Make psql reject attempts to set special variables to invalid values.

Previously, if the user set a special variable such as ECHO to an
unrecognized value, psql would bleat but store the new value anyway, and
then fall back to a default setting for the behavior controlled by the
variable.  This was agreed to be a not particularly good idea.  With
this patch, invalid values result in an error message and no change in
state.

(But this applies only to variables that affect psql's behavior; purely
informational variables such as ENCODING can still be set to random
values.)

To do this, modify the API for psql's assign-hook functions so that they
can return an OK/not OK result, and give them the responsibility for
printing error messages when they reject a value.  Adjust the APIs for
ParseVariableBool and ParseVariableNum to support the new behavior
conveniently.

In passing, document the variable VERSION, which had somehow escaped that.
And improve the quite-inadequate commenting in psql/variables.c.

Daniel Vérité, reviewed by Rahila Syed, some further tweaking by me

Discussion: https://postgr.es/m/7356e741-fa59-4146-a8eb-cf95fd6b21fb@mm

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

@@ -3078,10 +3078,8 @@ bar
     by <application>psql</application>. They represent certain option
     settings that can be changed at run time by altering the value of
     the variable, or in some cases represent changeable state of
-    <application>psql</application>. Although
-    you can use these variables for other purposes, this is not
-    recommended, as the program behavior might grow really strange
-    really quickly. By convention, all specially treated variables' names
+    <application>psql</application>.
+    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
@@ -3170,12 +3168,11 @@ bar
         start-up, use the switch <option>-a</option>. If set to
         <literal>queries</literal>,
         <application>psql</application> prints each query to standard output
-        as it is sent to the server. The switch for this is
+        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 is <option>-b</option>. If unset, or if set to
-        <literal>none</literal> (or any other value than those above) then
-        no queries are displayed.
+        for this behavior is <option>-b</option>. If unset, or if set to
+        <literal>none</literal>, then no queries are displayed.
         </para>
         </listitem>
       </varlistentry>
@@ -3201,6 +3198,9 @@ bar
         <listitem>
         <para>
         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.
         </para>
         </listitem>
       </varlistentry>
@@ -3241,9 +3241,8 @@ 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> (or any other value
-         than those above), all lines read in interactive mode are
-         saved on the history list.
+         unset, or if set to <literal>none</literal> (the default), all lines
+         read in interactive mode are saved on the history list.
         </para>
         <note>
         <para>
@@ -3312,7 +3311,7 @@ bar
          to an interactive session of <application>psql</application>
          will terminate the application. If set to a numeric value,
          that many <acronym>EOF</> characters are ignored before the
-         application terminates.  If the variable is set but has no
+         application terminates.  If the variable is set but not to a
          numeric value, the default is 10.
         </para>
         <note>
@@ -3477,6 +3476,16 @@ bar
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>VERSION</varname></term>
+        <listitem>
+        <para>
+        This variable is set at program start-up to
+        reflect <application>psql</>'s version.  It can be unset or changed.
+        </para>
+        </listitem>
+      </varlistentry>
+
     </variablelist>
 
    </refsect3>

src/bin/psql/command.c

@@ -248,32 +248,38 @@ exec_command(const char *cmd,
 				   *opt2,
 				   *opt3,
 				   *opt4;
-		enum trivalue reuse_previous;
+		enum trivalue reuse_previous = TRI_DEFAULT;
 
 		opt1 = read_connect_arg(scan_state);
 		if (opt1 != NULL && strncmp(opt1, prefix, sizeof(prefix) - 1) == 0)
 		{
-			reuse_previous =
-				ParseVariableBool(opt1 + sizeof(prefix) - 1, prefix) ?
-				TRI_YES : TRI_NO;
+			bool		on_off;
 
+			success = ParseVariableBool(opt1 + sizeof(prefix) - 1,
+										"-reuse-previous",
+										&on_off);
+			if (success)
+			{
+				reuse_previous = on_off ? TRI_YES : TRI_NO;
 				free(opt1);
 				opt1 = read_connect_arg(scan_state);
 			}
-		else
-			reuse_previous = TRI_DEFAULT;
+		}
 
+		if (success)			/* give up if reuse_previous was invalid */
+		{
 			opt2 = read_connect_arg(scan_state);
 			opt3 = read_connect_arg(scan_state);
 			opt4 = read_connect_arg(scan_state);
 
 			success = do_connect(reuse_previous, opt1, opt2, opt3, opt4);
 
