libpq: threadsafety for SSL certificate callback

In order to make the errorhandling code in backend libpq be thread-
safe the global variable used by the certificate verification call-
back need to be replaced with passing private data.

This moves the threadsafety needle a little but forwards, the call
to strerror_r also needs to be replaced with the error buffer made
thread local.  This is left as future work for when add the thread
primitives required for this to the tree.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/353226C7-97A1-4507-A380-36AA92983AE6@yesql.se

src/backend/libpq/be-secure-openssl.c

@@ -87,8 +87,14 @@ static bool ssl_is_server_start;
 static int	ssl_protocol_version_to_openssl(int v);
 static const char *ssl_protocol_version_to_string(int v);
 
-/* for passing data back from verify_cb() */
-static const char *cert_errdetail;
+struct CallbackErr
+{
+	/*
+	 * Storage for passing certificate verification error logging from the
+	 * callback.
+	 */
+	char	   *cert_errdetail;
+};
 
 /* ------------------------------------------------------------ */
 /*						 Public interface						*/
@@ -443,6 +449,7 @@ be_tls_open_server(Port *port)
 	int			waitfor;
 	unsigned long ecode;
 	bool		give_proto_hint;
+	static struct CallbackErr err_context;
 
 	Assert(!port->ssl);
 	Assert(!port->peer);
@@ -477,6 +484,10 @@ be_tls_open_server(Port *port)
 						SSLerrmessage(ERR_get_error()))));
 		return -1;
 	}
+
+	err_context.cert_errdetail = NULL;
+	SSL_set_ex_data(port->ssl, 0, &err_context);
+
 	port->ssl_in_use = true;
 
 aloop:
@@ -576,7 +587,7 @@ aloop:
 						(errcode(ERRCODE_PROTOCOL_VIOLATION),
 						 errmsg("could not accept SSL connection: %s",
 								SSLerrmessage(ecode)),
-						 cert_errdetail ? errdetail_internal("%s", cert_errdetail) : 0,
+						 err_context.cert_errdetail ? errdetail_internal("%s", err_context.cert_errdetail) : 0,
 						 give_proto_hint ?
 						 errhint("This may indicate that the client does not support any SSL protocol version between %s and %s.",
 								 ssl_min_protocol_version ?
@@ -585,7 +596,8 @@ aloop:
 								 ssl_max_protocol_version ?
 								 ssl_protocol_version_to_string(ssl_max_protocol_version) :
 								 MAX_OPENSSL_TLS_VERSION) : 0));
-				cert_errdetail = NULL;
+				if (err_context.cert_errdetail)
+					pfree(err_context.cert_errdetail);
 				break;
 			case SSL_ERROR_ZERO_RETURN:
 				ereport(COMMERROR,
@@ -1209,6 +1221,8 @@ verify_cb(int ok, X509_STORE_CTX *ctx)
 	const char *errstring;
 	StringInfoData str;
 	X509	   *cert;
+	SSL		   *ssl;
+	struct CallbackErr *cb_err;
 
 	if (ok)
 	{
@@ -1221,6 +1235,13 @@ verify_cb(int ok, X509_STORE_CTX *ctx)
 	errcode = X509_STORE_CTX_get_error(ctx);
 	errstring = X509_verify_cert_error_string(errcode);
 
+	/*
+	 * Extract the current SSL and CallbackErr object to use for passing error
+	 * detail back from the callback.
+	 */
+	ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
+	cb_err = (struct CallbackErr *) SSL_get_ex_data(ssl, 0);
+
 	initStringInfo(&str);
 	appendStringInfo(&str,
 					 _("Client certificate verification failed at depth %d: %s."),
@@ -1271,7 +1292,7 @@ verify_cb(int ok, X509_STORE_CTX *ctx)
 	}
 
 	/* Store our detail message to be logged later. */
-	cert_errdetail = str.data;
+	cb_err->cert_errdetail = str.data;
 
 	return ok;
 }