Implement support for BAZELISK_FORMAT_URL

This new configuration setting provides a format-like string to compute the
URL from which to fetch Bazel.  Takes precedence over BAZELISK_BASE_URL as
this is a more general concept.

Fixes #423.

README.md

@@ -77,11 +77,21 @@ By default Bazelisk retrieves Bazel releases, release candidates and binaries bu
 
 As mentioned in the previous section, the `<FORK>/<VERSION>` version format allows you to use your own Bazel fork hosted on GitHub:
 
-If you want to create a fork with your own releases, you have to follow the naming conventions that we use in `bazelbuild/bazel` for the binary file names.
+If you want to create a fork with your own releases, you should follow the naming conventions that we use in `bazelbuild/bazel` for the binary file names as this results in predictable URLs that are similar to the official ones.
 The URL format looks like `https://github.com/<FORK>/bazel/releases/download/<VERSION>/<FILENAME>`.
 
 You can also override the URL by setting the environment variable `$BAZELISK_BASE_URL`. Bazelisk will then append `/<VERSION>/<FILENAME>` to the base URL instead of using the official release server. Bazelisk will read file [`~/.netrc`](https://everything.curl.dev/usingcurl/netrc) for credentials for Basic authentication.
 
+If for any reason none of this works, you can also override the URL format altogether by setting the environment variable `$BAZELISK_FORMAT_URL`. This variable takes a format-like string with placeholders and performs the following replacements to compute the download URL:
+
+- `%e`: Extension suffix, such as the empty string or `.exe`.
+- `%h`: Value of `BAZELISK_VERIFY_SHA256`, respecting uppercase/lowercase characters.
+- `%m`: Machine architecture name, such as `arm64` or `x86_64`.
+- `%o`: Operating system name, such as `darwin` or `linux`.
+- `%v`: Bazel version as determined by Bazelisk.
+- `%%`: Literal `%` for escaping purposes.
+- All other characters after `%` are reserved for future use and result in a processing error.
+
 ## Ensuring that your developers use Bazelisk rather than Bazel
 
 Bazel installers typically provide Bazel's [shell wrapper script] as the `bazel` on the PATH.

bazelisk_test.sh

@@ -197,7 +197,20 @@ function test_bazel_version_from_file() {
       (echo "FAIL: Expected to find 'Build label: 5.0.0' in the output of 'bazelisk version'"; exit 1)
 }
 
-function test_bazel_version_from_url() {
+function test_bazel_version_from_format_url() {
+  setup
+
+  echo "0.19.0" > .bazelversion
+
+  BAZELISK_FORMAT_URL="https://github.com/bazelbuild/bazel/releases/download/%v/bazel-%v-%o-%m%e" \
+      BAZELISK_HOME="$BAZELISK_HOME" \
+          bazelisk version 2>&1 | tee log
+
+  grep "Build label: 0.19.0" log || \
+      (echo "FAIL: Expected to find 'Build label: 0.19.0' in the output of 'bazelisk version'"; exit 1)
+}
+
+function test_bazel_version_from_base_url() {
   setup
 
   echo "0.19.0" > .bazelversion
@@ -427,8 +440,12 @@ if [[ $BAZELISK_VERSION == "GO" ]]; then
   test_bazel_last_rc
   echo
 
-  echo "# test_bazel_version_from_url"
-  test_bazel_version_from_url
+  echo "# test_bazel_version_from_format_url"
+  test_bazel_version_from_format_url
+  echo
+
+  echo "# test_bazel_version_from_base_url"
+  test_bazel_version_from_base_url
   echo
 
   echo "# test_bazel_version_prefer_environment_to_bazeliskrc"

core/BUILD

@@ -19,6 +19,9 @@ go_library(
 
 go_test(
     name = "go_default_test",
-    srcs = ["core_test.go"],
+    srcs = [
+        "core_test.go",
+        "repositories_test.go",
+    ],
     embed = [":go_default_library"],
 )

core/core.go

@@ -422,8 +422,14 @@ func downloadBazelIfNecessary(version string, baseDirectory string, repos *Repos
 	}
 
 	var tmpDestPath string
-	if url := GetEnvOrConfig(BaseURLEnv); url != "" {
-		tmpDestPath, err = repos.DownloadFromBaseURL(url, version, destDir, tmpDestFile)
+	baseURL := GetEnvOrConfig(BaseURLEnv)
+	formatURL := GetEnvOrConfig(FormatURLEnv)
+	if baseURL != "" && formatURL != "" {
+		return "", fmt.Errorf("cannot set %s and %s at once", BaseURLEnv, FormatURLEnv)
+	} else if formatURL != "" {
+		tmpDestPath, err = repos.DownloadFromFormatURL(formatURL, version, destDir, tmpDestFile)
+	} else if baseURL != "" {
+		tmpDestPath, err = repos.DownloadFromBaseURL(baseURL, version, destDir, tmpDestFile)
 	} else {
 		tmpDestPath, err = downloader(destDir, tmpDestFile)
 	}