-		free(opt1);
 			free(opt2);
 			free(opt3);
 			free(opt4);
 		}
+		free(opt1);
+	}
 
 	/* \cd */
 	else if (strcmp(cmd, "cd") == 0)
@@ -1208,10 +1214,7 @@ exec_command(const char *cmd,
 
 			if (result &&
 				!SetVariable(pset.vars, opt, result))
-			{
-				psql_error("\\%s: error while setting variable\n", cmd);
 				success = false;
-			}
 
 			if (result)
 				free(result);
@@ -1325,10 +1328,8 @@ exec_command(const char *cmd,
 			}
 
 			if (!SetVariable(pset.vars, opt0, newval))
-			{
-				psql_error("\\%s: error while setting variable\n", cmd);
 				success = false;
-			}
+
 			free(newval);
 		}
 		free(opt0);
@@ -1564,7 +1565,7 @@ exec_command(const char *cmd,
 												 OT_NORMAL, NULL, false);
 
 		if (opt)
-			pset.timing = ParseVariableBool(opt, "\\timing");
+			success = ParseVariableBool(opt, "\\timing", &pset.timing);
 		else
 			pset.timing = !pset.timing;
 		if (!pset.quiet)
@@ -1589,10 +1590,8 @@ exec_command(const char *cmd,
 			success = false;
 		}
 		else if (!SetVariable(pset.vars, opt, NULL))
-		{
-			psql_error("\\%s: error while setting variable\n", cmd);
 			success = false;
-		}
+
 		free(opt);
 	}
 
@@ -2593,7 +2592,6 @@ do_pset(const char *param, const char *value, printQueryOpt *popt, bool quiet)
 			psql_error("\\pset: allowed formats are unaligned, aligned, wrapped, html, asciidoc, latex, latex-longtable, troff-ms\n");
 			return false;
 		}
-
 	}
 
 	/* set table line style */
@@ -2612,7 +2610,6 @@ do_pset(const char *param, const char *value, printQueryOpt *popt, bool quiet)
 			psql_error("\\pset: allowed line styles are ascii, old-ascii, unicode\n");
 			return false;
 		}
-
 	}
 
 	/* set unicode border line style */
@@ -2665,7 +2662,6 @@ do_pset(const char *param, const char *value, printQueryOpt *popt, bool quiet)
 	{
 		if (value)
 			popt->topt.border = atoi(value);
-
 	}
 
 	/* set expanded/vertical mode */
@@ -2676,7 +2672,17 @@ do_pset(const char *param, const char *value, printQueryOpt *popt, bool quiet)
 		if (value && pg_strcasecmp(value, "auto") == 0)
 			popt->topt.expanded = 2;
 		else if (value)
-			popt->topt.expanded = ParseVariableBool(value, param);
+		{
+			bool		on_off;
+
+			if (ParseVariableBool(value, NULL, &on_off))
+				popt->topt.expanded = on_off ? 1 : 0;
+			else
+			{
+				PsqlVarEnumError(param, value, "on, off, auto");
+				return false;
+			}
+		}
 		else
 			popt->topt.expanded = !popt->topt.expanded;
 	}
@@ -2685,7 +2691,7 @@ do_pset(const char *param, const char *value, printQueryOpt *popt, bool quiet)
 	else if (strcmp(param, "numericlocale") == 0)
 	{
 		if (value)
-			popt->topt.numericLocale = ParseVariableBool(value, param);
+			return ParseVariableBool(value, param, &popt->topt.numericLocale);
 		else
 			popt->topt.numericLocale = !popt->topt.numericLocale;
 	}
@@ -2740,7 +2746,7 @@ do_pset(const char *param, const char *value, printQueryOpt *popt, bool quiet)
 	else if (strcmp(param, "t") == 0 || strcmp(param, "tuples_only") == 0)
 	{
 		if (value)
-			popt->topt.tuples_only = ParseVariableBool(value, param);
+			return ParseVariableBool(value, param, &popt->topt.tuples_only);
 		else
 			popt->topt.tuples_only = !popt->topt.tuples_only;
 	}
@@ -2772,10 +2778,14 @@ do_pset(const char *param, const char *value, printQueryOpt *popt, bool quiet)
 			popt->topt.pager = 2;
 		else if (value)
 		{
-			if (ParseVariableBool(value, param))
-				popt->topt.pager = 1;
-			else
-				popt->topt.pager = 0;
+			bool		on_off;
+
+			if (!ParseVariableBool(value, NULL, &on_off))
+			{
+				PsqlVarEnumError(param, value, "on, off, always");
+				return false;
+			}
+			popt->topt.pager = on_off ? 1 : 0;
 		}
 		else if (popt->topt.pager == 1)
 			popt->topt.pager = 0;
@@ -2794,7 +2804,7 @@ do_pset(const char *param, const char *value, printQueryOpt *popt, bool quiet)
 	else if (strcmp(param, "footer") == 0)
 	{
 		if (value)
-			popt->topt.default_footer = ParseVariableBool(value, param);
+			return ParseVariableBool(value, param, &popt->topt.default_footer);
 		else
 			popt->topt.default_footer = !popt->topt.default_footer;
 	}

