Enable SSL library detection via PQsslAttribute()

Currently, libpq client code must have a connection handle
before it can query the "library" SSL attribute.  This poses
problems if the client needs to know what SSL library is in
use before constructing a connection string.

Allow PQsslAttribute(NULL, "library") to return the library
in use -- currently, just "OpenSSL" or NULL. The new behavior
is announced with the LIBPQ_HAS_SSL_LIBRARY_DETECTION feature
macro, allowing clients to differentiate between a libpq that
was compiled without SSL support and a libpq that's just too
old to tell.

Author: Jacob Champion <[email protected]>
Reviewed-by: Robert Haas <[email protected]>
Reviewed-by: Daniel Gustafsson <[email protected]>
Discussion: https://postgr.es/m/[email protected]

7 files changed, 65 insertions, 4 deletions

diff --git a/src/interfaces/libpq/Makefile b/src/interfaces/libpq/Makefile
index 3c53393fa41..89bf5e01260 100644
--- a/src/interfaces/libpq/Makefile
+++ b/src/interfaces/libpq/Makefile
@@ -13,6 +13,7 @@ subdir = src/interfaces/libpq
 top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
+export with_ssl
 
 PGFILEDESC = "PostgreSQL Access Library"
 
diff --git a/src/interfaces/libpq/fe-secure-openssl.c b/src/interfaces/libpq/fe-secure-openssl.c
index d81218a4ccd..d3bf57b850a 100644
--- a/src/interfaces/libpq/fe-secure-openssl.c
+++ b/src/interfaces/libpq/fe-secure-openssl.c
@@ -1631,14 +1631,14 @@ PQsslAttributeNames(PGconn *conn)
 const char *
 PQsslAttribute(PGconn *conn, const char *attribute_name)
 {
+	if (strcmp(attribute_name, "library") == 0)
+		return "OpenSSL";
+
 	if (!conn)
 		return NULL;
 	if (conn->ssl == NULL)
 		return NULL;
 
-	if (strcmp(attribute_name, "library") == 0)
-		return "OpenSSL";
-
 	if (strcmp(attribute_name, "key_bits") == 0)
 	{
 		static char sslbits_str[12];
diff --git a/src/interfaces/libpq/libpq-fe.h b/src/interfaces/libpq/libpq-fe.h
index 20eb855abc1..7986445f1a9 100644
--- a/src/interfaces/libpq/libpq-fe.h
+++ b/src/interfaces/libpq/libpq-fe.h
@@ -36,6 +36,8 @@ extern "C"
 #define LIBPQ_HAS_PIPELINING 1
 /* Indicates presence of PQsetTraceFlags; also new PQtrace output format */
 #define LIBPQ_HAS_TRACE_FLAGS 1
+/* Indicates that PQsslAttribute(NULL, "library") is useful */
+#define LIBPQ_HAS_SSL_LIBRARY_DETECTION 1
 
 /*
  * Option flags for PQcopyResult
diff --git a/src/interfaces/libpq/t/002_api.pl b/src/interfaces/libpq/t/002_api.pl
new file mode 100644
index 00000000000..7c6c5788a0c
--- /dev/null
+++ b/src/interfaces/libpq/t/002_api.pl
@@ -0,0 +1,20 @@
+# Copyright (c) 2022, PostgreSQL Global Development Group
+use strict;
+use warnings;
+
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Test PQsslAttribute(NULL, "library")
+my ($out, $err) = run_command(['testclient', '--ssl']);
+
+if ($ENV{with_ssl} eq 'openssl')
+{
+	is($out, 'OpenSSL', 'PQsslAttribute(NULL, "library") returns "OpenSSL"');
+}
+else
+{
+	is($err, 'SSL is not enabled', 'PQsslAttribute(NULL, "library") returns NULL');
+}
+
+done_testing();
diff --git a/src/interfaces/libpq/test/.gitignore b/src/interfaces/libpq/test/.gitignore
index 5e803d8816a..4b17210483d 100644
--- a/src/interfaces/libpq/test/.gitignore
+++ b/src/interfaces/libpq/test/.gitignore
@@ -1 +1,2 @@
+/testclient
 /uri-regress
diff --git a/src/interfaces/libpq/test/Makefile b/src/interfaces/libpq/test/Makefile
index 54212159065..1d45be0c375 100644
--- a/src/interfaces/libpq/test/Makefile
+++ b/src/interfaces/libpq/test/Makefile
@@ -11,7 +11,7 @@ endif
 override CPPFLAGS := -I$(libpq_srcdir) $(CPPFLAGS)
 LDFLAGS_INTERNAL += $(libpq_pgport)
 
-PROGS = uri-regress
+PROGS = testclient uri-regress
 
 all: $(PROGS)
 
diff --git a/src/interfaces/libpq/test/testclient.c b/src/interfaces/libpq/test/testclient.c
new file mode 100644
index 00000000000..2c730d83fa6
--- /dev/null
+++ b/src/interfaces/libpq/test/testclient.c
@@ -0,0 +1,37 @@
+/*
+ * testclient.c
+ *		A test program for the libpq public API
+ *
+ * Copyright (c) 2022, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		src/interfaces/libpq/test/testclient.c
+ */
+
+#include "postgres_fe.h"
+
+#include "libpq-fe.h"
+
+static void
+print_ssl_library()
+{
+	const char *lib = PQsslAttribute(NULL, "library");
+
+	if (!lib)
+		fprintf(stderr, "SSL is not enabled\n");
+	else
+		printf("%s\n", lib);
+}
+
+int
+main(int argc, char *argv[])
+{
+	if ((argc > 1) && !strcmp(argv[1], "--ssl"))
+	{
+		print_ssl_library();
+		return 0;
+	}
+
+	printf("currently only --ssl is supported\n");
+	return 1;
+}