Show error for unsupported arm64 platforms (#347)

Fixes bazelbuild/bazel#16061

platforms/platforms.go

@@ -31,13 +31,17 @@ var supportedPlatforms = map[string]*platform{
 
 // GetPlatform returns a Bazel CI-compatible platform identifier for the current operating system.
 // TODO(fweikert): raise an error for unsupported platforms
-func GetPlatform() string {
+func GetPlatform() (string, error) {
 	platform := supportedPlatforms[runtime.GOOS]
 	arch := runtime.GOARCH
-	if arch == "arm64" && platform.HasArm64Binary {
-		return platform.Name + "_arm64"
+	if arch == "arm64" {
+		if platform.HasArm64Binary {
+			return platform.Name + "_arm64", nil
+		}
+		return "", fmt.Errorf("arm64 %s is unsupported", runtime.GOOS)
 	}
-	return platform.Name
+
+	return platform.Name, nil
 }
 
 // DetermineExecutableFilenameSuffix returns the extension for binaries on the current operating system.

repositories/gcs.go

@@ -236,7 +236,11 @@ func (gcs *GCSRepo) GetLastGreenCommit(bazeliskHome string, downstreamGreen bool
 // DownloadAtCommit downloads a Bazel binary built at the given commit into the specified location and returns the absolute path.
 func (gcs *GCSRepo) DownloadAtCommit(commit, destDir, destFile string) (string, error) {
 	log.Printf("Using unreleased version at commit %s", commit)
-	url := fmt.Sprintf("%s/%s/%s/bazel", nonCandidateBaseURL, platforms.GetPlatform(), commit)
+	platform, err := platforms.GetPlatform()
+	if err != nil {
+		return "", err
+	}
+	url := fmt.Sprintf("%s/%s/%s/bazel", nonCandidateBaseURL, platform, commit)
 	return httputil.DownloadBinary(url, destDir, destFile)
 }