Support file:// protocol for BAZELISK_BASE_URL

README.md

@@ -86,6 +86,8 @@ The URL format looks like `https://github.com/<FORK>/bazel/releases/download/<VE
 
 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 you want to use the releases stored on the local disk, set the URL as `file://` followed by the local disk path. On Windows, escape `\` in the path by `%5C`.
+
 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`.

httputil/httputil.go

@@ -14,6 +14,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"strconv"
+	"strings"
 	"time"
 
 	netrc "github.com/bgentry/go-netrc/netrc"
@@ -78,7 +79,41 @@ func ReadRemoteFile(url string, auth string) ([]byte, http.Header, error) {
 	return body, res.Header, nil
 }
 
+type LocalFileError struct{ err error }
+
+func (e *LocalFileError) Error() string { return e.err.Error() }
+func (e *LocalFileError) Unwrap() error { return e.err }
+
+// Handles file:// URLs by reading files from disk.
+func readLocalFile(urlStr string) (*http.Response, error) {
+	urlStr = strings.TrimPrefix(urlStr, "file://")
+	path, err := url.PathUnescape(urlStr)
+	if err != nil {
+		return nil, &LocalFileError{err: fmt.Errorf("invalid file url %q: %w", urlStr, err)}
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, &LocalFileError{err: fmt.Errorf("could not open %q: %w", path, err)}
+	}
+	var size int64 = -1
+	if fi, statErr := f.Stat(); statErr == nil {
+		size = fi.Size()
+	}
+	return &http.Response{
+		StatusCode:    200,
+		Status:        "200 OK",
+		Header:        make(http.Header),
+		Body:          f,
+		ContentLength: size,
+		Request:       &http.Request{Method: "GET", URL: &url.URL{Scheme: "file", Path: path}},
+	}, nil
+}
+
 func get(url, auth string) (*http.Response, error) {
+	if strings.HasPrefix(url, "file://") {
+		return readLocalFile(url)
+	}
+
 	req, err := http.NewRequest("GET", url, nil)
 	if err != nil {
 		return nil, fmt.Errorf("could not create request: %v", err)
@@ -127,6 +162,10 @@ func get(url, auth string) (*http.Response, error) {
 func shouldRetry(res *http.Response, err error) bool {
 	// Retry if the client failed to speak HTTP.
 	if err != nil {
+		var nre *LocalFileError
+		if errors.As(err, &nre) {
+			return false
+		}
 		return true
 	}
 	// For HTTP: only retry on non-permanent/fatal errors.

httputil/httputil_test.go

@@ -3,6 +3,9 @@ package httputil
 import (
 	"errors"
 	"net/http"
+	"net/url"
+	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"testing"
@@ -251,3 +254,42 @@ func TestNoRetryOnPermanentError(t *testing.T) {
 		t.Fatalf("Expected no retries for permanent error, but got %d", clock.TimesSlept())
 	}
 }
+
+func TestReadLocalFile(t *testing.T) {
+	tmpDir := t.TempDir()
+	path := filepath.Join(tmpDir, "payload.txt")
+	want := "hello from disk"
+	if err := os.WriteFile(path, []byte(want), 0644); err != nil {
+		t.Fatalf("failed to write temp file: %v", err)
+	}
+	fileURL := (&url.URL{Scheme: "file", Path: path}).String()
+
+	body, _, err := ReadRemoteFile(fileURL, "")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got := string(body); got != want {
+		t.Fatalf("expected body %q, but got %q", want, got)
+	}
+}
+
+func TestReadLocalFileNotFound(t *testing.T) {
+	clock := newFakeClock()
+	RetryClock = clock
+	MaxRetries = 10
+	MaxRequestDuration = time.Hour
+
+	missingPath := filepath.Join(t.TempDir(), "does-not-exist.txt")
+	fileURL := (&url.URL{Scheme: "file", Path: missingPath}).String()
+
+	_, _, err := ReadRemoteFile(fileURL, "")
+	if err == nil {
+		t.Fatal("expected error for missing file")
+	}
+	if !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("expected os.ErrNotExist, got %v", err)
+	}
+	if clock.TimesSlept() != 0 {
+		t.Fatalf("expected no retries for file:// error, but slept %d times", clock.TimesSlept())
+	}
+}