Fix #10981 - Wrong usage of the write() system call in std.file.write() and in std.file.copy()

[vindexbit]

For the write system call (as with read), it is normal behavior to return fewer bytes than requested.

A more serious issue was found in std.file.copy:

cenforce(
    core.sys.posix.unistd.read(fdr, buf, toxfer) == toxfer
    && core.sys.posix.unistd.write(fdw, buf, toxfer) == toxfer,
    f, fromz);

The problem here is that an error may occur during the write call (for example, ENOSPC), but the programmer receives no information about it. Moreover, in my case, I encountered a phantom ENOENT error reported for the source file, which is clearly incorrect.

The behavior of the function has now been significantly improved.

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10982"