core/repositories.go

@@ -13,6 +13,9 @@ import (
 const (
 	// BaseURLEnv is the name of the environment variable that stores the base URL for downloads.
 	BaseURLEnv = "BAZELISK_BASE_URL"
+
+	// FormatURLEnv is the name of the environment variable that stores the format string to generate URLs for downloads.
+	FormatURLEnv = "BAZELISK_FORMAT_URL"
 )
 
 // DownloadFunc downloads a specific Bazel binary to the given location and returns the absolute path.
@@ -232,6 +235,65 @@ func (r *Repositories) DownloadFromBaseURL(baseURL, version, destDir, destFile s
 	return httputil.DownloadBinary(url, destDir, destFile)
 }
 
+func BuildURLFromFormat(formatURL, version string) (string, error) {
+	osName, err := platforms.DetermineOperatingSystem()
+	if err != nil {
+		return "", err
+	}
+
+	machineName, err := platforms.DetermineArchitecture(osName, version)
+	if err != nil {
+		return "", err
+	}
+
+	var b strings.Builder
+	b.Grow(len(formatURL) * 2) // Approximation.
+	for i := 0; i < len(formatURL); i++ {
+		ch := formatURL[i]
+		if ch == '%' {
+			i++
+			if i == len(formatURL) {
+				return "", errors.New("trailing %")
+			}
+
+			ch = formatURL[i]
+			switch ch {
+			case 'e':
+				b.WriteString(platforms.DetermineExecutableFilenameSuffix())
+			case 'h':
+				b.WriteString(GetEnvOrConfig("BAZELISK_VERIFY_SHA256"))
+			case 'm':
+				b.WriteString(machineName)
+			case 'o':
+				b.WriteString(osName)
+			case 'v':
+				b.WriteString(version)
+			case '%':
+				b.WriteByte('%')
+			default:
+				return "", fmt.Errorf("unknown placeholder %%%c", ch)
+			}
+		} else {
+			b.WriteByte(ch)
+		}
+	}
+	return b.String(), nil
+}
+
+// DownloadFromFormatURL can download Bazel binaries from a specific URL while ignoring the predefined repositories.
+func (r *Repositories) DownloadFromFormatURL(formatURL, version, destDir, destFile string) (string, error) {
+	if formatURL == "" {
+		return "", fmt.Errorf("%s is not set", FormatURLEnv)
+	}
+
+	url, err := BuildURLFromFormat(formatURL, version)
+	if err != nil {
+		return "", err
+	}
+
+	return httputil.DownloadBinary(url, destDir, destFile)
+}
+
 // CreateRepositories creates a new Repositories instance with the given repositories. Any nil repository will be replaced by a dummy repository that raises an error whenever a download is attempted.
 func CreateRepositories(releases ReleaseRepo, candidates CandidateRepo, fork ForkRepo, commits CommitRepo, rolling RollingRepo, supportsBaseURL bool) *Repositories {
 	repos := &Repositories{supportsBaseURL: supportsBaseURL}

