add a library to report build numbers without causing rebuilds

Allow native modules to specify use_version_lib, which will make
an android::build::GetBuildNumber() function available.  For host
builds, the function will return the build number at the time that
the module was linked.  For device modules it will return the
value of the ro.build.version.incremental property.

Bug: 71719137
Test: build_version_test
Test: m build_version_test && touch build/make/core/Makefile build/soong/cc/libbuildversion/tests/build_version_test.cpp && m build_version_test shows different build numbers for binary and library tests.
Change-Id: I6f7d40b7574bb8206866c4e39bad9c710c796e32

cc/linker.go

@@ -18,6 +18,7 @@ import (
 	"android/soong/android"
 	"fmt"
 
+	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 )
 
@@ -95,6 +96,9 @@ type BaseLinkerProperties struct {
 			Exclude_static_libs []string
 		}
 	}
+
+	// make android::build:GetBuildNumber() available containing the build ID.
+	Use_version_lib *bool `android:"arch_variant"`
 }
 
 func NewBaseLinker() *baseLinker {
@@ -136,6 +140,10 @@ func (linker *baseLinker) linkerDeps(ctx BaseModuleContext, deps Deps) Deps {
 	deps.ReexportSharedLibHeaders = append(deps.ReexportSharedLibHeaders, linker.Properties.Export_shared_lib_headers...)
 	deps.ReexportGeneratedHeaders = append(deps.ReexportGeneratedHeaders, linker.Properties.Export_generated_headers...)
 
+	if Bool(linker.Properties.Use_version_lib) {
+		deps.WholeStaticLibs = append(deps.WholeStaticLibs, "libbuildversion")
+	}
+
 	if ctx.useVndk() {
 		deps.SharedLibs = removeListFromList(deps.SharedLibs, linker.Properties.Target.Vendor.Exclude_shared_libs)
 		deps.ReexportSharedLibHeaders = removeListFromList(deps.ReexportSharedLibHeaders, linker.Properties.Target.Vendor.Exclude_shared_libs)
@@ -278,3 +286,31 @@ func (linker *baseLinker) link(ctx ModuleContext,
 	flags Flags, deps PathDeps, objs Objects) android.Path {
 	panic(fmt.Errorf("baseLinker doesn't know how to link"))
 }
+
+// Injecting version symbols
+// Some host modules want a version number, but we don't want to rebuild it every time.  Optionally add a step
+// after linking that injects a constant placeholder with the current version number.
+
+func init() {
+	pctx.HostBinToolVariable("symbolInjectCmd", "symbol_inject")
+}
+
+var injectVersionSymbol = pctx.AndroidStaticRule("injectVersionSymbol",
+	blueprint.RuleParams{
+		Command: "$symbolInjectCmd -i $in -o $out -s soong_build_number " +
+			"-from 'SOONG BUILD NUMBER PLACEHOLDER' -v $buildNumberFromFile",
+		CommandDeps: []string{"$symbolInjectCmd"},
+	},
+	"buildNumberFromFile")
+
+func (linker *baseLinker) injectVersionSymbol(ctx ModuleContext, in android.Path, out android.WritablePath) {
+	ctx.Build(pctx, android.BuildParams{
+		Rule:        injectVersionSymbol,
+		Description: "inject version symbol",
+		Input:       in,
+		Output:      out,
+		Args: map[string]string{
+			"buildNumberFromFile": ctx.Config().BuildNumberFromFile(),
+		},
+	})
+}