src/bin/psql/common.c

@@ -841,7 +841,6 @@ StoreQueryTuple(const PGresult *result)
 
 			if (!SetVariable(pset.vars, varname, value))
 			{
-				psql_error("could not set variable \"%s\"\n", varname);
 				free(varname);
 				success = false;
 				break;

src/bin/psql/input.c

@@ -541,7 +541,7 @@ finishInput(void)
 	{
 		int			hist_size;
 
-		hist_size = GetVariableNum(pset.vars, "HISTSIZE", 500, -1, true);
+		hist_size = GetVariableNum(pset.vars, "HISTSIZE", 500, -1);
 		(void) saveHistory(psql_history, hist_size);
 		free(psql_history);
 		psql_history = NULL;

src/bin/psql/mainloop.c

@@ -162,7 +162,7 @@ MainLoop(FILE *source)
 				/* This tries to mimic bash's IGNOREEOF feature. */
 				count_eof++;
 
-				if (count_eof < GetVariableNum(pset.vars, "IGNOREEOF", 0, 10, false))
+				if (count_eof < GetVariableNum(pset.vars, "IGNOREEOF", 0, 10))
 				{
 					if (!pset.quiet)
 						printf(_("Use \"\\q\" to leave %s.\n"), pset.progname);

src/bin/psql/startup.c

@@ -588,12 +588,8 @@ parse_psql_options(int argc, char *argv[], struct adhoc_opts * options)
 					{
 						*equal_loc = '\0';
 						if (!SetVariable(pset.vars, value, equal_loc + 1))
-						{
-							fprintf(stderr, _("%s: could not set variable \"%s\"\n"),
-									pset.progname, value);
 							exit(EXIT_FAILURE);
 					}
-					}
 
 					free(value);
 					break;
@@ -786,43 +782,47 @@ showVersion(void)
  * This isn't an amazingly good place for them, but neither is anywhere else.
  */
 
-static void
+static bool
 autocommit_hook(const char *newval)
 {
-	pset.autocommit = ParseVariableBool(newval, "AUTOCOMMIT");
+	return ParseVariableBool(newval, "AUTOCOMMIT", &pset.autocommit);
 }
 
-static void
+static bool
 on_error_stop_hook(const char *newval)
 {
-	pset.on_error_stop = ParseVariableBool(newval, "ON_ERROR_STOP");
+	return ParseVariableBool(newval, "ON_ERROR_STOP", &pset.on_error_stop);
 }
 
-static void
+static bool
 quiet_hook(const char *newval)
 {
-	pset.quiet = ParseVariableBool(newval, "QUIET");
+	return ParseVariableBool(newval, "QUIET", &pset.quiet);
 }
 
-static void
+static bool
 singleline_hook(const char *newval)
 {
-	pset.singleline = ParseVariableBool(newval, "SINGLELINE");
+	return ParseVariableBool(newval, "SINGLELINE", &pset.singleline);
 }
 
-static void
+static bool
 singlestep_hook(const char *newval)
 {
-	pset.singlestep = ParseVariableBool(newval, "SINGLESTEP");
+	return ParseVariableBool(newval, "SINGLESTEP", &pset.singlestep);
 }
 
