Revert changes for SSL compression in libpq

This partially reverts 096bbf7 and 9d2d457, undoing the libpq changes as
it could cause breakages in distributions that share one single libpq
version across multiple major versions of Postgres for extensions and
applications linking to that.

Note that the backend is unchanged here, and it still disables SSL
compression while simplifying the underlying catalogs that tracked if
compression was enabled or not for a SSL connection.

Per discussion with Tom Lane and Daniel Gustafsson.

Discussion: https://postgr.es/m/YEbq15JKJwIX+S6m@paquier.xyz

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -163,11 +163,11 @@ ALTER SERVER testserver1 OPTIONS (
 	keepalives_interval 'value',
 	tcp_user_timeout 'value',
 	-- requiressl 'value',
+	sslcompression 'value',
 	sslmode 'value',
 	sslcert 'value',
 	sslkey 'value',
 	sslrootcert 'value',
-	sslcompression 'value',
 	sslcrl 'value',
 	--requirepeer 'value',
 	krbsrvname 'value',

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -177,11 +177,11 @@ ALTER SERVER testserver1 OPTIONS (
 	keepalives_interval 'value',
 	tcp_user_timeout 'value',
 	-- requiressl 'value',
+	sslcompression 'value',
 	sslmode 'value',
 	sslcert 'value',
 	sslkey 'value',
 	sslrootcert 'value',
-	sslcompression 'value',
 	sslcrl 'value',
 	--requirepeer 'value',
 	krbsrvname 'value',

doc/src/sgml/libpq.sgml

@@ -1640,7 +1640,26 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
       <term><literal>sslcompression</literal></term>
       <listitem>
        <para>
-        Ignored (formerly, this specified whether to attempt SSL compression).
+        If set to 1, data sent over SSL connections will be compressed.  If
+        set to 0, compression will be disabled.  The default is 0.  This
+        parameter is ignored if a connection without SSL is made.
+       </para>
+
+       <para>
+        SSL compression is nowadays considered insecure and its use is no
+        longer recommended.  <productname>OpenSSL</productname> 1.1.0 disables
+        compression by default, and many operating system distributions
+        disable it in prior versions as well, so setting this parameter to on
+        will not have any effect if the server does not accept compression.
+        <productname>PostgreSQL</productname> 14 disables compression
+        completely in the backend.
+       </para>
+
+       <para>
+        If security is not a primary concern, compression can improve
+        throughput if the network is the bottleneck.  Disabling compression
+        can improve response time and throughput if CPU performance is the
+        limiting factor.
        </para>
       </listitem>
      </varlistentry>
@@ -2533,7 +2552,9 @@ const char *PQsslAttribute(const PGconn *conn, const char *attribute_name);
          <term><literal>compression</literal></term>
           <listitem>
            <para>
-            SSL compression is no longer supported, always returns "off".
+            If SSL compression is in use, returns the name of the compression
+            algorithm, or "on" if compression is used but the algorithm is
+            not known. If compression is not in use, returns "off".
            </para>
           </listitem>
          </varlistentry>
@@ -7168,6 +7189,16 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
      </para>
     </listitem>
 
+    <listitem>
+     <para>
+      <indexterm>
+       <primary><envar>PGSSLCOMPRESSION</envar></primary>
+      </indexterm>
+      <envar>PGSSLCOMPRESSION</envar> behaves the same as the <xref
+      linkend="libpq-connect-sslcompression"/> connection parameter.
+     </para>
+    </listitem>
+
     <listitem>
      <para>
       <indexterm>

src/bin/psql/command.c

@@ -3509,6 +3509,7 @@ printSSLInfo(void)
 	const char *protocol;
 	const char *cipher;
 	const char *bits;
+	const char *compression;
 
 	if (!PQsslInUse(pset.db))
 		return;					/* no SSL */
@@ -3516,11 +3517,13 @@ printSSLInfo(void)
 	protocol = PQsslAttribute(pset.db, "protocol");
 	cipher = PQsslAttribute(pset.db, "cipher");
 	bits = PQsslAttribute(pset.db, "key_bits");
+	compression = PQsslAttribute(pset.db, "compression");
 
