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this was apparently omitted long ago out of a lack of understanding of
its importance and the fact that POSIX doesn't specify it. despite not
being officially standardized, however, it turns out that at least
AIX, glibc, NetBSD, OpenBSD, QNX, and Solaris document and support it.
in certain usage cases, such as implementing a DNS gateway on top of
the stub resolver interfaces, it's necessary to distinguish the case
where a name does not exit (NxDomain) from one where it exists but has
no addresses (or other records) of the requested type (NODATA). in
fact, even the legacy gethostbyname API had this distinction, which we
were previously unable to support correctly because the backend lacked
it.
apart from fixing an important functionality gap, adding this
distinction helps clarify to users how search domain fallback works
(falling back in cases corresponding to EAI_NONAME, not in ones
corresponding to EAI_NODATA), a topic that has been a source of
ongoing confusion and frustration.
as a result of this change, EAI_NONAME is no longer a valid universal
error code for getaddrinfo in the case where AI_ADDRCONFIG has
suppressed use of all address families. in order to return an accurate
result in this case, getaddrinfo is modified to still perform at least
one lookup. this will almost surely fail (with a network error, since
there is no v4 or v6 network to query DNS over) unless a result comes
from the hosts file or from ip literal parsing, but in case it does
succeed, the result is replaced by EAI_NODATA.
glibc has a related error code, EAI_ADDRFAMILY, that could be used for
the AI_ADDRCONFIG case and certain NODATA cases, but distinguishing
them properly in full generality seems to require additional DNS
queries that are otherwise not useful. on glibc, it is only used for
ip literals with mismatching family, not for DNS or hosts file results
where the name has addresses only in the opposite family. since this
seems misleading and inconsistent, and since EAI_NODATA already covers
the semantic case where the "name" exists but doesn't have any
addresses in the requested family, we do not adopt EAI_ADDRFAMILY at
this time. this could be changed at some point if desired, but the
logic for getting all the corner cases with AI_ADDRCONFIG right is
slightly nontrivial.
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EAI_MEMORY is not possible because the resolver backend does not
allocate. if it did, it would be necessary for us to explicitly return
ENOMEM as the error, since errno is not guaranteed to reflect the
error cause except in the case of EAI_SYSTEM, so the existing code was
not correct anyway.
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these functions are horribly underspecified, inconsistent between
historical systems, and should never have been included. however, the
signatures we have match the glibc ones, and the glibc behavior is to
treat NxDomain and NODATA results as a success condition, not an
ENOENT error.
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this case statement was accidently left behind when this function
was refactored in commit e8f39ca4898237cf71657500f0b11534c47a0521.
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mistakenly ordering strings before addresses in the result buffer
broke the alignment that the preceding code had set up.
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according to the documentation in the man pages, the GNU extension
functions gethostbyaddr_r, gethostbyname_r and gethostbyname2_r are
guaranteed to set the result pointer to NULL in case of error or no
result.
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these changes reduce the size of the function somewhat and remove many
of its dependencies, including free. in principle it should now be
async-signal-safe, but this has not been verified in detail.
minor changes to error handling are also made.
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they're supposed to return an error code rather than using errno.
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