-static void
+static bool
 fetch_count_hook(const char *newval)
 {
-	pset.fetch_count = ParseVariableNum(newval, -1, -1, false);
+	if (newval == NULL)
+		pset.fetch_count = -1;	/* default value */
+	else if (!ParseVariableNum(newval, "FETCH_COUNT", &pset.fetch_count))
+		return false;
+	return true;
 }
 
-static void
+static bool
 echo_hook(const char *newval)
 {
 	if (newval == NULL)
@@ -837,39 +837,57 @@ echo_hook(const char *newval)
 		pset.echo = PSQL_ECHO_NONE;
 	else
 	{
-		psql_error("unrecognized value \"%s\" for \"%s\"; assuming \"%s\"\n",
-				   newval, "ECHO", "none");
-		pset.echo = PSQL_ECHO_NONE;
+		PsqlVarEnumError("ECHO", newval, "none, errors, queries, all");
+		return false;
 	}
+	return true;
 }
 
-static void
+static bool
 echo_hidden_hook(const char *newval)
 {
 	if (newval == NULL)
 		pset.echo_hidden = PSQL_ECHO_HIDDEN_OFF;
 	else if (pg_strcasecmp(newval, "noexec") == 0)
 		pset.echo_hidden = PSQL_ECHO_HIDDEN_NOEXEC;
-	else if (ParseVariableBool(newval, "ECHO_HIDDEN"))
-		pset.echo_hidden = PSQL_ECHO_HIDDEN_ON;
-	else	/* ParseVariableBool printed msg if needed */
-		pset.echo_hidden = PSQL_ECHO_HIDDEN_OFF;
+	else
+	{
+		bool		on_off;
+
+		if (ParseVariableBool(newval, NULL, &on_off))
+			pset.echo_hidden = on_off ? PSQL_ECHO_HIDDEN_ON : PSQL_ECHO_HIDDEN_OFF;
+		else
+		{
+			PsqlVarEnumError("ECHO_HIDDEN", newval, "on, off, noexec");
+			return false;
+		}
+	}
+	return true;
 }
 
-static void
+static bool
 on_error_rollback_hook(const char *newval)
 {
 	if (newval == NULL)
 		pset.on_error_rollback = PSQL_ERROR_ROLLBACK_OFF;
 	else if (pg_strcasecmp(newval, "interactive") == 0)
 		pset.on_error_rollback = PSQL_ERROR_ROLLBACK_INTERACTIVE;
-	else if (ParseVariableBool(newval, "ON_ERROR_ROLLBACK"))
-		pset.on_error_rollback = PSQL_ERROR_ROLLBACK_ON;
-	else	/* ParseVariableBool printed msg if needed */
-		pset.on_error_rollback = PSQL_ERROR_ROLLBACK_OFF;
+	else
+	{
+		bool		on_off;
+
+		if (ParseVariableBool(newval, NULL, &on_off))
+			pset.on_error_rollback = on_off ? PSQL_ERROR_ROLLBACK_ON : PSQL_ERROR_ROLLBACK_OFF;
+		else
+		{
+			PsqlVarEnumError("ON_ERROR_ROLLBACK", newval, "on, off, interactive");
+			return false;
+		}
+	}
+	return true;
 }
 
-static void
+static bool
 comp_keyword_case_hook(const char *newval)
 {
 	if (newval == NULL)
@@ -884,13 +902,14 @@ comp_keyword_case_hook(const char *newval)
 		pset.comp_case = PSQL_COMP_CASE_LOWER;
 	else
 	{
-		psql_error("unrecognized value \"%s\" for \"%s\"; assuming \"%s\"\n",
-				   newval, "COMP_KEYWORD_CASE", "preserve-upper");
-		pset.comp_case = PSQL_COMP_CASE_PRESERVE_UPPER;
+		PsqlVarEnumError("COMP_KEYWORD_CASE", newval,
+						 "lower, upper, preserve-lower, preserve-upper");
+		return false;
 	}
+	return true;
 }
 
-static void
+static bool
 histcontrol_hook(const char *newval)
 {
 	if (newval == NULL)
@@ -905,31 +924,35 @@ histcontrol_hook(const char *newval)
 		pset.histcontrol = hctl_none;
 	else
 	{
-		psql_error("unrecognized value \"%s\" for \"%s\"; assuming \"%s\"\n",
-				   newval, "HISTCONTROL", "none");
-		pset.histcontrol = hctl_none;
+		PsqlVarEnumError("HISTCONTROL", newval,
+						 "none, ignorespace, ignoredups, ignoreboth");
+		return false;
 	}
+	return true;
 }
 
