Process aliases

[mat-hek]

TODO:

• support aliases in spawn_opt
• reply_demonitor
• C docs
• erlang docs
• process reference serialisation

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later

[mat-hek]

@bettio it seems that CI sometimes fails because it cannot find erlang:alias. I added it to erlang.erl, nifs.gperf and nifs.c, am I missing something?

[bettio]

For consistency with the work we are doing lately I suggest renaming all PID_REF and pid_reference to PROCESS_REF and process_reference

[bettio]

nitpicking/very low priority comment: just for consistency I would suggest moving first the 4 branch:
#if TERM_BYTES == 4 ... #elif TERM_BYTES == 8.
Same applies to the other functions as well.

[bettio]

We are using uint32_t for process id.

[mat-hek]

Are you sure? It's int32 here: https://github.com/atomvm/AtomVM/blob/main/src/libAtomVM/context.h#L106 and in many other places

[bettio]

We should also test unhappy paths, such as:

1. double/triple unalias
2. alias a dead process
3. unalias a dead process

In addition multiple aliases should be tested, this is legit but is should be properly tested.

[bettio]

We should use term_is_nonempty_list to avoid issues with non proper lists.

[bettio]

here we check that last list item is nil.

[bettio]

Let's use PascalCase. See AVMCCS-N004 rule in our coding style guide.
(There is still some old code that has to be migrated to updated style).

[mat-hek]

@bettio added more tests and missing features. The CI is failing, but it seems unrelated to the changes

[bettio]

Looks mostly good: there is only one change related to IS_NULL_PTR that is required. I suggest also rebasing on top of main, since main has a lot of fixes for flaky tests.

[bettio]

We should not use !IS_NULL_PTR (x) since it is a shorthand for UNLIKELY(x == NULL). So we must use it only when it is unlikely that the pointer is null (that is mostly for error handling purposes).

[mat-hek]

Done, but there are many occurrences of !IS_NULL_PTR in the code

[pguyot]

esp32 tests are crashing

[pguyot]

My main concern so far is that monitors are now extended to 2 or 3 words instead of 1 or 2. This may break some code that took monitors and passed it around as references, as the term_to_ref_ticks/term_from_ref_ticks is a common pattern. A similar breakage existed with resources being larger references if one tries to pass a resource where a 1/2 words ref is expected, but it is less likely as resources were not references so far. This may be the crash cause of esp32 tests.

port.erl

call(Port, Message, Timeout) ->
    MonitorRef = monitor(port, Port),
    Port ! {'$call', {self(), MonitorRef}, Message},
    Result =
        receive
            {'DOWN', MonitorRef, port, Port, normal} ->
                {error, noproc};
            {'DOWN', MonitorRef, port, Port, Reason} ->
                {error, Reason};
            out_of_memory ->
                out_of_memory;
            {MonitorRef, Ret} ->
                Ret
        after Timeout ->
            {error, timeout}
        end,
    demonitor(MonitorRef, [flush]),
    Result.

uart_driver.c:

static void uart_driver_do_read(Context *ctx, GenMessage gen_message)
{
    GlobalContext *glb = ctx->global;
    struct UARTData *uart_data = ctx->platform_data;
    term pid = gen_message.pid;
    term ref = gen_message.ref;
    uint64_t ref_ticks = term_to_ref_ticks(ref);

[pguyot]

This is not enforced by style guide but usually we don't quote atoms when it's not required.

[pguyot]

This is not what OTP does and there probably is a good reason.
Regular monitors are smaller than aliases.

7> Pid = spawn(fun() -> receive ok -> ok end end).
<0.94.0>
8> monitor(process, Pid).
#Ref<0.3008598808.1200095248.57512>
9> monitor(process, Pid, [{alias, explicit_unalias}]).
#Ref<0.0.11651.3008598808.1200160784.57533>
10> make_ref().
#Ref<0.3008598808.1200095248.57553>

[pguyot]

This was fixed for regular refs, we probably want ref_ticks >> 32

[pguyot]

get_otp_version/0 already exists in this module.

Also there is a poor usage (fixed in #2025 )

    OTPVersion = get_otp_version(),
    if
        OTPVersion =:= atomvm orelse OTPVersion >= 26 ->
            B == erlang:term_to_binary(A);
        true ->
            true
    end.

be sure to rather do:

    OTPVersion = get_otp_version(),
    if
        OTPVersion >= 26 ->
            B == erlang:term_to_binary(A);
        true ->
            true
    end.

as atoms sort higher than integers.

[mat-hek]

Also there is a poor usage

I would call it 'a readable way' :P

[mat-hek]

@pguyot the reason for the STM32 tests failing was that the size of a regular reference is actually 3 terms there, which means process reference takes 4 terms, and it probably conflicts with resource reference size, which is always 4 terms. I changed the process reference size to 5, but that's not a perfect solution :P So we need to come up with another way of distinguishing references - do you have anything in mind?

I also added RefData as a more universal representation of references than ref_ticks. WYDT? Would it make sense for it to support resource references too?