"current" is implicitly added to stubs.versions

So far, when a library `libfoo` has `stubs.versions: ["10", "11"]`, then
`shared_libs: ["libfoo"]` is linked to the version 11 of the stub.

This requires the author of `libfoo` to manually update the property
whenever a new version is introduced. Otherwise, clients are not able
to use the newly added APIs because the latest stub is for an old
version.

This change eliminates the need for manual updating. "current" version
is always implicitly added to `stubs.versions`. It is added even when
nothing is set on the property, if `stubs.symbol_file` is set. i.e.

```
cc_library {
    name: "libfoo",
    stubs: {
        symbol_file: "libfoo.map.txt",
	// no versions: [...] needed
    },
}

cc_library {
    name: "a_client",
    shared_libs: ["libfoo"],
    apex_available: ["myapex"],
    min_sdk_version: "29",
}

apex {
    name: "myapex",
    native_shared_libraries: ["a_client"],
    min_sdk_version: "29",
}
```

`a_client` links to the "current" stub of `libfoo` that has all symbols
shown in the map file.

Note that, above doesn't mean that the client has unlimited access to
APIs that are introduced even after the min_sdk_version of the client
(29 in this example). The use of such APIs still has to be guarded with
`__builtin_available` check.

Bug: N/A
Test: m
Change-Id: I70bb1600c18e74d36c6b24c3569d2149f02aaf96

cc/library.go

@@ -64,7 +64,8 @@ type LibraryProperties struct {
 		// symbols that are exported for stubs variant of this library.
 		Symbol_file *string `android:"path"`
 
-		// List versions to generate stubs libs for.
+		// List versions to generate stubs libs for. The version name "current" is always
+		// implicitly added.
 		Versions []string
 	}
 
@@ -170,6 +171,8 @@ type LibraryMutatedProperties struct {
 
 	// This variant is a stubs lib
 	BuildStubs bool `blueprint:"mutated"`
+	// This variant is the latest version
+	IsLatestVersion bool `blueprint:"mutated"`
 	// Version of the stubs lib
 	StubsVersion string `blueprint:"mutated"`
 	// List of all stubs versions associated with an implementation lib
@@ -777,7 +780,7 @@ type libraryInterface interface {
 
 type versionedInterface interface {
 	buildStubs() bool
-	setBuildStubs()
+	setBuildStubs(isLatest bool)
 	hasStubsVariants() bool
 	setStubsVersion(string)
 	stubsVersion() string
@@ -1495,7 +1498,7 @@ func (library *libraryDecorator) install(ctx ModuleContext, file android.Path) {
 			if ctx.isVndk() && !ctx.IsVndkExt() {
 				return
 			}
-		} else if len(library.Properties.Stubs.Versions) > 0 && !ctx.Host() && ctx.directlyInAnyApex() {
+		} else if library.hasStubsVariants() && !ctx.Host() && ctx.directlyInAnyApex() {
 			// Bionic libraries (e.g. libc.so) is installed to the bootstrap subdirectory.
 			// The original path becomes a symlink to the corresponding file in the
 			// runtime APEX.
@@ -1611,11 +1614,29 @@ func (library *libraryDecorator) symbolFileForAbiCheck(ctx ModuleContext) *strin
 }
 
 func (library *libraryDecorator) hasStubsVariants() bool {
-	return len(library.Properties.Stubs.Versions) > 0
+	// Just having stubs.symbol_file is enough to create a stub variant. In that case
+	// the stub for the future API level is created.
+	return library.Properties.Stubs.Symbol_file != nil ||
+		len(library.Properties.Stubs.Versions) > 0
 }
 
 func (library *libraryDecorator) stubsVersions(ctx android.BaseMutatorContext) []string {
-	return library.Properties.Stubs.Versions
+	if !library.hasStubsVariants() {
+		return nil
+	}
+
+	// Future API level is implicitly added if there isn't
+	vers := library.Properties.Stubs.Versions
+	if inList(android.FutureApiLevel.String(), vers) {
+		return vers
+	}
+	// In some cases, people use the raw value "10000" in the versions property.
+	// We shouldn't add the future API level in that case, otherwise there will
+	// be two identical versions.
+	if inList(strconv.Itoa(android.FutureApiLevel.FinalOrFutureInt()), vers) {
+		return vers
+	}
+	return append(vers, android.FutureApiLevel.String())
 }
 
 func (library *libraryDecorator) setStubsVersion(version string) {
@@ -1626,8 +1647,9 @@ func (library *libraryDecorator) stubsVersion() string {
 	return library.MutatedProperties.StubsVersion
 }
 
-func (library *libraryDecorator) setBuildStubs() {
+func (library *libraryDecorator) setBuildStubs(isLatest bool) {
 	library.MutatedProperties.BuildStubs = true
+	library.MutatedProperties.IsLatestVersion = isLatest
 }
 
 func (library *libraryDecorator) setAllStubsVersions(versions []string) {
@@ -1639,8 +1661,7 @@ func (library *libraryDecorator) allStubsVersions() []string {
 }
 
 func (library *libraryDecorator) isLatestStubVersion() bool {
-	versions := library.Properties.Stubs.Versions
-	return versions[len(versions)-1] == library.stubsVersion()
+	return library.MutatedProperties.IsLatestVersion
 }
 
 func (library *libraryDecorator) availableFor(what string) bool {
@@ -1883,7 +1904,8 @@ func createVersionVariations(mctx android.BottomUpMutatorContext, versions []str
 			c.stl = nil
 			c.Properties.PreventInstall = true
 			lib := moduleLibraryInterface(m)
-			lib.setBuildStubs()
+			isLatest := i == (len(versions) - 1)
+			lib.setBuildStubs(isLatest)
 
 			if variants[i] != "" {
 				// A non-LLNDK stubs module is hidden from make and has a dependency from the