-static void
+static bool
 prompt1_hook(const char *newval)
 {
 	pset.prompt1 = newval ? newval : "";
+	return true;
 }
 
-static void
+static bool
 prompt2_hook(const char *newval)
 {
 	pset.prompt2 = newval ? newval : "";
+	return true;
 }
 
-static void
+static bool
 prompt3_hook(const char *newval)
 {
 	pset.prompt3 = newval ? newval : "";
+	return true;
 }
 
-static void
+static bool
 verbosity_hook(const char *newval)
 {
 	if (newval == NULL)
@@ -942,16 +965,16 @@ verbosity_hook(const char *newval)
 		pset.verbosity = PQERRORS_VERBOSE;
 	else
 	{
-		psql_error("unrecognized value \"%s\" for \"%s\"; assuming \"%s\"\n",
-				   newval, "VERBOSITY", "default");
-		pset.verbosity = PQERRORS_DEFAULT;
+		PsqlVarEnumError("VERBOSITY", newval, "default, terse, verbose");
+		return false;
 	}
 
 	if (pset.db)
 		PQsetErrorVerbosity(pset.db, pset.verbosity);
+	return true;
 }
 
-static void
+static bool
 show_context_hook(const char *newval)
 {
 	if (newval == NULL)
@@ -964,13 +987,13 @@ show_context_hook(const char *newval)
 		pset.show_context = PQSHOW_CONTEXT_ALWAYS;
 	else
 	{
-		psql_error("unrecognized value \"%s\" for \"%s\"; assuming \"%s\"\n",
-				   newval, "SHOW_CONTEXT", "errors");
-		pset.show_context = PQSHOW_CONTEXT_ERRORS;
+		PsqlVarEnumError("SHOW_CONTEXT", newval, "never, errors, always");
+		return false;
 	}
 
 	if (pset.db)
 		PQsetErrorContextVisibility(pset.db, pset.show_context);
+	return true;
 }
 
 

src/bin/psql/variables.c

@@ -58,6 +58,11 @@ CreateVariableSpace(void)
 	return ptr;
 }
 
+/*
+ * Get string value of variable, or NULL if it's not defined.
+ *
+ * Note: result is valid until variable is next assigned to.
+ */
 const char *
 GetVariable(VariableSpace space, const char *name)
 {
@@ -79,94 +84,121 @@ GetVariable(VariableSpace space, const char *name)
 }
 
 /*
- * Try to interpret "value" as boolean value.
+ * Try to interpret "value" as a boolean value, and if successful,
+ * store it in *result.  Otherwise don't clobber *result.
  *
  * Valid values are: true, false, yes, no, on, off, 1, 0; as well as unique
  * prefixes thereof.
  *
  * "name" is the name of the variable we're assigning to, to use in error
  * report if any.  Pass name == NULL to suppress the error report.
+ *
+ * Return true when "value" is syntactically valid, false otherwise.
  */
 bool
-ParseVariableBool(const char *value, const char *name)
+ParseVariableBool(const char *value, const char *name, bool *result)
 {
 	size_t		len;
+	bool		valid = true;
 
 	if (value == NULL)
-		return false;			/* not set -> assume "off" */
+	{
+		*result = false;		/* not set -> assume "off" */
+		return valid;
+	}
 
 	len = strlen(value);
 
