Replacement for /sbin/init that launches an Erlang/OTP Release
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README.md

erlinit

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This is a replacement for /sbin/init that launches an Erlang/OTP release. It is intentionally minimalist as it expects Erlang/OTP to be in charge of application initialization and supervision. It can be thought of as a simple Erlang/OTP release start script with some basic system initialization.

Release setup

A system should contain only one Erlang/OTP release under the /srv/erlang directory. A typical directory hierarchy would be:

/srv/erlang/my_app*
     ├── lib
     │   ├── my_app-0.0.1
     │   │   ├── ebin
     │   │   └── priv
     │   ├── kernel-2.16.2
     │   │   └── ebin
     │   └── stdlib-1.19.2
     │       └── ebin
     └── releases
         ├── 1
         │   ├── my_app.boot
         │   ├── my_app.rel
         │   ├── my_app.script
         │   ├── sys.config
         │   └── vm.args
         └── RELEASES

In the above release hierarchy, the directory my_app at the base is optional. If there are multiple releases, the first one is used.

Currently, erlinit runs the Erlang VM found in /usr/lib/erlang so it is important that the release and the VM match versions. As would be expected, the sys.config and vm.args are used, so it is possible to configure the system via files in the release.

Configuration and Command line options

erlinit pulls its configuration from both the commandline and the file /etc/erlinit.config. The commandline comes from the Linux kernel arguments that that are left over after the kernel processes them. Look at the bootloader configuration (e.g. U-Boot) and the Linux kernel configuration (in the case of default args) to see how to modify these.

The erlinit.config file is parsed line by line. If a line starts with a #, it is ignored. Parameters are passed via the file similar to a commandline. For example, the following is a valid /etc/erlinit.config:

# erlinit.config example

# Enable UTF-8 filename handling
-e LANG=en_US.UTF-8;LANGUAGE=en

# Uncomment to enable verbose prints
-v

The following lists the options:

-b, --boot <path>
    Specify a specific .boot file for the Erlang VM to load
    Normally, the .boot file is automatically detected. The .boot extension is
    optional. A relative path is relative to the release directory.

-c, --ctty <tty[n]>
    Force the controlling terminal (ttyAMA0, tty1, etc.)

-d, --uniqueid-exec <program and arguments>
    Run the specified program to get a unique id for the board. This is useful with -n

-e, --env <VAR=value;VAR2=Value2...>
    Set additional environment variables

--gid <id>
    Run the Erlang VM under the specified group ID

--graceful-shutdown-timeout <milliseconds>
    After the application signals that it wants to reboot, poweroff, or halt,
    wait this many milliseconds for it to cleanup and exit.

-h, --hang-on-exit
    Hang the system if Erlang exits. The default is to reboot.

-H, --reboot-on-exit
    Reboot when Erlang exits.

--hang-on-fatal
    Hang if a fatal error is detected in erlinit.

-m, --mount <dev:path:type:flags:options>
    Mount the specified path. See mount(8) and fstab(5) for fields
    Specify multiple times for more than one path to mount.

-n, --hostname-pattern <pattern>
    Specify a hostname for the system. The pattern is a printf(3)
    pattern. It is passed a unique ID for the board. E.g., "nerves-%.4s"

--pre-run-exec <program and arguments>
    Run the specified command before Erlang starts

--poweroff-on-exit
    Power off when Erlang exits. This is similar to --hang-on-exit except it's for
    platforms without a reset button or an easy way to restart

--poweroff-on-fatal
    Power off if a fatal error is detected in erlinit.

--reboot-on-fatal
    Reboot if a fatal error is detected in erlinit. This is the default.

-r, --release-path <path1[:path2...]>
    A colon-separated lists of paths to search for
    Erlang releases. The default is /srv/erlang.

--run-on-exit <program and arguments>
    Run the specified command on exit.

-s, --alternate-exec <program and arguments>
    Run another program that starts Erlang up. The arguments to `erlexec` are passed afterwards.

