ssl: clear the error queue before SSL_read/write/accept()

If other modules or libraries do not clear the OpenSSL error queue after
a failed operation, other code that relies on SSL_get_error() -- in
particular, code that deals with SSL_ERROR_WANT_READ/WRITE logic -- will
malfunction later on. To prevent this, explicitly clear the error queue
before calls like SSL_read/write/accept().

PR: 60223
Submitted by: Paul Spangler <paul.spangler ni.com>

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1769332 13f79535-47bb-0310-9956-ffa450edef68

modules/ssl/ssl_engine_io.c

@@ -689,6 +689,11 @@ static apr_status_t ssl_io_input_read(bio_filter_in_ctx_t *inctx,
             break;
         }
 
+        /* We rely on SSL_get_error() after the read, which requires an empty
+         * error queue before the read in order to work properly.
+         */
+        ERR_clear_error();
+
         /* SSL_read may not read because we haven't taken enough data
          * from the stack.  This is where we want to consider all of
          * the blocking and SPECULATIVE semantics
@@ -866,6 +871,11 @@ static apr_status_t ssl_filter_write(ap_filter_t *f,
         return APR_EGENERAL;
     }
 
+    /* We rely on SSL_get_error() after the write, which requires an empty error
+     * queue before the write in order to work properly.
+     */
+    ERR_clear_error();
+
     outctx = (bio_filter_out_ctx_t *)BIO_get_data(filter_ctx->pbioWrite);
     res = SSL_write(filter_ctx->pssl, (unsigned char *)data, len);
 
@@ -1343,6 +1353,11 @@ static apr_status_t ssl_io_filter_handshake(ssl_filter_ctx_t *filter_ctx)
         return APR_SUCCESS;
     }
 
+    /* We rely on SSL_get_error() after the accept, which requires an empty
+     * error queue before the accept in order to work properly.
+     */
+    ERR_clear_error();
+
     if ((n = SSL_accept(filter_ctx->pssl)) <= 0) {
         bio_filter_in_ctx_t *inctx = (bio_filter_in_ctx_t *)
                                      BIO_get_data(filter_ctx->pbioRead);