Verify write permission to executable file before update

internal/update/update.go

@@ -67,13 +67,19 @@ func update(ctx context.Context, p *core.Printer, timeout time.Duration, silent
 func updateInner(ctx context.Context, p *core.Printer, silent bool) error {
 	c := client.NewClient(client.ClientConfig{})
 
-	// Get the current version by calling `fetch --version` so that if the
-	// executable was updated while we were waiting for the update lock,
-	// we have the most up-to-date local version.
+	// Get the current executable path and verify that we have write
+	// permission in order to replace the file.
 	exePath, err := getExecutablePath()
 	if err != nil {
 		return err
 	}
+	if !canReplaceFile(exePath) {
+		return errNoWritePermission(exePath)
+	}
+
+	// Get the current version by calling `fetch --version` so that if the
+	// executable was updated while we were waiting for the update lock,
+	// we have the most up-to-date local version.
 	version, err := getExeVersion(ctx, exePath)
 	if err != nil {
 		return err
@@ -459,3 +465,17 @@ func (err errNoReleaseArtifact) PrintTo(p *core.Printer) {
 	p.Reset()
 	p.WriteString("'")
 }
+
+type errNoWritePermission string
+
+func (err errNoWritePermission) Error() string {
+	return fmt.Sprintf("the current process does not have write permission to '%s'", string(err))
+}
+
+func (err errNoWritePermission) PrintTo(p *core.Printer) {
+	p.WriteString("the current process does not have write permission to '")
+	p.Set(core.Dim)
+	p.WriteString(string(err))
+	p.Reset()
+	p.WriteString("'")
+}

internal/update/update_unix.go

@@ -97,3 +97,9 @@ func tryLockFile(f *os.File) (bool, error) {
 func unlockFile(f *os.File) error {
 	return unix.Flock(int(f.Fd()), unix.LOCK_UN)
 }
+
+// canReplaceFile returns true if this process can replace the file at the
+// provided location.
+func canReplaceFile(path string) bool {
+	return unix.Access(path, unix.W_OK) == nil
+}

internal/update/update_windows.go

@@ -245,3 +245,8 @@ func unlockFile(f *os.File) error {
 	var ol windows.Overlapped
 	return windows.UnlockFileEx(windows.Handle(f.Fd()), 0, allBytes, allBytes, &ol)
 }
+
+// canReplaceFile always returns true on windows.
+func canReplaceFile(path string) bool {
+	return true
+}