erts: Fix child_setup partial read

[garazdawi]

When calling read it may return a partial result. So we make
sure that all the data is read before returning.

The problem has been observed when reading the ACK message,
but we use the new routine for all reads in the child so that
we know that it will work everywhere.

[github-actions]

CT Test Results

       3 files     125 suites   39m 20s ⏱️
1 439 tests 1 397 ✔️ 42 💤 0 ❌
1 737 runs  1 678 ✔️ 59 💤 0 ❌

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[sverker]

In main there is a read(sigchld_pipe[0], ibuff, sizeof(ibuff));.
Should that also use read_all?

What about a write_all? write can also return partial results according to docs.

[garazdawi]

In main there is a read(sigchld_pipe[0], ibuff, sizeof(ibuff));.
Should that also use read_all?

Yes, might as well change that one also.

What about a write_all? write can also return partial results according to docs.

I've never seen a write do a partial write in practice... though I suppose it does not hurt to be extra safe.

[garazdawi]

Fixed, please review again @sverker.

[sverker]

Ok, except I don't get the point of that EPIPE condition in neither write_all or read_all.

[garazdawi]

except I don't get the point of that EPIPE condition in neither write_all or read_all.

when 0 is read/written that is usually an indication that the pipe has been closed. Testcases for the old implementation of child_setup checked that this was the error code returned, so to be compatible I've had to return EPIPE for all of those as well. Strictly speaking I think it may only be one or two of the reads where is behaviour can be observed by the user, but I decided to add it for all just to be consistent.