-	if (pg_strncasecmp(value, "true", len) == 0)
-		return true;
-	else if (pg_strncasecmp(value, "false", len) == 0)
-		return false;
-	else if (pg_strncasecmp(value, "yes", len) == 0)
-		return true;
-	else if (pg_strncasecmp(value, "no", len) == 0)
-		return false;
+	if (len > 0 && pg_strncasecmp(value, "true", len) == 0)
+		*result = true;
+	else if (len > 0 && pg_strncasecmp(value, "false", len) == 0)
+		*result = false;
+	else if (len > 0 && pg_strncasecmp(value, "yes", len) == 0)
+		*result = true;
+	else if (len > 0 && pg_strncasecmp(value, "no", len) == 0)
+		*result = false;
 	/* 'o' is not unique enough */
 	else if (pg_strncasecmp(value, "on", (len > 2 ? len : 2)) == 0)
-		return true;
+		*result = true;
 	else if (pg_strncasecmp(value, "off", (len > 2 ? len : 2)) == 0)
-		return false;
+		*result = false;
 	else if (pg_strcasecmp(value, "1") == 0)
-		return true;
+		*result = true;
 	else if (pg_strcasecmp(value, "0") == 0)
-		return false;
+		*result = false;
 	else
 	{
-		/* NULL is treated as false, so a non-matching value is 'true' */
+		/* string is not recognized; don't clobber *result */
 		if (name)
-			psql_error("unrecognized value \"%s\" for \"%s\"; assuming \"%s\"\n",
-					   value, name, "on");
-		return true;
+			psql_error("unrecognized value \"%s\" for \"%s\": boolean expected\n",
+					   value, name);
+		valid = false;
 	}
+	return valid;
 }
 
-
 /*
- * Read numeric variable, or defaultval if it is not set, or faultval if its
- * value is not a valid numeric string.  If allowtrail is false, this will
- * include the case where there are trailing characters after the number.
+ * Try to interpret "value" as an integer value, and if successful,
+ * store it in *result.  Otherwise don't clobber *result.
+ *
+ * "name" is the name of the variable we're assigning to, to use in error
+ * report if any.  Pass name == NULL to suppress the error report.
+ *
+ * Return true when "value" is syntactically valid, false otherwise.
  */
-int
-ParseVariableNum(const char *val,
-				 int defaultval,
-				 int faultval,
-				 bool allowtrail)
+bool
+ParseVariableNum(const char *value, const char *name, int *result)
 {
-	int			result;
+	char	   *end;
+	long		numval;
 
-	if (!val)
-		result = defaultval;
-	else if (!val[0])
-		result = faultval;
+	if (value == NULL)
+		return false;
+	errno = 0;
+	numval = strtol(value, &end, 0);
+	if (errno == 0 && *end == '\0' && end != value && numval == (int) numval)
+	{
+		*result = (int) numval;
+		return true;
+	}
 	else
 	{
-		char	   *end;
-
-		result = strtol(val, &end, 0);
-		if (!allowtrail && *end)
-			result = faultval;
+		/* string is not recognized; don't clobber *result */
+		if (name)
+			psql_error("invalid value \"%s\" for \"%s\": integer expected\n",
+					   value, name);
+		return false;
 	}
-
-	return result;
 }
 
+/*
+ * Read integer value of the numeric variable named "name".
+ *
+ * Return defaultval if it is not set, or faultval if its value is not a
+ * valid integer.  (No error message is issued.)
+ */
 int
 GetVariableNum(VariableSpace space,
 			   const char *name,
 			   int defaultval,
-			   int faultval,
-			   bool allowtrail)
+			   int faultval)
 {
 	const char *val;
+	int			result;
 
 	val = GetVariable(space, name);
-	return ParseVariableNum(val, defaultval, faultval, allowtrail);
+	if (!val)
+		return defaultval;
+
+	if (ParseVariableNum(val, NULL, &result))
+		return result;
+	else
+		return faultval;
 }
 
+/*
+ * Print values of all variables.
+ */
 void
 PrintVariables(VariableSpace space)
 {
@@ -184,17 +216,28 @@ PrintVariables(VariableSpace space)
 	}
 }
 
+/*
+ * Set the variable named "name" to value "value",
+ * or delete it if "value" is NULL.
+ *
+ * Returns true if successful, false if not; in the latter case a suitable
+ * error message has been printed, except for the unexpected case of
+ * space or name being NULL.
+ */
 bool
 SetVariable(VariableSpace space, const char *name, const char *value)
 {
 	struct _variable *current,
 			   *previous;
 
