Support generating sdk snapshots that can be selected by Soong config vars

Allows the builder of an sdk snapshot to specify a Soong config
variable that can be used to control whether the snapshot's prebuilts
are used or not.

Bug: 193523070
Test: m nothing
Change-Id: Ib09500ba61befc1202dff61dc06847c730ba9253

sdk/update.go

@@ -35,6 +35,37 @@ import (
 //     By default every unversioned module in the generated snapshot has prefer: false. Building it
 //     with SOONG_SDK_SNAPSHOT_PREFER=true will force them to use prefer: true.
 //
+// SOONG_SDK_SNAPSHOT_USE_SOURCE_CONFIG_VAR
+//     If set this specifies the Soong config var that can be used to control whether the prebuilt
+//     modules from the generated snapshot or the original source modules. Values must be a colon
+//     separated pair of strings, the first of which is the Soong config namespace, and the second
+//     is the name of the variable within that namespace.
+//
+//     The config namespace and var name are used to set the `use_source_config_var` property. That
+//     in turn will cause the generated prebuilts to use the soong config variable to select whether
+//     source or the prebuilt is used.
+//     e.g. If an sdk snapshot is built using:
+//       m SOONG_SDK_SNAPSHOT_USE_SOURCE_CONFIG_VAR=acme:build_from_source sdkextensions-sdk
+//     Then the resulting snapshot will include:
+//       use_source_config_var: {
+//         config_namespace: "acme",
+//         var_name: "build_from_source",
+//       }
+//
+//     Assuming that the config variable is defined in .mk using something like:
+//       $(call add_soong_config_namespace,acme)
+//       $(call add_soong_config_var_value,acme,build_from_source,true)
+//
+//     Then when the snapshot is unpacked in the repository it will have the following behavior:
+//       m droid - will use the sdkextensions-sdk prebuilts if present. Otherwise, it will use the
+//           sources.
+//       m SOONG_CONFIG_acme_build_from_source=true droid - will use the sdkextensions-sdk
+//            sources, if present. Otherwise, it will use the prebuilts.
+//
+//     This is a temporary mechanism to control the prefer flags and will be removed once a more
+//     maintainable solution has been implemented.
+//     TODO(b/174997203): Remove when no longer necessary.
+//
 // SOONG_SDK_SNAPSHOT_VERSION
 //     This provides control over the version of the generated snapshot.
 //
@@ -760,6 +791,8 @@ func (t unversionedToVersionedTransformation) transformModule(module *bpModule)
 	module.insertAfter("name", "sdk_member_name", name)
 	// Remove the prefer property if present as versioned modules never need marking with prefer.
 	module.removeProperty("prefer")
+	// Ditto for use_source_config_var
+	module.removeProperty("use_source_config_var")
 	return module
 }
 
@@ -1627,13 +1660,24 @@ func (s *sdk) createMemberSnapshot(ctx *memberContext, member *sdkMember, bpModu
 		// snapshot to be created that sets prefer: true.
 		// TODO(b/174997203): Remove once the ability to select the modules to prefer can be done
 		//  dynamically at build time not at snapshot generation time.
-		prefer := ctx.sdkMemberContext.Config().IsEnvTrue("SOONG_SDK_SNAPSHOT_PREFER")
+		config := ctx.sdkMemberContext.Config()
+		prefer := config.IsEnvTrue("SOONG_SDK_SNAPSHOT_PREFER")
 
 		// Set prefer. Setting this to false is not strictly required as that is the default but it does
 		// provide a convenient hook to post-process the generated Android.bp file, e.g. in tests to
 		// check the behavior when a prebuilt is preferred. It also makes it explicit what the default
 		// behavior is for the module.
 		bpModule.insertAfter("name", "prefer", prefer)
+
+		configVar := config.Getenv("SOONG_SDK_SNAPSHOT_USE_SOURCE_CONFIG_VAR")
+		if configVar != "" {
+			parts := strings.Split(configVar, ":")
+			cfp := android.ConfigVarProperties{
+				Config_namespace: proptools.StringPtr(parts[0]),
+				Var_name:         proptools.StringPtr(parts[1]),
+			}
+			bpModule.insertAfter("prefer", "use_source_config_var", cfp)
+		}
 	}
 
 	// Group the variants by os type.