diff --git a/src/backend/libpq/be-secure-openssl.c b/src/backend/libpq/be-secure-openssl.c
index 600966347e..95cceeed7a 100644
--- a/src/backend/libpq/be-secure-openssl.c
+++ b/src/backend/libpq/be-secure-openssl.c
@@ -78,7 +78,7 @@ static DH  *tmp_dh_cb(SSL *s, int is_export, int keylength);
 static int	verify_cb(int, X509_STORE_CTX *);
 static void info_cb(const SSL *ssl, int type, int args);
 static void initialize_ecdh(void);
-static const char *SSLerrmessage(void);
+static const char *SSLerrmessage(unsigned long ecode);
 
 static char *X509_NAME_to_cstring(X509_NAME *name);
 
@@ -182,7 +182,7 @@ be_tls_init(void)
 		if (!SSL_context)
 			ereport(FATAL,
 					(errmsg("could not create SSL context: %s",
-							SSLerrmessage())));
+							SSLerrmessage(ERR_get_error()))));
 
 		/*
 		 * Disable OpenSSL's moving-write-buffer sanity check, because it
@@ -198,7 +198,7 @@ be_tls_init(void)
 			ereport(FATAL,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
 				  errmsg("could not load server certificate file \"%s\": %s",
-						 ssl_cert_file, SSLerrmessage())));
+						 ssl_cert_file, SSLerrmessage(ERR_get_error()))));
 
 		if (stat(ssl_key_file, &buf) != 0)
 			ereport(FATAL,
@@ -251,12 +251,12 @@ be_tls_init(void)
 										SSL_FILETYPE_PEM) != 1)
 			ereport(FATAL,
 					(errmsg("could not load private key file \"%s\": %s",
-							ssl_key_file, SSLerrmessage())));
+							ssl_key_file, SSLerrmessage(ERR_get_error()))));
 
 		if (SSL_CTX_check_private_key(SSL_context) != 1)
 			ereport(FATAL,
 					(errmsg("check of private key failed: %s",
-							SSLerrmessage())));
+							SSLerrmessage(ERR_get_error()))));
 	}
 
 	/* set up ephemeral DH keys, and disallow SSL v2/v3 while at it */
@@ -285,7 +285,7 @@ be_tls_init(void)
 			(root_cert_list = SSL_load_client_CA_file(ssl_ca_file)) == NULL)
 			ereport(FATAL,
 					(errmsg("could not load root certificate file \"%s\": %s",
-							ssl_ca_file, SSLerrmessage())));
+							ssl_ca_file, SSLerrmessage(ERR_get_error()))));
 	}
 
 	/*----------
@@ -316,7 +316,7 @@ be_tls_init(void)
 			else
 				ereport(FATAL,
 						(errmsg("could not load SSL certificate revocation list file \"%s\": %s",
-								ssl_crl_file, SSLerrmessage())));
+								ssl_crl_file, SSLerrmessage(ERR_get_error()))));
 		}
 	}
 
@@ -353,6 +353,7 @@ be_tls_open_server(Port *port)
 	int			r;
 	int			err;
 	int			waitfor;
+	unsigned long ecode;
 
 	Assert(!port->ssl);
 	Assert(!port->peer);
@@ -362,7 +363,7 @@ be_tls_open_server(Port *port)
 		ereport(COMMERROR,
 				(errcode(ERRCODE_PROTOCOL_VIOLATION),
 				 errmsg("could not initialize SSL connection: %s",
-						SSLerrmessage())));
+						SSLerrmessage(ERR_get_error()))));
 		return -1;
 	}
 	if (!my_SSL_set_fd(port, port->sock))
@@ -370,16 +371,36 @@ be_tls_open_server(Port *port)
 		ereport(COMMERROR,
 				(errcode(ERRCODE_PROTOCOL_VIOLATION),
 				 errmsg("could not set SSL socket: %s",
-						SSLerrmessage())));
+						SSLerrmessage(ERR_get_error()))));
 		return -1;
 	}
 	port->ssl_in_use = true;
 
