ssl-protocol-version-hints-2.patch

diff --git a/src/backend/libpq/be-secure-openssl.c b/src/backend/libpq/be-secure-openssl.c
index 8adf64c78e..778b166753 100644
--- a/src/backend/libpq/be-secure-openssl.c
+++ b/src/backend/libpq/be-secure-openssl.c
@@ -72,6 +72,7 @@ static bool dummy_ssl_passwd_cb_called = false;
 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);
 
 /* ------------------------------------------------------------ */
 /*						 Public interface						*/
@@ -365,6 +366,7 @@ be_tls_open_server(Port *port)
 	int			err;
 	int			waitfor;
 	unsigned long ecode;
+	bool		give_proto_hint;
 
 	Assert(!port->ssl);
 	Assert(!port->peer);
@@ -451,10 +453,48 @@ aloop:
 							 errmsg("could not accept SSL connection: EOF detected")));
 				break;
 			case SSL_ERROR_SSL:
+				switch (ERR_GET_REASON(ecode))
+				{
+						/*
+						 * NO_PROTOCOLS_AVAILABLE occurs if the client and
+						 * server min/max protocol version limits don't
+						 * overlap.  UNSUPPORTED_PROTOCOL,
+						 * WRONG_VERSION_NUMBER, and
+						 * TLSV1_ALERT_PROTOCOL_VERSION have been observed
+						 * when trying to communicate with an old OpenSSL
+						 * library.  It's not very clear what would make
+						 * OpenSSL return the other codes listed here, but a
+						 * hint about protocol versions seems like it's
+						 * appropriate for all.
+						 */
+					case SSL_R_NO_PROTOCOLS_AVAILABLE:
+					case SSL_R_UNSUPPORTED_PROTOCOL:
+					case SSL_R_BAD_PROTOCOL_VERSION_NUMBER:
+					case SSL_R_UNKNOWN_SSL_VERSION:
+					case SSL_R_UNSUPPORTED_SSL_VERSION:
+					case SSL_R_VERSION_TOO_HIGH:
+					case SSL_R_VERSION_TOO_LOW:
+					case SSL_R_WRONG_SSL_VERSION:
+					case SSL_R_WRONG_VERSION_NUMBER:
+					case SSL_R_TLSV1_ALERT_PROTOCOL_VERSION:
+						give_proto_hint = true;
+						break;
+					default:
+						give_proto_hint = false;
+						break;
+				}
 				ereport(COMMERROR,
 						(errcode(ERRCODE_PROTOCOL_VIOLATION),
 						 errmsg("could not accept SSL connection: %s",
-								SSLerrmessage(ecode))));
+								SSLerrmessage(ecode)),
+						 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 ?
+								 ssl_protocol_version_to_string(ssl_min_protocol_version) :
+								 MIN_OPENSSL_TLS_VERSION,
+								 ssl_max_protocol_version ?
+								 ssl_protocol_version_to_string(ssl_max_protocol_version) :
+								 MAX_OPENSSL_TLS_VERSION) : 0));
 				break;
 			case SSL_ERROR_ZERO_RETURN:
 				ereport(COMMERROR,
@@ -1328,6 +1368,29 @@ ssl_protocol_version_to_openssl(int v)
 	return -1;
 }
 
+/*
+ * Likewise provide a mapping to strings.
+ */
+static const char *
+ssl_protocol_version_to_string(int v)
+{
+	switch (v)
+	{
+		case PG_TLS_ANY:
+			return "any";
+		case PG_TLS1_VERSION:
+			return "TLSv1";
+		case PG_TLS1_1_VERSION:
+			return "TLSv1.1";
+		case PG_TLS1_2_VERSION:
+			return "TLSv1.2";
+		case PG_TLS1_3_VERSION:
+			return "TLSv1.3";
+	}
+
+	return "(unrecognized)";
+}
+
 
 static void
 default_openssl_tls_init(SSL_CTX *context, bool isServerStart)
diff --git a/src/include/common/openssl.h b/src/include/common/openssl.h
index 47fa129994..e4ef53ef70 100644
--- a/src/include/common/openssl.h
+++ b/src/include/common/openssl.h
@@ -17,12 +17,30 @@
 #ifdef USE_OPENSSL
 #include <openssl/ssl.h>
 
