fix(android): say what the sdk lookup checked, not just to set ANDROID_HOME

internal/android/android.go

@@ -247,7 +247,9 @@ func EnvWithAndroidPlatformTools(env []string) []string {
 // AdbBinary locates the adb binary via PATH or known Android SDK locations.
 func AdbBinary() (string, error) { return findAndroidTool("adb", "platform-tools") }
 
-func emulatorBinary() (string, error) { return findAndroidTool("emulator", "emulator") }
+// EmulatorBinary locates the emulator binary via PATH or known Android SDK
+// locations.
+func EmulatorBinary() (string, error) { return findAndroidTool("emulator", "emulator") }
 
 // findAndroidTool locates a binary from the Android SDK. It checks PATH,
 // then $ANDROID_HOME/<subdir>/<name> and $ANDROID_SDK_ROOT/<subdir>/<name>,
@@ -264,7 +266,36 @@ func findAndroidTool(name, subdir string) (string, error) {
 		}
 		tried = append(tried, candidate)
 	}
-	return "", fmt.Errorf("could not locate %q: not on PATH and not under any known Android SDK root (set $ANDROID_HOME to point at your SDK; tried %v)", name, tried)
+	return "", fmt.Errorf(
+		"%s not found: not on PATH, and not at [%s]; %s\nput %s on PATH, or point $ANDROID_HOME at an Android SDK that has %s/%s",
+		name, strings.Join(tried, ", "), sdkRootStatus(), name, subdir, name,
+	)
+}
+
+// sdkRootStatus reports what the SDK root variables hold, so a lookup failure
+// says whether they were unset or pointed somewhere that is not an SDK instead
+// of leaving the reader to work out which from a list of paths.
+func sdkRootStatus() string {
+	var reported []string
+	for _, variable := range []string{"ANDROID_HOME", "ANDROID_SDK_ROOT"} {
+		value := os.Getenv(variable)
+		if value == "" {
+			continue
+		}
+		info, err := os.Stat(value)
+		switch {
+		case err != nil:
+			reported = append(reported, fmt.Sprintf("$%s=%s does not exist", variable, value))
+		case !info.IsDir():
+			reported = append(reported, fmt.Sprintf("$%s=%s is not a directory", variable, value))
+		default:
+			reported = append(reported, fmt.Sprintf("$%s=%s", variable, value))
+		}
+	}
+	if len(reported) == 0 {
+		return "$ANDROID_HOME and $ANDROID_SDK_ROOT are unset"
+	}
+	return strings.Join(reported, ", ")
 }
 
 func androidSDKCandidates() []string {
@@ -283,11 +314,20 @@ func androidSDKCandidates() []string {
 		addRoot(filepath.Join(home, "Library", "Android", "sdk"))
 		addRoot(filepath.Join(home, "Android", "Sdk"))
 	}
-	addRoot("/opt/homebrew/share/android-commandlinetools")
-	addRoot("/usr/local/share/android-commandlinetools")
+	for _, root := range standardSDKRoots {
+		addRoot(root)
+	}
 	return roots
 }
 
+// standardSDKRoots are the install locations checked after the environment and
+// the home directory. A var so a resolution test can point it at a fixture
+// instead of whatever SDK the host running the test happens to have.
+var standardSDKRoots = []string{
+	"/opt/homebrew/share/android-commandlinetools",
+	"/usr/local/share/android-commandlinetools",
+}
+
 func listAdbDevices(ctx context.Context) ([]string, error) {
 	adb, err := AdbBinary()
 	if err != nil {
@@ -317,7 +357,7 @@ func parseAdbDevices(output string) []string {
 }
 
 func listAVDs(ctx context.Context) ([]string, error) {
-	emulator, err := emulatorBinary()
+	emulator, err := EmulatorBinary()
 	if err != nil {
 		return nil, err
 	}
@@ -382,7 +422,7 @@ func pickAVD(requested string, available []string) (string, error) {
 }
 
 func bootAVD(_ context.Context, name string) error {
-	emulator, err := emulatorBinary()
+	emulator, err := EmulatorBinary()
 	if err != nil {
 		return err
 	}

internal/android/android_test.go

@@ -1,6 +1,8 @@
 package android
 
 import (
+	"os"
+	"path/filepath"
 	"reflect"
 	"slices"
 	"strings"
@@ -136,6 +138,114 @@ func TestPickAVD_NoneAvailable(t *testing.T) {
 	}
 }
 
