gh-127257: ssl: Raise OSError for ERR_LIB_SYS (GH-127361)

From the ERR_raise manpage:

    ERR_LIB_SYS

        This "library code" indicates that a system error is
        being reported.  In this case, the reason code given
        to `ERR_raise()` and `ERR_raise_data()` *must* be
        `errno(3)`.


This PR only handles ERR_LIB_SYS for the high-lever error types
SSL_ERROR_SYSCALL and SSL_ERROR_SSL, i.e., not the ones where
OpenSSL indicates it has some more information about the issue.

Author: encukou

Misc/NEWS.d/next/Library/2024-11-28-14-14-46.gh-issue-127257.n6-jU9.rst

@@ -0,0 +1,2 @@
+In :mod:`ssl`, system call failures that OpenSSL reports using
+``ERR_LIB_SYS`` are now raised as :exc:`OSError`.

Modules/_ssl.c

@@ -667,6 +667,11 @@ PySSL_SetError(PySSLSocket *sslsock, const char *filename, int lineno)
                         ERR_GET_REASON(e) == SSL_R_CERTIFICATE_VERIFY_FAILED) {
                     type = state->PySSLCertVerificationErrorObject;
                 }
+                if (ERR_GET_LIB(e) == ERR_LIB_SYS) {
+                    // A system error is being reported; reason is set to errno
+                    errno = ERR_GET_REASON(e);
+                    return PyErr_SetFromErrno(PyExc_OSError);
+                }
                 p = PY_SSL_ERROR_SYSCALL;
             }
             break;
@@ -692,6 +697,11 @@ PySSL_SetError(PySSLSocket *sslsock, const char *filename, int lineno)
                 errstr = "EOF occurred in violation of protocol";
             }
 #endif
+            if (ERR_GET_LIB(e) == ERR_LIB_SYS) {
+                // A system error is being reported; reason is set to errno
+                errno = ERR_GET_REASON(e);
+                return PyErr_SetFromErrno(PyExc_OSError);
+            }
             break;
         }
         default: