[musl] netlink.c: missing handling of EAGAIN and EWOULDBLOCK

[Waldek Kozaczuk <jwkozaczuk@gmail.com>]

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Hi,

Very recently we implemented minimal rnetlink support on OSv side which
allowed us to finally switch to the netlink-based implementation of
getifaddrs() and if_nameindex().

However, I noticed that the function __netlink_enumerate() in
https://github.com/ifduyue/musl/blob/master/src/network/netlink.c uses
MSG_DONTWAIT flag when calling recv() which may fail with EAGAIN or
EWOULDBLOCK and there is no error/retry handling for that. I actually saw
both functions fail occasionally on OSv.

One way to fix is to add missing error handling. But another simpler
solution is to stop using MSG_DONTWAIT altogether and force recv() to
block. In other words, the line:

r = recv(fd, u.buf, sizeof(u.buf), MSG_DONTWAIT);

should change to:

r = recv(fd, u.buf, sizeof(u.buf), 0);

For time being we are applying a header trick on OSv side to re-define
MSG_DONTWAIT as 0 when compiling those specific musl sources.

Waldek

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