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Capistrano & Puma; neither a valid executable name nor an absolute path

When attempting to deploy a Rails application using the puma web sever using the systemd functionality in the capistrano puma gem, we may receive the error message:

Neither a valid executable name nor an absolute path

When attempting to start the systemd service. This most often occurs when using the capistrano rbenv plugin. This is because the Capistrano rbenv plugin modifies the SSHKit.config.command_map[:bundle] path to include the RBENV_ROOT and RBENV_VERSION environment variables at the start of the bundle path. Systemd doesn't support Exec command starting with environment variables, instead requiring them to be in separate Environment lines.

We can fix this by overriding the puma.server.erb template with a new systemd unit file as follows:

Description=Puma HTTP Server for <%= "#{fetch(:application)} (#{fetch(:stage)})" %>

<%="User=#{puma_user(@role)}" if fetch(:puma_systemctl_user) == :system %>
WorkingDirectory=<%= current_path %>
ExecStart=/usr/local/rbenv/bin/rbenv exec bundle exec puma -C <%= fetch(:puma_conf) %>
ExecReload=/bin/kill -USR1 $MAINPID
ExecStop=/bin/kill -TSTP $MAINPID
StandardOutput=append:<%= fetch(:puma_access_log) %>
StandardError=append:<%= fetch(:puma_error_log) %>
<%="EnvironmentFile=#{fetch(:puma_service_unit_env_file)}" if fetch(:puma_service_unit_env_file) %>
<% fetch(:puma_service_unit_env_vars, []).each do |environment_variable| %>
<%="Environment=#{environment_variable}" %>
<% end %>

Environment=RBENV_VERSION=<%= fetch(:rbenv_ruby) %>


SyslogIdentifier=puma_<%= fetch(:application) %>_<%= fetch(:stage) %>

WantedBy=<%=(fetch(:puma_systemctl_user) == :system) ? "multi-user.target" : "default.target"%>

Note that this hardcodes the path to rbenv so if the path is different, for example because it's a user install not a system install, this will need updating.

This unit file also adds an ExecReload option to allow us to use systemd for zero downtime deploys.

For a fully working example see this repository.

There's more information in this github issue.