core/repositories_test.go

@@ -0,0 +1,78 @@
+package core
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/bazelbuild/bazelisk/platforms"
+)
+
+func TestBuildURLFromFormat(t *testing.T) {
+	osName, err := platforms.DetermineOperatingSystem()
+	if err != nil {
+		t.Fatalf("Cannot get operating system name: %v", err)
+	}
+
+	version := "6.0.0"
+
+	machineName, err := platforms.DetermineArchitecture(osName, version)
+	if err != nil {
+		t.Fatalf("Cannot get machine architecture name: %v", err)
+	}
+
+	suffix := platforms.DetermineExecutableFilenameSuffix()
+
+	previousSha256, hadSha256 := os.LookupEnv("BAZELISK_VERIFY_SHA256")
+	sha256 := "SomeSha256ValueThatIsIrrelevant"
+	if err := os.Setenv("BAZELISK_VERIFY_SHA256", sha256); err != nil {
+		t.Fatalf("Failed to set BAZELISK_VERIFY_SHA256")
+	}
+	defer func() {
+		if hadSha256 {
+			os.Setenv("BAZELISK_VERIFY_SHA256", previousSha256)
+		} else {
+			os.Unsetenv("BAZELISK_VERIFY_SHA256")
+		}
+	}()
+
+	type test struct {
+		format  string
+		want    string
+		wantErr error
+	}
+
+	tests := []test{
+		{format: "", want: ""},
+		{format: "no/placeholders", want: "no/placeholders"},
+
+		{format: "%", wantErr: errors.New("trailing %")},
+		{format: "%%", want: "%"},
+		{format: "%%%%", want: "%%"},
+		{format: "invalid/trailing/%", wantErr: errors.New("trailing %")},
+		{format: "escaped%%placeholder", want: "escaped%placeholder"},
+
+		{format: "foo-%e-bar", want: fmt.Sprintf("foo-%s-bar", suffix)},
+		{format: "foo-%h-bar", want: fmt.Sprintf("foo-%s-bar", sha256)},
+		{format: "foo-%m-bar", want: fmt.Sprintf("foo-%s-bar", machineName)},
+		{format: "foo-%o-bar", want: fmt.Sprintf("foo-%s-bar", osName)},
+		{format: "foo-%v-bar", want: fmt.Sprintf("foo-%s-bar", version)},
+
+		{format: "repeated %v %m %v", want: fmt.Sprintf("repeated %s %s %s", version, machineName, version)},
+
+		{format: "https://real.example.com/%e/%m/%o/%v#%%20trailing", want: fmt.Sprintf("https://real.example.com/%s/%s/%s/%s#%%20trailing", suffix, machineName, osName, version)},
+	}
+
+	for _, tc := range tests {
+		got, err := BuildURLFromFormat(tc.format, version)
+		if fmt.Sprintf("%v", err) != fmt.Sprintf("%v", tc.wantErr) {
+			if got != "" {
+				t.Errorf("format '%s': got non-empty '%s' on error", tc.format, got)
+			}
+			t.Errorf("format '%s': got error %v, want error %v", tc.format, err, tc.wantErr)
+		} else if got != tc.want {
+			t.Errorf("format '%s': got %s, want %s", tc.format, got, tc.want)
+		}
+	}
+}

httputil/BUILD

@@ -10,12 +10,12 @@ go_library(
         "fake.go",
         "httputil.go",
     ],
+    importpath = "github.com/bazelbuild/bazelisk/httputil",
+    visibility = ["//visibility:public"],
     deps = [
+        "@com_github_bgentry_go_netrc//:go_default_library",
         "@com_github_mitchellh_go_homedir//:go_default_library",
-        "@com_github_bgentry_go_netrc//:go_default_library"
     ],
-    importpath = "github.com/bazelbuild/bazelisk/httputil",
-    visibility = ["//visibility:public"],
 )
 
 go_test(

platforms/BUILD

@@ -16,3 +16,9 @@ go_test(
     srcs = ["platforms_test.go"],
     embed = [":go_default_library"],
 )
+
+go_test(
+    name = "go_default_test",
+    srcs = ["platforms_test.go"],
+    embed = [":go_default_library"],
+)

platforms/platforms.go

@@ -54,8 +54,7 @@ func DetermineExecutableFilenameSuffix() string {
 	return filenameSuffix
 }
 
-// DetermineBazelFilename returns the correct file name of a local Bazel binary.
-func DetermineBazelFilename(version string, includeSuffix bool) (string, error) {
+func DetermineArchitecture(osName, version string) (string, error) {
 	var machineName string
 	switch runtime.GOARCH {
 	case "amd64":
@@ -66,16 +65,32 @@ func DetermineBazelFilename(version string, includeSuffix bool) (string, error)
 		return "", fmt.Errorf("unsupported machine architecture \"%s\", must be arm64 or x86_64", runtime.GOARCH)
 	}
 
-	var osName string
+	if osName == "darwin" {
+		machineName = DarwinFallback(machineName, version)
+	}
+
+	return machineName, nil
+}
+
+func DetermineOperatingSystem() (string, error) {
 	switch runtime.GOOS {
 	case "darwin", "linux", "windows":
-		osName = runtime.GOOS
+		return runtime.GOOS, nil
 	default:
 		return "", fmt.Errorf("unsupported operating system \"%s\", must be Linux, macOS or Windows", runtime.GOOS)
 	}
+}
 
-	if osName == "darwin" {
-		machineName = DarwinFallback(machineName, version)
+// DetermineBazelFilename returns the correct file name of a local Bazel binary.
+func DetermineBazelFilename(version string, includeSuffix bool) (string, error) {
+	osName, err := DetermineOperatingSystem()
+	if err != nil {
+		return "", err
+	}
+
+	machineName, err := DetermineArchitecture(osName, version)
+	if err != nil {
+		return "", err
 	}
 
 	var filenameSuffix string