Add support for abstract Unix-domain sockets

This is a variant of the normal Unix-domain sockets that don't use the
file system but a separate "abstract" namespace.  At the user
interface, such sockets are represented by names starting with "@".
Supported on Linux and Windows right now.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/6dee8574-b0ad-fc49-9c8c-2edc796f0033@2ndquadrant.com

2 files changed, 27 insertions, 2 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 3795c570048..f810789ea82 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -749,6 +749,21 @@ include_dir 'conf.d'
         An empty value
         specifies not listening on any Unix-domain sockets, in which case
         only TCP/IP sockets can be used to connect to the server.
+       </para>
+
+       <para>
+        A value that starts with <literal>@</literal> specifies that a
+        Unix-domain socket in the abstract namespace should be created
+        (currently supported on Linux and Windows).  In that case, this value
+        does not specify a <quote>directory</quote> but a prefix from which
+        the actual socket name is computed in the same manner as for the
+        file-system namespace.  While the abstract socket name prefix can be
+        chosen freely, since it is not a file-system location, the convention
+        is to nonetheless use file-system-like values such as
+        <literal>@/tmp</literal>.
+       </para>
+
+       <para>
         The default value is normally
         <filename>/tmp</filename>, but that can be changed at build time.
         On Windows, the default is empty, which means no Unix-domain socket is
@@ -763,6 +778,7 @@ include_dir 'conf.d'
         named <literal>.s.PGSQL.<replaceable>nnnn</replaceable>.lock</literal> will be
         created in each of the <varname>unix_socket_directories</varname> directories.
         Neither file should ever be removed manually.
+        For sockets in the abstract namespace, no lock file is created.
        </para>
       </listitem>
      </varlistentry>
@@ -787,7 +803,8 @@ include_dir 'conf.d'
 
        <para>
         This parameter is not supported on Windows.  Any setting will be
-        ignored.
+        ignored.  Also, sockets in the abstract namespace have no file owner,
+        so this setting is also ignored in that case.
        </para>
       </listitem>
      </varlistentry>
@@ -834,6 +851,11 @@ include_dir 'conf.d'
         similar effect by pointing <varname>unix_socket_directories</varname> to a
         directory having search permission limited to the desired audience.
        </para>
+
+       <para>
+        Sockets in the abstract namespace have no file permissions, so this
+        setting is also ignored in that case.
+       </para>
       </listitem>
      </varlistentry>
 
diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 9d4b6ab4a89..06bd412044c 100644
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -1031,7 +1031,10 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
         communication; the value is the name of the directory in which the
         socket file is stored.  (On Unix, an absolute path name begins with a
         slash.  On Windows, paths starting with drive letters are also
-        recognized.)  The default behavior when <literal>host</literal> is not
+        recognized.)  If the host name starts with <literal>@</literal>, it is
+        taken as a Unix-domain socket in the abstract namespace (currently
+        supported on Linux and Windows).
+        The default behavior when <literal>host</literal> is not
         specified, or is empty, is to connect to a Unix-domain
         socket<indexterm><primary>Unix domain socket</primary></indexterm> in
         <filename>/tmp</filename> (or whatever socket directory was specified