Improve psql's behavior for \set and \unset of its control variables.

This commit improves on the results of commit 511ae62 in two ways:

1. It restores the historical behavior that "\set FOO" is interpreted
as setting FOO to "on", if FOO is a boolean control variable.  We
already found one test script that was expecting that behavior, and
the psql documentation certainly does nothing to discourage people
from assuming that would work, since it often says just "if FOO is set"
when describing the effects of a boolean variable.  However, now this
case will result in actually setting FOO to "on", not an empty string.

2. It arranges for an "\unset" of a control variable to set the value
back to its default value, rather than becoming apparently undefined.
The control variables are also initialized that way at psql startup.

In combination, these things guarantee that a control variable always
has a displayable value that reflects what psql is actually doing.
That is a pretty substantial usability improvement.

The implementation involves adding a second type of variable hook function
that is able to replace a proposed new value (including NULL) with another
one.  We could alternatively have complicated the API of the assign hook,
but this way seems better since many variables can share the same
substitution hook function.

Also document the actual behavior of these variables more fully,
including covering assorted behaviors that were there before but
never documented.

This patch also includes some minor cleanup that should have been in
511ae62 but was missed.

Patch by me, but it owes a lot to discussions with Daniel Vérité.

Discussion: https://postgr.es/m/9572.1485821620@sss.pgh.pa.us

Author: tglsfdc

doc/src/sgml/ref/psql-ref.sgml

@@ -455,8 +455,8 @@ EOF
       any, by an equal sign on the command line. To unset a variable,
       leave off the equal sign. To set a variable with an empty value,
       use the equal sign but leave off the value. These assignments are
-      done during a very early stage of start-up, so variables reserved
-      for internal purposes might get overwritten later.
+      done during command line processing, so variables that reflect
+      connection state will get overwritten later.
       </para>
       </listitem>
     </varlistentry>
@@ -2692,7 +2692,7 @@ lo_import 152801
         class="parameter">name</replaceable> to <replaceable
         class="parameter">value</replaceable>, or if more than one value
         is given, to the concatenation of all of them. If only one
-        argument is given, the variable is set with an empty value. To
+        argument is given, the variable is set to an empty-string value. To
         unset a variable, use the <command>\unset</command> command.
         </para>
 
@@ -2709,9 +2709,11 @@ lo_import 152801
         </para>
 
         <para>
-        Although you are welcome to set any variable to anything you
-        want, <application>psql</application> treats several variables
-        as special. They are documented in the section about variables.
+        Certain variables are special, in that they
+        control <application>psql</application>'s behavior or are
+        automatically set to reflect connection state.  These variables are
+        documented in <xref linkend="APP-PSQL-variables"
+        endterm="APP-PSQL-variables-title">, below.
         </para>
 
         <note>
@@ -2835,6 +2837,14 @@ testdb=&gt; <userinput>\setenv LESS -imx4F</userinput>
         Unsets (deletes) the <application>psql</> variable <replaceable
         class="parameter">name</replaceable>.
         </para>
+
+        <para>
+        Most variables that control <application>psql</application>'s behavior
+        cannot be unset; instead, an <literal>\unset</> command is interpreted
+        as setting them to their default values.
+        See <xref linkend="APP-PSQL-variables"
+        endterm="APP-PSQL-variables-title">, below.
+        </para>
         </listitem>
       </varlistentry>
 
@@ -3053,7 +3063,7 @@ bar
 
     <para>
     If you call <command>\set</command> without a second argument, the
-    variable is set, with an empty string as value. To unset (i.e., delete)
+    variable is set to an empty-string value. To unset (i.e., delete)
     a variable, use the command <command>\unset</command>.  To show the
     values of all variables, call <command>\set</command> without any argument.
     </para>
@@ -3082,8 +3092,23 @@ bar
     By convention, all specially treated variables' names
     consist of all upper-case ASCII letters (and possibly digits and
     underscores). To ensure maximum compatibility in the future, avoid
-    using such variable names for your own purposes. A list of all specially
-    treated variables follows.
+    using such variable names for your own purposes.
+   </para>
+
+   <para>
+    Variables that control <application>psql</application>'s behavior
+    generally cannot be unset or set to invalid values.  An <literal>\unset</>
+    command is allowed but is interpreted as setting the variable to its
+    default value.  A <literal>\set</> command without a second argument is
+    interpreted as setting the variable to <literal>on</>, for control
+    variables that accept that value, and is rejected for others.  Also,
+    control variables that accept the values <literal>on</>
+    and <literal>off</> will also accept other common spellings of Boolean
+    values, such as <literal>true</> and <literal>false</>.
+   </para>
+
+   <para>
+    The specially treated variables are:
    </para>
 