+/*
+ * OpenSSL doesn't provide any very nice way to identify the min/max
+ * protocol versions the library supports, so we fake it as best we can.
+ * Note in particular that this doesn't account for restrictions that
+ * might be specified in the installation's openssl.cnf.
+ */
+#define MIN_OPENSSL_TLS_VERSION  "TLSv1"
+
+#if defined(TLS1_3_VERSION)
+#define MAX_OPENSSL_TLS_VERSION  "TLSv1.3"
+#elif defined(TLS1_2_VERSION)
+#define MAX_OPENSSL_TLS_VERSION  "TLSv1.2"
+#elif defined(TLS1_1_VERSION)
+#define MAX_OPENSSL_TLS_VERSION  "TLSv1.1"
+#else
+#define MAX_OPENSSL_TLS_VERSION  "TLSv1"
+#endif
+
 /* src/common/protocol_openssl.c */
 #ifndef SSL_CTX_set_min_proto_version
 extern int	SSL_CTX_set_min_proto_version(SSL_CTX *ctx, int version);
 extern int	SSL_CTX_set_max_proto_version(SSL_CTX *ctx, int version);
 #endif
 
-#endif
+#endif							/* USE_OPENSSL */
 
 #endif							/* COMMON_OPENSSL_H */
diff --git a/src/interfaces/libpq/fe-secure-openssl.c b/src/interfaces/libpq/fe-secure-openssl.c
index 2d813ef5f9..10fa09020d 100644
--- a/src/interfaces/libpq/fe-secure-openssl.c
+++ b/src/interfaces/libpq/fe-secure-openssl.c
@@ -1304,6 +1304,42 @@ open_client_SSL(PGconn *conn)
 									  libpq_gettext("SSL error: %s\n"),
 									  err);
 					SSLerrfree(err);
+					switch (ERR_GET_REASON(ecode))
+					{
+							/*
+							 * NO_PROTOCOLS_AVAILABLE occurs if the client and
+							 * server min/max protocol version limits don't
+							 * overlap.  UNSUPPORTED_PROTOCOL,
+							 * WRONG_VERSION_NUMBER, and
+							 * TLSV1_ALERT_PROTOCOL_VERSION have been observed
+							 * when trying to communicate with an old OpenSSL
+							 * library.  It's not very clear what would make
+							 * OpenSSL return the other codes listed here, but
+							 * a hint about protocol versions seems like it's
+							 * appropriate for all.
+							 */
+						case SSL_R_NO_PROTOCOLS_AVAILABLE:
+						case SSL_R_UNSUPPORTED_PROTOCOL:
+						case SSL_R_BAD_PROTOCOL_VERSION_NUMBER:
+						case SSL_R_UNKNOWN_SSL_VERSION:
+						case SSL_R_UNSUPPORTED_SSL_VERSION:
+						case SSL_R_VERSION_TOO_HIGH:
+						case SSL_R_VERSION_TOO_LOW:
+						case SSL_R_WRONG_SSL_VERSION:
+						case SSL_R_WRONG_VERSION_NUMBER:
+						case SSL_R_TLSV1_ALERT_PROTOCOL_VERSION:
+							appendPQExpBuffer(&conn->errorMessage,
+											  libpq_gettext("This may indicate that the server does not support any SSL protocol version between %s and %s.\n"),
+											  conn->ssl_min_protocol_version ?
+											  conn->ssl_min_protocol_version :
+											  MIN_OPENSSL_TLS_VERSION,
+											  conn->ssl_max_protocol_version ?
+											  conn->ssl_max_protocol_version :
+											  MAX_OPENSSL_TLS_VERSION);
+							break;
+						default:
+							break;
+					}
 					pgtls_close(conn);
 					return PGRES_POLLING_FAILED;
 				}