pctx, ctx)
+	rule.Command().Text("echo").Flag(proptools.ShellEscape(";; " + c.stem())).Text(">").Output(header)
+	rule.Build(c.Name()+"_header", "Generate cil header")
+	srcPaths = append(android.Paths{header}, srcPaths...)
+
 	ctx.Build(pctx, android.BuildParams{
 		Rule:        android.Cat,
 		Inputs:      srcPaths,