tls: include X509 error string when verify result is not x509_V_OK.

[niedbalski]

Description

Users with expired CA never got to understand the root cause by reading the log message, although this information is available on the x509 error details. (https://x509errors.org/)

[2024/09/16 10:00:38] [error] [tls] error: unexpected EOF with reason: certificate verify failed

This patch adds the X509_verify_cert_error_string to the log message when SSL verification result != X509_V_OK.

With this patch applied, the following information is returned

[2024/10/28 22:35:46] [error] [tls] certificate verification failed, reason: unable to get local issuer certificate (X509 code: 20)

Testing

Configuration I have used.

[SERVICE]
    log_level debug

[CUSTOM]
    name calyptia
    api_key xxx
    fleet_name test-jorge
    calyptia_tls.verify on
    calyptia_host cloud-api-staging.calyptia.com

Documentation

N/A

Backporting

• Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[cosmo0920]

I found other code style issues. Basically, the added diffs are properly handled for errors. But, we need to /* ... */ style of one line comments.

[niedbalski]

@edsiper @cosmo0920 fixed the issues, will provide some unit tests to ensure this is not breaking.