Confusing "Resource temporarily unavailable" error on timeout with OpenSSL backend

Consider this example:

#include <git2.h>
#include <assert.h>
#include <stdio.h>

int
main ()
{
  int err;
  err = git_libgit2_init ();
  assert (err == 1);

  git_clone_options opts;
  err = git_clone_init_options (&opts, GIT_CLONE_OPTIONS_VERSION);
  assert (err == 0);

  err = git_libgit2_opts (GIT_OPT_SET_SERVER_TIMEOUT, 1);
  assert (err == 0);

  git_repository *repo;
  err = git_clone (&repo, "https://git.guix.gnu.org/guix.git", "/tmp/example",
		   &opts);
  assert (err != 0);

  const git_error *gerr;
  gerr = giterr_last ();
  fprintf (stderr, "last error: %s\n", gerr->message);

  return 0;
}

When running it (using libgit2 1.9.2 on Linux, with the OpenSSL backend), I get:

last error: SSL error: syscall failure: Resource temporarily unavailable

... which is somewhat confusing.

Here's what we see from strace:

socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC, IPPROTO_TCP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(443), sin_addr=inet_addr("46.224.9.33")}, 16) = 0
fcntl(3, F_GETFL)                       = 0x2 (flags O_RDWR)
fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
...
sendto(3, "\26\3\1\6\4\1\0\6\0\3\3m\205\362N]I5|\3452\341b\v\316`9'*/\225?"..., 1545, 0, NULL, 0) = 1545
recvfrom(3, 0x19d1853, 5, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=3, events=POLLIN}], 1, 1)     = 0 (Timeout)
close(3)                                = 0

This corresponds to socket_read, calling recv then poll.

The issue here is that the error reported ("Resource temporarily available") is the wrong one; it should say "Timeout" (or similar). socket_read does try to convey that error but it gets lost in the abstraction layers (BIO, streams, etc.).

Start by reproducing the example with the one-second server timeout and tracing socket_read through the BIO and stream abstraction layers. The work is done when the timeout is preserved and reported as a timeout rather than "Resource temporarily unavailable".