Add --dry-run support for --update to check for updates without installing

docs/cli-reference.md

@@ -456,7 +456,7 @@ Print detailed build information.
 
 ### `--update`
 
-Update fetch binary in place.
+Update fetch binary in place. Use with `--dry-run` to check for updates without installing.
 
 ### `--complete SHELL`
 
@@ -469,10 +469,11 @@ fetch --complete fish > ~/.config/fish/completions/fetch.fish
 
 ### `--dry-run`
 
-Print request information without sending.
+Print request information without sending. When used with `--update`, checks for the latest version without installing.
 
 ```sh
 fetch --dry-run -m POST -j '{"test": true}' example.com
+fetch --update --dry-run
 ```
 
 ## Environment Variables

integration/integration_test.go

@@ -1191,6 +1191,29 @@ func TestMain(t *testing.T) {
 		res = runFetch(t, fetchPath, "--version")
 		assertExitCode(t, 0, res)
 
+		// Test dry-run update when already on latest version.
+		newVersion.Store(&version)
+		dryRunModTime := getModTime(t, fetchPath)
+		res = runFetch(t, fetchPath, server.URL, "--update", "--dry-run")
+		assertExitCode(t, 0, res)
+		assertBufContains(t, res.stderr, "Already using the latest version")
+		if !getModTime(t, fetchPath).Equal(dryRunModTime) {
+			t.Fatal("binary was modified during dry-run update (same version)")
+		}
+
+		// Test dry-run update when a new version is available.
+		newVersion.Store(&newStr)
+		dryRunModTime = getModTime(t, fetchPath)
+		res = runFetch(t, fetchPath, server.URL, "--update", "--dry-run")
+		assertExitCode(t, 0, res)
+		assertBufContains(t, res.stderr, "Update available")
+		assertBufContains(t, res.stderr, newStr)
+		assertBufNotContains(t, res.stderr, "Updated fetch:")
+		assertBufNotContains(t, res.stderr, "Downloading")
+		if !getModTime(t, fetchPath).Equal(dryRunModTime) {
+			t.Fatal("binary was modified during dry-run update")
+		}
+
 		// Test the auto-update functionality.
 		res = runFetch(t, fetchPath, "--version", "--auto-update", "0s")
 		assertExitCode(t, 0, res)

internal/update/update.go

@@ -21,8 +21,8 @@ import (
 
 // Update checks the API for the latest fetch version and upgrades the current
 // executable in-place, returning the exit code to use.
-func Update(ctx context.Context, p *core.Printer, timeout time.Duration, silent bool) int {
-	err := update(ctx, p, timeout, silent)
+func Update(ctx context.Context, p *core.Printer, timeout time.Duration, silent bool, dryRun bool) int {
+	err := update(ctx, p, timeout, silent, dryRun)
 	if err == nil {
 		return 0
 	}
@@ -31,7 +31,7 @@ func Update(ctx context.Context, p *core.Printer, timeout time.Duration, silent
 	return 1
 }
 
-func update(ctx context.Context, p *core.Printer, timeout time.Duration, silent bool) error {
+func update(ctx context.Context, p *core.Printer, timeout time.Duration, silent bool, dryRun bool) error {
 	if timeout > 0 {
 		// Ensure the context is cancelled after the provided timeout.
 		var cancel context.CancelFunc
@@ -61,10 +61,10 @@ func update(ctx context.Context, p *core.Printer, timeout time.Duration, silent
 	}()
 
 	// Perform the update.
-	return updateInner(ctx, p, silent)
+	return updateInner(ctx, p, silent, dryRun)
 }
 
-func updateInner(ctx context.Context, p *core.Printer, silent bool) error {
+func updateInner(ctx context.Context, p *core.Printer, silent bool, dryRun bool) error {
 	c := client.NewClient(client.ClientConfig{})
 
 	// Get the current executable path and verify that we have write
@@ -104,6 +104,20 @@ func updateInner(ctx context.Context, p *core.Printer, silent bool) error {
 		return nil
 	}
 
+	if dryRun {
+		if !silent {
+			p.WriteString("Update available: ")
+			p.WriteString(version)
+			p.WriteString(" -> ")
+			p.Set(core.Bold)
+			p.WriteString(latest.TagName)
+			p.Reset()
+			p.WriteString("\n")
+			p.Flush()
+		}
+		return nil
+	}
+
 	// Look for the artifact URL for our OS and architecture.
 	artifactURL := getArtifactURL(latest)
 	if artifactURL == "" {

main.go

@@ -96,7 +96,7 @@ func main() {
 	if app.Update {
 		p := handle.Stderr()
 		timeout := getValue(app.Cfg.Timeout)
-		status := update.Update(ctx, p, timeout, verbosity == core.VSilent)
+		status := update.Update(ctx, p, timeout, verbosity == core.VSilent, app.DryRun)
 		os.Exit(status)
 	}