netutils: smtp: Treat recv EOF as an error

[Old-Ding]

Summary

recv() returns zero when the peer closes a TCP connection cleanly. The SMTP response path only treated negative returns as errors, so an EOF could leave stale or empty response data to be checked as if a server reply had arrived.

Return ERROR on zero-length receives at each SMTP response point.

Validation

• git diff --check origin/master..HEAD
• git show --check --stat --oneline HEAD
• git ls-files --eol netutils/smtp/smtp.c

[xiaoxiang781216]

@Old-Ding please fix:

../nuttx/tools/checkpatch.sh -c -u -m -g 4d5f60005c6d91c76e4a585fe3102aab50119ec6..HEAD
Error: /home/runner/work/nuttx-apps/nuttx-apps/apps/netutils/smtp/smtp.c:6:79: error: Long line found
Used config files:
    1: .codespellrc
Some checks failed. For contributing guidelines, see:
  https://github.com/apache/nuttx/blob/master/CONTRIBUTING.md

[cederom]

Thank you @Old-Ding :-)

• When no data received (recv() returns 0) do we want to return error are you sure? It looks there should be no processing but no error too? This looks like a functional change?
• Please add valid email as git commit signature.

[Old-Ding]

Updated in 0c9459b2: wrapped the long SPDX copyright line that checkpatch reported and added Signed-off-by to the follow-up commit. Both commits now use the same Old-Ding GitHub noreply identity.

For the recv() == 0 behavior: this function expects a required SMTP server reply before every next protocol step. A zero return from recv() means the peer performed an orderly shutdown, so no reply was received for that step. Treating that as ERROR keeps the transaction from continuing after EOF and avoids checking whatever was already in psmtp->buffer from an earlier reply.

[cederom]

• Thank you @Old-Ding for clarification that 0 means EOT.
• Please take a look at https://github.com/apache/nuttx/blob/master/CONTRIBUTING.md:
  • Please provide build and runtime logs confirming the fix is operational and works as expected.
  • Each git commit needs a message.
  • Each git commit needs a valid email signature.

[Old-Ding]

Updated in ab9dc070:

• Amended the follow-up commit so it has a descriptive message body and Signed-off-by using the Old-Ding GitHub noreply address.
• check, Lint, labeler, and the Build setup/select jobs are passing now; the remaining Build matrix jobs are still running/queued.

I do not have a local NuttX build/runtime environment on this Windows host (gcc, make, and cmake are not installed), so I am waiting for the GitHub Actions build logs. A runtime log still needs a real SMTP test environment; I have not attached one yet.

[cederom]

"Old-Ding GitHub noreply address" is not a valid email address, please use email address that anyone can contact you when needed.