-	if (!space)
+	if (!space || !name)
 		return false;
 
 	if (!valid_variable_name(name))
+	{
+		psql_error("invalid variable name: \"%s\"\n", name);
 		return false;
+	}
 
 	if (!value)
 		return DeleteVariable(space, name);
@@ -205,13 +248,30 @@ SetVariable(VariableSpace space, const char *name, const char *value)
 	{
 		if (strcmp(current->name, name) == 0)
 		{
-			/* found entry, so update */
-			if (current->value)
-				free(current->value);
-			current->value = pg_strdup(value);
+			/*
+			 * Found entry, so update, unless hook returns false.  The hook
+			 * may need the passed value to have the same lifespan as the
+			 * variable, so allocate it right away, even though we'll have to
+			 * free it again if the hook returns false.
+			 */
+			char	   *new_value = pg_strdup(value);
+			bool		confirmed;
+
 			if (current->assign_hook)
-				(*current->assign_hook) (current->value);
-			return true;
+				confirmed = (*current->assign_hook) (new_value);
+			else
+				confirmed = true;
+
+			if (confirmed)
+			{
+				if (current->value)
+					pg_free(current->value);
+				current->value = new_value;
+			}
+			else
+				pg_free(new_value);		/* current->value is left unchanged */
+
+			return confirmed;
 		}
 	}
 
@@ -226,19 +286,29 @@ SetVariable(VariableSpace space, const char *name, const char *value)
 }
 
 /*
- * This both sets a hook function, and calls it on the current value (if any)
+ * Attach an assign hook function to the named variable.
+ *
+ * If the variable doesn't already exist, create it with value NULL,
+ * just so we have a place to store the hook function.  (Externally,
+ * this isn't different from it not being defined.)
+ *
+ * The hook is immediately called on the variable's current value.  This is
+ * meant to let it update any derived psql state.  If the hook doesn't like
+ * the current value, it will print a message to that effect, but we'll ignore
+ * it.  Generally we do not expect any such failure here, because this should
+ * get called before any user-supplied value is assigned.
  */
-bool
+void
 SetVariableAssignHook(VariableSpace space, const char *name, VariableAssignHook hook)
 {
 	struct _variable *current,
 			   *previous;
 
-	if (!space)
-		return false;
+	if (!space || !name)
+		return;
 
 	if (!valid_variable_name(name))
-		return false;
+		return;
 
 	for (previous = space, current = space->next;
 		 current;
@@ -248,8 +318,8 @@ SetVariableAssignHook(VariableSpace space, const char *name, VariableAssignHook
 		{
 			/* found entry, so update */
 			current->assign_hook = hook;
-			(*hook) (current->value);
-			return true;
+			(void) (*hook) (current->value);
+			return;
 		}
 	}
 
@@ -260,16 +330,24 @@ SetVariableAssignHook(VariableSpace space, const char *name, VariableAssignHook
 	current->assign_hook = hook;
 	current->next = NULL;
 	previous->next = current;
-	(*hook) (NULL);
-	return true;
+	(void) (*hook) (NULL);
 }
 
+/*
+ * Convenience function to set a variable's value to "on".
+ */
 bool
 SetVariableBool(VariableSpace space, const char *name)
 {
 	return SetVariable(space, name, "on");
 }
 
+/*
+ * Attempt to delete variable.
+ *
+ * If unsuccessful, print a message and return "false".
+ * Deleting a nonexistent variable is not an error.
+ */
 bool
 DeleteVariable(VariableSpace space, const char *name)
 {
@@ -277,7 +355,7 @@ DeleteVariable(VariableSpace space, const char *name)
 			   *previous;
 
 	if (!space)
-		return false;
+		return true;
 
 	for (previous = space, current = space->next;
 		 current;
@@ -285,14 +363,21 @@ DeleteVariable(VariableSpace space, const char *name)
 	{
 		if (strcmp(current->name, name) == 0)
 		{
+			if (current->assign_hook)
+			{
+				/* Allow deletion only if hook is okay with NULL value */
+				if (!(*current->assign_hook) (NULL))
+					return false;		/* message printed by hook */
 				if (current->value)
 					free(current->value);
 				current->value = NULL;
-			/* Physically delete only if no hook function to remember */
-			if (current->assign_hook)
-				(*current->assign_hook) (NULL);
+				/* Don't delete entry, or we'd forget the hook function */
+			}
 			else
 			{
+				/* We can delete the entry as well as its value */
+				if (current->value)
+					free(current->value);
 				previous->next = current->next;
 				free(current->name);
 				free(current);
@@ -303,3 +388,16 @@ DeleteVariable(VariableSpace space, const char *name)
 
 	return true;
 }
+
+/*
+ * Emit error with suggestions for variables or commands
+ * accepting enum-style arguments.
+ * This function just exists to standardize the wording.
+ * suggestions should follow the format "fee, fi, fo, fum".
+ */
+void
+PsqlVarEnumError(const char *name, const char *value, const char *suggestions)
+{
+	psql_error("unrecognized value \"%s\" for \"%s\"\nAvailable values are: %s.\n",
+			   value, name, suggestions);
+}