-t, --print-timing
    Print out when erlinit starts and when it launches Erlang (for
    benchmarking)

--uid <id>
    Run the Erlang VM under the specified user ID

--update-clock
    Force the system clock to at least the build date/time of erlinit.

-v, --verbose
    Enable verbose prints

--warn-unused-tty
    Print a message on ttys receiving kernel logs, but not an Erlang console

--working_directory <path>
    Set the working directory

Rebooting or hanging when the Erlang VM exits

In production, if the Erlang VM exits for any reason, the desired behavior is usually to reboot. This is the default. When developing your app, you'll quickly find that this is frustrating since it makes it more difficult to gather debug information. The following other options are available:

  1. -h or --hang-on-exit - erlinit instructs the kernel to halt. On most systems this will cause the kernel to hang. Some systems reboot after a long delay - for example, a watchdog timer could trigger a reboot.
  2. --poweroff-on-exit - erlinit instructs the kernel to power off.
  3. --run-on-exit - erlinit runs the specified program on exit.

The 3rd option can be particularly useful for debugging since it allows you to manually collect debug data. For example, specifying --run-on-exit /bin/sh launches a shell. Another use is to invoke a program that reverts back to a known good version of the application. When the command exits, erlinit will either reboot, hang, or poweroff depending on whether --hang-on-exit or --poweroff-on-exit were passed.

Read-only root file systems

By default erlinit keeps the root filesystem mounted read-only. This is useful since it significantly reduces the chance of corrupting the root filesystem at runtime. If applications need to write to disk, they can always mount a writable partition and have code that handles corruptions on it. Recovering from a corrupt root filesystem is harder. During development, though, working with a read-only root filesystem can be a pain so an alternative is to remount it read-write. Applications can do this, but another approach is to update the application to reference the files in development from /tmp or a writable partition. For example, the Erlang code search path can be updated at runtime to references new directories for code. The relsync program does this to dynamically update Erlang code via the Erlang distribution protocol.

Logging

erlinit logs to /dev/kmsg and the messages can be viewed by running dmesg. For debug purposes and if /dev/kmsg cannot be opened, logging goes to stderr. This latter situation isn't desirable for normal use since writing to stderr can block. In some scenarios, it can block indefinitely (e.g., logging to a gadget serial device).

Debugging erlinit

Since erlinit is the first user process run, it can be a little tricky to debug when things go wrong. Hopefully this won't happen to you, but if it does, try passing '-v' in the kernel arguments so that erlinit runs in verbose mode. If it looks like the Erlang runtime is being started, but it crashes or hangs midway without providing any usable console output, try passing -s "/usr/bin/strace -f" in the config file or via kernel arguments to run strace on the initialization process. Be sure to add the strace program to /usr/bin. Sometimes you need to sift through the strace output to find the missing library or file that couldn't be loaded. When debugged, please consider contributing a fix back to help other erlinit users.

Filesystem mounting notes

It is possible to mount filesystems before the Erlang VM is started. This is useful if the Erlang release is not on the root filesystem and to support logging to a writable filesystem. This mechanism is not intended to support all types of mounts as the error conditions and handling are usually better handled at the application level. Additionally, it is good practice to check that a filesystem has mounted successfully in an application just so that if an error occurred, the filesystem can be fixed or reformatted. Obviously, logging or loading an alternative Erlang release will not be available if this happens, but at least the system can recover for the next reboot.

Typical mount commandline arguments look like:

-m /dev/mmcblk0p4:/mnt:vfat::utf8

This mounts /dev/mccblk0p4 as a vfat filesystem to the /mnt directory. No flags are passed, and the utf8 option is passed to the vfat driver. See mount(8) for options.