 aloop:
+	/*
+	 * Prepare to call SSL_get_error() by clearing thread's OpenSSL error
+	 * queue.  In general, the current thread's error queue must be empty
+	 * before the TLS/SSL I/O operation is attempted, or SSL_get_error()
+	 * will not work reliably.  An extension may have failed to clear the
+	 * per-thread error queue following another call to an OpenSSL I/O
+	 * routine.
+	 */
+	ERR_clear_error();
 	r = SSL_accept(port->ssl);
 	if (r <= 0)
 	{
 		err = SSL_get_error(port->ssl, r);
+
+		/*
+		 * Other clients of OpenSSL in the backend may fail to call
+		 * ERR_get_error(), but we always do, so as to not cause problems
+		 * for OpenSSL clients that don't call ERR_clear_error()
+		 * defensively.  Be sure that this happens by calling now.
+		 * SSL_get_error() relies on the OpenSSL per-thread error queue
+		 * being intact, so this is the earliest possible point
+		 * ERR_get_error() may be called.
+		 */
+		ecode = ERR_get_error();
 		switch (err)
 		{
 			case SSL_ERROR_WANT_READ:
@@ -413,7 +434,7 @@ aloop:
 				ereport(COMMERROR,
 						(errcode(ERRCODE_PROTOCOL_VIOLATION),
 						 errmsg("could not accept SSL connection: %s",
-								SSLerrmessage())));
+								SSLerrmessage(ecode))));
 				break;
 			case SSL_ERROR_ZERO_RETURN:
 				ereport(COMMERROR,
@@ -522,10 +543,13 @@ be_tls_read(Port *port, void *ptr, size_t len, int *waitfor)
 {
 	ssize_t		n;
 	int			err;
+	unsigned long ecode;
 
 	errno = 0;
+	ERR_clear_error();
 	n = SSL_read(port->ssl, ptr, len);
 	err = SSL_get_error(port->ssl, n);
+	ecode = (err != SSL_ERROR_NONE || n < 0) ? ERR_get_error() : 0;
 	switch (err)
 	{
 		case SSL_ERROR_NONE:
@@ -552,7 +576,7 @@ be_tls_read(Port *port, void *ptr, size_t len, int *waitfor)
 		case SSL_ERROR_SSL:
 			ereport(COMMERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
-					 errmsg("SSL error: %s", SSLerrmessage())));
+					 errmsg("SSL error: %s", SSLerrmessage(ecode))));
 			/* fall through */
 		case SSL_ERROR_ZERO_RETURN:
 			errno = ECONNRESET;
@@ -579,10 +603,13 @@ be_tls_write(Port *port, void *ptr, size_t len, int *waitfor)
 {
 	ssize_t		n;
 	int			err;
+	unsigned long ecode;
 
 	errno = 0;
+	ERR_clear_error();
 	n = SSL_write(port->ssl, ptr, len);
 	err = SSL_get_error(port->ssl, n);
+	ecode = (err != SSL_ERROR_NONE || n < 0) ? ERR_get_error() : 0;
 	switch (err)
 	{
 		case SSL_ERROR_NONE:
@@ -609,7 +636,7 @@ be_tls_write(Port *port, void *ptr, size_t len, int *waitfor)
 		case SSL_ERROR_SSL:
 			ereport(COMMERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
-					 errmsg("SSL error: %s", SSLerrmessage())));
+					 errmsg("SSL error: %s", SSLerrmessage(ecode))));
 			/* fall through */
 		case SSL_ERROR_ZERO_RETURN:
 			errno = ECONNRESET;
@@ -765,7 +792,8 @@ load_dh_file(int keylength)
 	{
 		if (DH_check(dh, &codes) == 0)
 		{
-			elog(LOG, "DH_check error (%s): %s", fnbuf, SSLerrmessage());
+			elog(LOG, "DH_check error (%s): %s", fnbuf,
+				 SSLerrmessage(ERR_get_error()));
 			return NULL;
 		}
 		if (codes & DH_CHECK_P_NOT_PRIME)
@@ -805,7 +833,7 @@ load_dh_buffer(const char *buffer, size_t len)
 	if (dh == NULL)
 		ereport(DEBUG2,
 				(errmsg_internal("DH load buffer: %s",
-								 SSLerrmessage())));
+								 SSLerrmessage(ERR_get_error()))));
 	BIO_free(bio);
 