-	printf(_("SSL connection (protocol: %s, cipher: %s, bits: %s)\n"),
+	printf(_("SSL connection (protocol: %s, cipher: %s, bits: %s, compression: %s)\n"),
 		   protocol ? protocol : _("unknown"),
 		   cipher ? cipher : _("unknown"),
-		   bits ? bits : _("unknown"));
+		   bits ? bits : _("unknown"),
+		   (compression && strcmp(compression, "off") != 0) ? _("on") : _("off"));
 }
 
 /*

src/interfaces/libpq/fe-connect.c

@@ -275,12 +275,9 @@ static const internalPQconninfoOption PQconninfoOptions[] = {
 		"SSL-Mode", "", 12,		/* sizeof("verify-full") == 12 */
 	offsetof(struct pg_conn, sslmode)},
 
-	/*
-	 * "sslcompression" is no longer used, but keep it present for backwards
-	 * compatibility.
-	 */
-	{"sslcompression", NULL, NULL, NULL,
-	"SSL-Compression", "", 1, -1},
+	{"sslcompression", "PGSSLCOMPRESSION", "0", NULL,
+		"SSL-Compression", "", 1,
+	offsetof(struct pg_conn, sslcompression)},
 
 	{"sslcert", "PGSSLCERT", NULL, NULL,
 		"SSL-Client-Cert", "", 64,
@@ -4054,6 +4051,8 @@ freePGconn(PGconn *conn)
 		free(conn->sslcrl);
 	if (conn->sslcrldir)
 		free(conn->sslcrldir);
+	if (conn->sslcompression)
+		free(conn->sslcompression);
 	if (conn->requirepeer)
 		free(conn->requirepeer);
 	if (conn->ssl_min_protocol_version)

src/interfaces/libpq/fe-secure-openssl.c

@@ -1257,8 +1257,13 @@ initialize_SSL(PGconn *conn)
 	if (have_rootcert)
 		SSL_set_verify(conn->ssl, SSL_VERIFY_PEER, verify_cb);
 
-	/* disable SSL compression */
+	/*
+	 * Set compression option if necessary.
+	 */
+	if (conn->sslcompression && conn->sslcompression[0] == '0')
 		SSL_set_options(conn->ssl, SSL_OP_NO_COMPRESSION);
+	else
+		SSL_clear_options(conn->ssl, SSL_OP_NO_COMPRESSION);
 
 	return 0;
 }
@@ -1548,12 +1553,8 @@ PQsslAttribute(PGconn *conn, const char *attribute_name)
 	if (strcmp(attribute_name, "cipher") == 0)
 		return SSL_get_cipher(conn->ssl);
 
-	/*
-	 * SSL compression is disabled, so even if connecting to an older server
-	 * which still supports it, it will not be active.
-	 */
 	if (strcmp(attribute_name, "compression") == 0)
-		return "off";
+		return SSL_get_current_compression(conn->ssl) ? "on" : "off";
 
 	if (strcmp(attribute_name, "protocol") == 0)
 		return SSL_get_version(conn->ssl);

src/interfaces/libpq/libpq-int.h

@@ -358,6 +358,7 @@ struct pg_conn
 	char	   *keepalives_count;	/* maximum number of TCP keepalive
 									 * retransmits */
 	char	   *sslmode;		/* SSL mode (require,prefer,allow,disable) */
+	char	   *sslcompression; /* SSL compression (0 or 1) */
 	char	   *sslkey;			/* client key filename */
 	char	   *sslcert;		/* client certificate filename */
 	char	   *sslpassword;	/* client key file password */

src/test/ssl/t/001_ssltests.pl

@@ -17,7 +17,7 @@ if ($ENV{with_ssl} ne 'openssl')
 }
 else
 {
-	plan tests => 101;
+	plan tests => 100;
 }
 
 #### Some configuration
@@ -157,13 +157,6 @@ test_connect_fails(
 	qr/root certificate file "invalid" does not exist/,
 	"connect without server root cert sslmode=verify-full");
 
-# Test deprecated SSL parameters, still accepted for backwards
-# compatibility.
-test_connect_ok(
-	$common_connstr,
-	"sslrootcert=invalid sslmode=require sslcompression=1 requiressl=1",
-	"connect with deprecated connection parameters");
-
 # Try with wrong root cert, should fail. (We're using the client CA as the
 # root, but the server's key is signed by the server CA.)
 test_connect_fails($common_connstr,