Hostnames

erlinit can set the hostname of the system so that it is available when Erlang starts up. The hostname can be hardcoded in the /etc/hostname file or it can be set by parameters to erlinit. Additionally, it's possible to specify a part of the name to be based on a unique ID or other information present on the file system. The -n argument is used to specify the hostname string. It is a printf(3) formatted string that is passed a string argument. The string argument is found by running the command specified by -d. For example, if a command is available the prints a unique identifier to stdout, it can be used to define the hostname:

-d "getmyid -args" -n erl-%.4s

If the getmyid program returns 012345, then the hostname would be erl-0123.

Another use would be for the program specified by -d to just return the hostname. The configuration would look like:

-d "getmyhostname -args" -n %s

In theory, the getmyhostname program could read an EEPROM or some file on a writable partition to return the hostname.

Root disk naming

If you have multiple memory cards, SSDs, or other devices connected, it's possible that Linux will enumerate those devices in a nondeterministic order. This can be mitigated by using udev to populate the /dev/disks/by-* directories, but even this can be inconvenient when you just want to refer to the drive that provides the root filesystem. To address this, erlinit creates /dev/rootdisk0, /dev/rootdisk0p1, etc. and symlinks them to the expected devices. For example, if your root file system is on /dev/mmcblk0p1, you'll get a symlink from /dev/rootdisk0p1 to /dev/mmcblk0p1 and the whole disk will be /dev/rootdisk0. Similarly, if the root filesystem is on /dev/sdb1, you'd still get /dev/rootdisk0p1 and /dev/rootdisk0 and they'd by symlinked to /dev/sdb1 and /dev/sdb respectively.

Chaining programs

It's possible for erlinit to run a program that launches erlexec so that various aspects of the Erlang VM can be modified in an advanced way. This is done by specifying -s or --alternate-exec. The program (and arguments) specified are invoked and the erlexec and other options that would have been run are passed as the last arguments.

One use is running strace on the Erlang VM as described in the debugging section. Another use is to capture the Erlang console to a pipe and redirect it to a GUI or web app. The dtach utility is useful for this. An example invocation is: --alternate-exec "/usr/bin/dtach -N /tmp/iex_prompt". See the dtach manpage for details.

Multiple consoles

Some targets such as the Raspberry Pi have multiple locations where the Erlang shell could be sent. Currently, erlinit only supports a console on one of the locations. This can cause some confusion and look like a hang. To address this, erlinit can print a warning message on the unused consoles using the --warn-unused-tty option. For example, if the user specifies that the Erlang shell is on ttyAMA0 (the UART port), a message will be printed on tty0 (the HDMI output).

Privilege

By default, erlinit starts the Erlang VM with superuser privilege. It is possible to reduce privilege by specifying the --uid and --gid options. Before doing this, make sure that your embedded Erlang/OTP application can support this. When dealing with hardware, it is quite easy to run into situations requiring elevated privileges.

Clocks

If you're running on a system without a real-time clock, the clock will report that it's 1970. Even if you have a real-time clock, a failure of the battery could still cause it to show a date in the 1980s or 1990s. erlinit isn't smart enough to fix this, but it can set a lower bound for the clock based on its build timestamp. Specify the --update-clock option to enable this. Additionally, this lower bound is set very early on in the boot process so nothing besides erlinit should see a decade's old time.

This option has the following caveats:

  1. Some projects have chosen to use the date to check whether NTP has synchronized the clock yet. If you are a person who did this, please consider a more direct route of checking NTP's synchronization status.
  2. Look into what fake-hwclock does if you need something better.
  3. If using this to guarantee a minimum timestamp so that SSL certificates work, be sure that the SSL certificates don't expire before the next firmware update. (Not that you don't have to do that anyway, but just a friendly reminder)

Hacking

It seems like there are an endless number of small tweaks to erlinit that yield meaningful improvements. Please post Github issues before starting on anything substantial since there's often another way.

To verify that your changes work, run make check to run erlinit through its regression tests. These tests should run fine on both OSX and Linux even though erlinit is intended to be run on a minimal embedded Linux system. See test/fixture for the shared library that's used to simulate erlinit being run as Linux's init process (pid 1).