Allow group access on PGDATA

Allow the cluster to be optionally init'd with read access for the
group.

This means a relatively non-privileged user can perform a backup of the
cluster without requiring write privileges, which enhances security.

The mode of PGDATA is used to determine whether group permissions are
enabled for directory and file creates.  This method was chosen as it's
simple and works well for the various utilities that write into PGDATA.

Changing the mode of PGDATA manually will not automatically change the
mode of all the files contained therein.  If the user would like to
enable group access on an existing cluster then changing the mode of all
the existing files will be required.  Note that pg_upgrade will
automatically change the mode of all migrated files if the new cluster
is init'd with the -g option.

Tests are included for the backend and all the utilities which operate
on the PG data directory to ensure that the correct mode is set based on
the data directory permissions.

Author: David Steele <david@pgmasters.net>
Reviewed-By: Michael Paquier, with discussion amongst many others.
Discussion: https://postgr.es/m/ad346fe6-b23e-59f1-ecb7-0e08390ad629%40pgmasters.net

4 files changed, 42 insertions, 0 deletions

diff --git a/doc/src/sgml/ref/initdb.sgml b/doc/src/sgml/ref/initdb.sgml
index 826dd91f729..10a8a86a030 100644
--- a/doc/src/sgml/ref/initdb.sgml
+++ b/doc/src/sgml/ref/initdb.sgml
@@ -77,6 +77,14 @@ PostgreSQL documentation
   </para>
 
   <para>
+    For security reasons the new cluster created by <command>initdb</command>
+    will only be accessible by the cluster owner by default.  The
+    <option>--allow-group-access</option> option allows any user in the same
+    group as the cluster owner to read files in the cluster.  This is useful
+    for performing backups as a non-privileged user.
+  </para>
+
+  <para>
    <command>initdb</command> initializes the database cluster's default
    locale and character set encoding. The character set encoding,
    collation order (<literal>LC_COLLATE</literal>) and character set classes
@@ -188,6 +196,17 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry id="app-initdb-allow-group-access" xreflabel="group access">
+      <term><option>-g</option></term>
+      <term><option>--allow-group-access</option></term>
+      <listitem>
+       <para>
+        Allows users in the same group as the cluster owner to read all cluster
+        files created by <command>initdb</command>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="app-initdb-data-checksums" xreflabel="data checksums">
       <term><option>-k</option></term>
       <term><option>--data-checksums</option></term>
diff --git a/doc/src/sgml/ref/pg_basebackup.sgml b/doc/src/sgml/ref/pg_basebackup.sgml
index 95045669c93..fc1edf48645 100644
--- a/doc/src/sgml/ref/pg_basebackup.sgml
+++ b/doc/src/sgml/ref/pg_basebackup.sgml
@@ -737,6 +737,12 @@ PostgreSQL documentation
    or later.
   </para>
 
+  <para>
+   <application>pg_basebackup</application> will preserve group permissions in
+   both the <literal>plain</literal> and <literal>tar</literal> formats if group
+   permissions are enabled on the source cluster.
+  </para>
+
  </refsect1>
 
  <refsect1>
diff --git a/doc/src/sgml/ref/pg_receivewal.sgml b/doc/src/sgml/ref/pg_receivewal.sgml
index e3f2ce1fcb7..a18ddd4bff1 100644
--- a/doc/src/sgml/ref/pg_receivewal.sgml
+++ b/doc/src/sgml/ref/pg_receivewal.sgml
@@ -425,6 +425,12 @@ PostgreSQL documentation
    not keep up with fetching the WAL data.
   </para>
 
+  <para>
+   <application>pg_receivewal</application> will preserve group permissions on
+   the received WAL files if group permissions are enabled on the source
+   cluster.
+  </para>
+
  </refsect1>
 
  <refsect1>
diff --git a/doc/src/sgml/ref/pg_recvlogical.sgml b/doc/src/sgml/ref/pg_recvlogical.sgml
index a79ca200849..141c5cddce1 100644
--- a/doc/src/sgml/ref/pg_recvlogical.sgml
+++ b/doc/src/sgml/ref/pg_recvlogical.sgml
@@ -400,6 +400,17 @@ PostgreSQL documentation
  </refsect1>
 
  <refsect1>
+  <title>Notes</title>
+
+  <para>
+   <application>pg_recvlogical</application> will preserve group permissions on
+   the received WAL files if group permissions are enabled on the source
+   cluster.
+  </para>
+
+ </refsect1>
+
+ <refsect1>
   <title>Examples</title>
 
   <para>