 	return dh;
@@ -971,26 +999,26 @@ initialize_ecdh(void)
 }
 
 /*
- * Obtain reason string for last SSL error
+ * Obtain reason string for passed SSL errcode
+ *
+ * ERR_get_error() is used by caller to get errcode to pass here.
  *
  * Some caution is needed here since ERR_reason_error_string will
  * return NULL if it doesn't recognize the error code.  We don't
  * want to return NULL ever.
  */
 static const char *
-SSLerrmessage(void)
+SSLerrmessage(unsigned long ecode)
 {
-	unsigned long errcode;
 	const char *errreason;
 	static char errbuf[32];
 
-	errcode = ERR_get_error();
-	if (errcode == 0)
+	if (ecode == 0)
 		return _("no SSL error reported");
-	errreason = ERR_reason_error_string(errcode);
+	errreason = ERR_reason_error_string(ecode);
 	if (errreason != NULL)
 		return errreason;
-	snprintf(errbuf, sizeof(errbuf), _("SSL error code %lu"), errcode);
+	snprintf(errbuf, sizeof(errbuf), _("SSL error code %lu"), ecode);
 	return errbuf;
 }
 
diff --git a/src/interfaces/libpq/fe-secure-openssl.c b/src/interfaces/libpq/fe-secure-openssl.c
index 133546b790..9392506dbd 100644
--- a/src/interfaces/libpq/fe-secure-openssl.c
+++ b/src/interfaces/libpq/fe-secure-openssl.c
@@ -70,7 +70,7 @@ static int verify_peer_name_matches_certificate_name(PGconn *conn,
 static void destroy_ssl_system(void);
 static int	initialize_SSL(PGconn *conn);
 static PostgresPollingStatusType open_client_SSL(PGconn *);
-static char *SSLerrmessage(void);
+static char *SSLerrmessage(unsigned long ecode);
 static void SSLerrfree(char *buf);
 
 static int	my_sock_read(BIO *h, char *buf, int size);
@@ -152,7 +152,7 @@ pgtls_open_client(PGconn *conn)
 			!SSL_set_app_data(conn->ssl, conn) ||
 			!my_SSL_set_fd(conn, conn->sock))
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 				   libpq_gettext("could not establish SSL connection: %s\n"),
@@ -209,11 +209,32 @@ pgtls_read(PGconn *conn, void *ptr, size_t len)
 	int			result_errno = 0;
 	char		sebuf[256];
 	int			err;
+	unsigned long ecode;
 
 rloop:
+	/*
+	 * Prepare to call SSL_get_error() by clearing thread's OpenSSL error
+	 * queue.  In general, the current thread's error queue must be empty
+	 * before the TLS/SSL I/O operation is attempted, or SSL_get_error()
+	 * will not work reliably.  Since the possibility exists that other
+	 * OpenSSL clients running in the same thread but not under our control
+	 * will fail to call ERR_get_error() themselves (after their own I/O
+	 * operations), pro-actively clear the per-thread error queue now.
+	 */
 	SOCK_ERRNO_SET(0);
+	ERR_clear_error();
 	n = SSL_read(conn->ssl, ptr, len);
 	err = SSL_get_error(conn->ssl, n);
+
+	/*
+	 * Other clients of OpenSSL may fail to call ERR_get_error(), but we
+	 * always do, so as to not cause problems for OpenSSL clients that
+	 * don't call ERR_clear_error() defensively.  Be sure that this
+	 * happens by calling now.  SSL_get_error() relies on the OpenSSL
+	 * per-thread error queue being intact, so this is the earliest
+	 * possible point ERR_get_error() may be called.
+	 */
+	ecode = (err != SSL_ERROR_NONE || n < 0) ? ERR_get_error() : 0;
 	switch (err)
 	{
 		case SSL_ERROR_NONE:
@@ -266,7 +287,7 @@ rloop:
 			break;
 		case SSL_ERROR_SSL:
 			{
-				char	   *errm = SSLerrmessage();
+				char	   *errm = SSLerrmessage(ecode);
 