+// fakeSDK writes an SDK layout holding only the named tools ("emulator/emulator"),
+// so a lookup test never resolves against the host's own SDK.
+func fakeSDK(t *testing.T, tools ...string) string {
+	t.Helper()
+	root := t.TempDir()
+	for _, tool := range tools {
+		path := filepath.Join(root, filepath.FromSlash(tool))
+		if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+			t.Fatalf("mkdir %s: %v", filepath.Dir(path), err)
+		}
+		if err := os.WriteFile(path, nil, 0o755); err != nil {
+			t.Fatalf("write %s: %v", path, err)
+		}
+	}
+	return root
+}
+
+// isolateSDKLookup closes every route to the host's own SDK: an empty PATH, no
+// root variables, a home with nothing under it, and the standard install
+// locations replaced by the given roots. It returns the fake home directory.
+func isolateSDKLookup(t *testing.T, roots ...string) string {
+	t.Helper()
+	home := t.TempDir()
+	t.Setenv("PATH", t.TempDir())
+	t.Setenv("ANDROID_HOME", "")
+	t.Setenv("ANDROID_SDK_ROOT", "")
+	t.Setenv("HOME", home)
+	original := standardSDKRoots
+	t.Cleanup(func() { standardSDKRoots = original })
+	standardSDKRoots = roots
+	return home
+}
+
+func TestAdbBinary_ResolvesUnderStandardSDKRootWithAndroidHomeUnset(t *testing.T) {
+	sdk := fakeSDK(t, "platform-tools/adb")
+	isolateSDKLookup(t, sdk)
+
+	adb, err := AdbBinary()
+	if err != nil {
+		t.Fatalf("AdbBinary: %v", err)
+	}
+	if want := filepath.Join(sdk, "platform-tools", "adb"); adb != want {
+		t.Errorf("AdbBinary() = %q, want %q", adb, want)
+	}
+}
+
+func TestEmulatorBinary_ResolvesUnderStandardSDKRootWithAndroidHomeUnset(t *testing.T) {
+	sdk := fakeSDK(t, "emulator/emulator")
+	isolateSDKLookup(t, sdk)
+
+	emulator, err := EmulatorBinary()
+	if err != nil {
+		t.Fatalf("EmulatorBinary: %v", err)
+	}
+	if want := filepath.Join(sdk, "emulator", "emulator"); emulator != want {
+		t.Errorf("EmulatorBinary() = %q, want %q", emulator, want)
+	}
+}
+
+func TestAdbBinary_UsesAndroidHome(t *testing.T) {
+	sdk := fakeSDK(t, "platform-tools/adb")
+	isolateSDKLookup(t)
+	t.Setenv("ANDROID_HOME", sdk)
+
+	adb, err := AdbBinary()
+	if err != nil {
+		t.Fatalf("AdbBinary: %v", err)
+	}
+	if want := filepath.Join(sdk, "platform-tools", "adb"); adb != want {
+		t.Errorf("AdbBinary() = %q, want %q", adb, want)
+	}
+}
+
+func TestAdbBinary_NoSDKAnywhereReportsWhereItLooked(t *testing.T) {
+	empty := t.TempDir()
+	home := isolateSDKLookup(t, empty)
+
+	_, err := AdbBinary()
+	if err == nil {
+		t.Fatal("expected an error with no SDK anywhere")
+	}
+	for _, want := range []string{
+		"adb not found: not on PATH",
+		filepath.Join(home, "Library", "Android", "sdk", "platform-tools", "adb"),
+		filepath.Join(empty, "platform-tools", "adb"),
+		"$ANDROID_HOME and $ANDROID_SDK_ROOT are unset",
+		"put adb on PATH, or point $ANDROID_HOME at an Android SDK that has platform-tools/adb",
+	} {
+		if !strings.Contains(err.Error(), want) {
+			t.Errorf("error %q missing %q", err, want)
+		}
+	}
+}
+
+func TestAdbBinary_AndroidHomePointingNowhereIsNamed(t *testing.T) {
+	isolateSDKLookup(t, t.TempDir())
+	missing := filepath.Join(t.TempDir(), "no-such-sdk")
+	t.Setenv("ANDROID_HOME", missing)
+
+	_, err := AdbBinary()
+	if err == nil {
+		t.Fatal("expected an error when ANDROID_HOME points nowhere")
+	}
+	if want := "$ANDROID_HOME=" + missing + " does not exist"; !strings.Contains(err.Error(), want) {
+		t.Errorf("error %q must say %q rather than leaving it in the tried list", err, want)
+	}
+}
+
 func TestPathContains(t *testing.T) {
 	path := "/usr/bin:/opt/tools:/usr/local/bin"
 	if !pathContains(path, "/opt/tools") {