     <variablelist>
@@ -3153,7 +3178,7 @@ bar
         <para>
         The name of the database you are currently connected to. This is
         set every time you connect to a database (including program
-        start-up), but can be unset.
+        start-up), but can be changed or unset.
         </para>
         </listitem>
       </varlistentry>
@@ -3171,8 +3196,8 @@ bar
         as it is sent to the server. The switch to select this behavior is
         <option>-e</option>. If set to <literal>errors</literal>, then only
         failed queries are displayed on standard error output. The switch
-        for this behavior is <option>-b</option>. If unset, or if set to
-        <literal>none</literal>, then no queries are displayed.
+        for this behavior is <option>-b</option>. If set to
+        <literal>none</literal> (the default), then no queries are displayed.
         </para>
         </listitem>
       </varlistentry>
@@ -3187,8 +3212,9 @@ bar
         <productname>PostgreSQL</productname> internals and provide
         similar functionality in your own programs. (To select this behavior
         on program start-up, use the switch <option>-E</option>.)  If you set
-        the variable to the value <literal>noexec</literal>, the queries are
+        this variable to the value <literal>noexec</literal>, the queries are
         just shown but are not actually sent to the server and executed.
+        The default value is <literal>off</>.
         </para>
         </listitem>
       </varlistentry>
@@ -3200,7 +3226,7 @@ bar
         The current client character set encoding.
         This is set every time you connect to a database (including
         program start-up), and when you change the encoding
-        with <literal>\encoding</>, but it can be unset.
+        with <literal>\encoding</>, but it can be changed or unset.
         </para>
         </listitem>
       </varlistentry>
@@ -3209,7 +3235,7 @@ bar
         <term><varname>FETCH_COUNT</varname></term>
         <listitem>
         <para>
-        If this variable is set to an integer value &gt; 0,
+        If this variable is set to an integer value greater than zero,
         the results of <command>SELECT</command> queries are fetched
         and displayed in groups of that many rows, rather than the
         default behavior of collecting the entire result set before
@@ -3220,6 +3246,13 @@ bar
         Keep in mind that when using this feature, a query might
         fail after having already displayed some rows.
         </para>
+
+        <para>
+        <varname>FETCH_COUNT</varname> is ignored if it is unset or does not
+        have a positive value.  It cannot be set to a value that is not
+        syntactically an integer.
+        </para>
+
         <tip>
         <para>
         Although you can use any output format with this feature,
@@ -3241,7 +3274,7 @@ bar
          list. If set to a value of <literal>ignoredups</literal>, lines
          matching the previous history line are not entered. A value of
          <literal>ignoreboth</literal> combines the two options. If
-         unset, or if set to <literal>none</literal> (the default), all lines
+         set to <literal>none</literal> (the default), all lines
          read in interactive mode are saved on the history list.
         </para>
         <note>
@@ -3257,8 +3290,12 @@ bar
         <term><varname>HISTFILE</varname></term>
         <listitem>
         <para>
-        The file name that will be used to store the history list. The default
-        value is <filename>~/.psql_history</filename>.  For example, putting:
+        The file name that will be used to store the history list.  If unset,
+        the file name is taken from the <envar>PSQL_HISTORY</envar>
+        environment variable.  If that is not set either, the default
+        is <filename>~/.psql_history</filename>,
+        or <filename>%APPDATA%\postgresql\psql_history</filename> on Windows.
+        For example, putting:
 <programlisting>
 \set HISTFILE ~/.psql_history- :DBNAME
 </programlisting>
@@ -3279,8 +3316,10 @@ bar
         <term><varname>HISTSIZE</varname></term>
         <listitem>
         <para>
-        The number of commands to store in the command history. The
-        default value is 500.
+        The maximum number of commands to store in the command history.
+        If unset, at most 500 commands are stored by default.
+        If set to a value that is negative or not an integer, no limit is
+        applied.
         </para>
         <note>
         <para>
@@ -3297,7 +3336,7 @@ bar
         <para>
         The database server host you are currently connected to. This is
         set every time you connect to a database (including program
-        start-up), but can be unset.
+        start-up), but can be changed or unset.
         </para>
         </listitem>
       </varlistentry>
@@ -3350,7 +3389,7 @@ bar
         generates an error, the error is ignored and the transaction
         continues. When set to <literal>interactive</>, such errors are only
         ignored in interactive sessions, and not when reading script
-        files. When unset or set to <literal>off</>, a statement in a
+        files. When set to <literal>off</> (the default), a statement in a
         transaction block that generates an error aborts the entire
         transaction. The error rollback mode works by issuing an
         implicit <command>SAVEPOINT</> for you, just before each command
@@ -3385,7 +3424,7 @@ bar
         <para>
         The database server port to which you are currently connected.
         This is set every time you connect to a database (including
-        program start-up), but can be unset.
+        program start-up), but can be changed or unset.
         </para>
         </listitem>
       </varlistentry>
@@ -3458,7 +3497,7 @@ bar
         <para>
         The database user you are currently connected as. This is set
         every time you connect to a database (including program
-        start-up), but can be unset.
+        start-up), but can be changed or unset.
         </para>
         </listitem>
       </varlistentry>
@@ -3481,7 +3520,7 @@ bar
         <listitem>
         <para>
         This variable is set at program start-up to
-        reflect <application>psql</>'s version.  It can be unset or changed.
+        reflect <application>psql</>'s version.  It can be changed or unset.
         </para>
         </listitem>
       </varlistentry>
@@ -4015,6 +4054,7 @@ PSQL_EDITOR_LINENUMBER_ARG='--line '
     </para>
     <para>
      The location of the history file can be set explicitly via
+     the <varname>HISTFILE</varname> <application>psql</> variable or
      the <envar>PSQL_HISTORY</envar> environment variable.
     </para>
    </listitem>