 				printfPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("SSL error: %s\n"), errm);
@@ -318,10 +339,13 @@ pgtls_write(PGconn *conn, const void *ptr, size_t len)
 	int			result_errno = 0;
 	char		sebuf[256];
 	int			err;
+	unsigned long ecode;
 
 	SOCK_ERRNO_SET(0);
+	ERR_clear_error();
 	n = SSL_write(conn->ssl, ptr, len);
 	err = SSL_get_error(conn->ssl, n);
+	ecode = (err != SSL_ERROR_NONE || n < 0) ? ERR_get_error() : 0;
 	switch (err)
 	{
 		case SSL_ERROR_NONE:
@@ -372,7 +396,7 @@ pgtls_write(PGconn *conn, const void *ptr, size_t len)
 			break;
 		case SSL_ERROR_SSL:
 			{
-				char	   *errm = SSLerrmessage();
+				char	   *errm = SSLerrmessage(ecode);
 
 				printfPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("SSL error: %s\n"), errm);
@@ -839,7 +863,7 @@ pgtls_init(PGconn *conn)
 		SSL_context = SSL_CTX_new(SSLv23_method());
 		if (!SSL_context)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 						 libpq_gettext("could not create SSL context: %s\n"),
@@ -1009,7 +1033,7 @@ initialize_SSL(PGconn *conn)
 #endif
 		if (SSL_CTX_use_certificate_chain_file(SSL_context, fnbuf) != 1)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 			   libpq_gettext("could not read certificate file \"%s\": %s\n"),
@@ -1024,7 +1048,7 @@ initialize_SSL(PGconn *conn)
 
 		if (SSL_use_certificate_file(conn->ssl, fnbuf, SSL_FILETYPE_PEM) != 1)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 			   libpq_gettext("could not read certificate file \"%s\": %s\n"),
@@ -1079,7 +1103,7 @@ initialize_SSL(PGconn *conn)
 			conn->engine = ENGINE_by_id(engine_str);
 			if (conn->engine == NULL)
 			{
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 					 libpq_gettext("could not load SSL engine \"%s\": %s\n"),
@@ -1091,7 +1115,7 @@ initialize_SSL(PGconn *conn)
 
 			if (ENGINE_init(conn->engine) == 0)
 			{
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 				libpq_gettext("could not initialize SSL engine \"%s\": %s\n"),
@@ -1107,7 +1131,7 @@ initialize_SSL(PGconn *conn)
 										   NULL, NULL);
 			if (pkey == NULL)
 			{
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("could not read private SSL key \"%s\" from engine \"%s\": %s\n"),
@@ -1121,7 +1145,7 @@ initialize_SSL(PGconn *conn)
 			}
 			if (SSL_use_PrivateKey(conn->ssl, pkey) != 1)
 			{
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("could not load private SSL key \"%s\" from engine \"%s\": %s\n"),
@@ -1177,7 +1201,7 @@ initialize_SSL(PGconn *conn)
 
 		if (SSL_use_PrivateKey_file(conn->ssl, fnbuf, SSL_FILETYPE_PEM) != 1)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 			   libpq_gettext("could not load private key file \"%s\": %s\n"),
@@ -1191,7 +1215,7 @@ initialize_SSL(PGconn *conn)
 	if (have_cert &&
 		SSL_check_private_key(conn->ssl) != 1)
 	{
-		char	   *err = SSLerrmessage();
+		char	   *err = SSLerrmessage(ERR_get_error());
 
