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

1 files changed, 33 insertions, 2 deletions

diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 2e0c06102ee..910e9a81eaf 100644
--- a/doc/src/sgml/libpq.sgml
+++ b/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>
@@ -7171,6 +7192,16 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
     <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>
        <primary><envar>PGSSLCERT</envar></primary>
       </indexterm>
       <envar>PGSSLCERT</envar> behaves the same as the <xref