 		printfPQExpBuffer(&conn->errorMessage,
 						  libpq_gettext("certificate does not match private key file \"%s\": %s\n"),
@@ -1229,7 +1253,7 @@ initialize_SSL(PGconn *conn)
 #endif
 		if (SSL_CTX_load_verify_locations(SSL_context, fnbuf, NULL) != 1)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 							  libpq_gettext("could not read root certificate file \"%s\": %s\n"),
@@ -1259,7 +1283,7 @@ initialize_SSL(PGconn *conn)
 				X509_STORE_set_flags(cvstore,
 						  X509_V_FLAG_CRL_CHECK | X509_V_FLAG_CRL_CHECK_ALL);
 #else
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("SSL library does not support CRL certificates (file \"%s\")\n"),
@@ -1327,11 +1351,14 @@ open_client_SSL(PGconn *conn)
 {
 	int			r;
 
+	ERR_clear_error();
 	r = SSL_connect(conn->ssl);
 	if (r <= 0)
 	{
 		int			err = SSL_get_error(conn->ssl, r);
+		unsigned long ecode;
 
+		ecode = ERR_get_error();
 		switch (err)
 		{
 			case SSL_ERROR_WANT_READ:
@@ -1356,7 +1383,7 @@ open_client_SSL(PGconn *conn)
 				}
 			case SSL_ERROR_SSL:
 				{
-					char	   *err = SSLerrmessage();
+					char	   *err = SSLerrmessage(ecode);
 
 					printfPQExpBuffer(&conn->errorMessage,
 									  libpq_gettext("SSL error: %s\n"),
@@ -1384,7 +1411,9 @@ open_client_SSL(PGconn *conn)
 	conn->peer = SSL_get_peer_certificate(conn->ssl);
 	if (conn->peer == NULL)
 	{
-		char	   *err = SSLerrmessage();
+		char	   *err;
+
+		err = SSLerrmessage(ERR_get_error());
 
 		printfPQExpBuffer(&conn->errorMessage,
 					libpq_gettext("certificate could not be obtained: %s\n"),
@@ -1456,7 +1485,9 @@ pgtls_close(PGconn *conn)
 
 
 /*
- * Obtain reason string for last SSL error
+ * Obtain reason string for passed SSL errcode
+ *
+ * ERR_get_error() is used by caller to get errcode to pass here.
  *
  * Some caution is needed here since ERR_reason_error_string will
  * return NULL if it doesn't recognize the error code.  We don't
@@ -1467,28 +1498,26 @@ static char ssl_nomem[] = "out of memory allocating error description";
 #define SSL_ERR_LEN 128
 
 static char *
-SSLerrmessage(void)
+SSLerrmessage(unsigned long ecode)
 {
-	unsigned long errcode;
 	const char *errreason;
 	char	   *errbuf;
 
 	errbuf = malloc(SSL_ERR_LEN);
 	if (!errbuf)
 		return ssl_nomem;
-	errcode = ERR_get_error();
-	if (errcode == 0)
+	if (ecode == 0)
 	{
 		snprintf(errbuf, SSL_ERR_LEN, libpq_gettext("no SSL error reported"));
 		return errbuf;
 	}
-	errreason = ERR_reason_error_string(errcode);
+	errreason = ERR_reason_error_string(ecode);
 	if (errreason != NULL)
 	{
 		strlcpy(errbuf, errreason, SSL_ERR_LEN);
 		return errbuf;
 	}
-	snprintf(errbuf, SSL_ERR_LEN, libpq_gettext("SSL error code %lu"), errcode);
+	snprintf(errbuf, SSL_ERR_LEN, libpq_gettext("SSL error code %lu"), ecode);
 	return errbuf;
 }