src/bin/psql/variables.h

@@ -3,25 +3,39 @@
  *
  * Copyright (c) 2000-2017, PostgreSQL Global Development Group
  *
+ * This implements a sort of variable repository.  One could also think of it
+ * as a cheap version of an associative array.  Each variable has a string
+ * name and a string value.  The value can't be NULL, or more precisely
+ * that's not distinguishable from the variable being unset.
+ *
  * src/bin/psql/variables.h
  */
 #ifndef VARIABLES_H
 #define VARIABLES_H
 
 /*
- * This implements a sort of variable repository. One could also think of it
- * as a cheap version of an associative array. In each one of these
- * datastructures you can store name/value pairs.  There can also be an
- * "assign hook" function that is called whenever the variable's value is
- * changed.
+ * Variables can be given "assign hook" functions.  The assign hook can
+ * prevent invalid values from being assigned, and can update internal C
+ * variables to keep them in sync with the variable's current value.
+ *
+ * A hook function is called before any attempted assignment, with the
+ * proposed new value of the variable (or with NULL, if an \unset is being
+ * attempted).  If it returns false, the assignment doesn't occur --- it
+ * should print an error message with psql_error() to tell the user why.
  *
- * An "unset" operation causes the hook to be called with newval == NULL.
+ * When a hook function is installed with SetVariableAssignHook(), it is
+ * called with the variable's current value (or with NULL, if it wasn't set
+ * yet).  But its return value is ignored in this case.  The hook should be
+ * set before any possibly-invalid value can be assigned.
+ */
+typedef bool (*VariableAssignHook) (const char *newval);
+
+/*
+ * Data structure representing one variable.
  *
  * Note: if value == NULL then the variable is logically unset, but we are
  * keeping the struct around so as not to forget about its hook function.
  */
-typedef void (*VariableAssignHook) (const char *newval);
-
 struct _variable
 {
 	char	   *name;
@@ -30,27 +44,31 @@ struct _variable
 	struct _variable *next;
 };
 
+/* Data structure representing a set of variables */
 typedef struct _variable *VariableSpace;
 
+
 VariableSpace CreateVariableSpace(void);
 const char *GetVariable(VariableSpace space, const char *name);
 
-bool		ParseVariableBool(const char *value, const char *name);
-int ParseVariableNum(const char *val,
-				 int defaultval,
-				 int faultval,
-				 bool allowtrail);
+bool ParseVariableBool(const char *value, const char *name,
+				  bool *result);
+
+bool ParseVariableNum(const char *value, const char *name,
+				 int *result);
+
 int GetVariableNum(VariableSpace space,
 			   const char *name,
 			   int defaultval,
-			   int faultval,
-			   bool allowtrail);
+			   int faultval);
 
 void		PrintVariables(VariableSpace space);
 
 bool		SetVariable(VariableSpace space, const char *name, const char *value);
-bool		SetVariableAssignHook(VariableSpace space, const char *name, VariableAssignHook hook);
+void		SetVariableAssignHook(VariableSpace space, const char *name, VariableAssignHook hook);
 bool		SetVariableBool(VariableSpace space, const char *name);
 bool		DeleteVariable(VariableSpace space, const char *name);
 
+void		PsqlVarEnumError(const char *name, const char *value, const char *suggestions);
+
 #endif   /* VARIABLES_H */

src/test/regress/expected/psql.out

@@ -2,6 +2,15 @@
 -- Tests for psql features that aren't closely connected to any
 -- specific server features
 --
+-- \set
+-- fail: invalid name
+\set invalid/name foo
+invalid variable name: "invalid/name"
+-- fail: invalid value for special variable
+\set AUTOCOMMIT foo
+unrecognized value "foo" for "AUTOCOMMIT": boolean expected
+\set FETCH_COUNT foo
+invalid value "foo" for "FETCH_COUNT": integer expected
 -- \gset
 select 10 as test01, 20 as test02, 'Hello' as test03 \gset pref01_
 \echo :pref01_test01 :pref01_test02 :pref01_test03
@@ -9,7 +18,7 @@ select 10 as test01, 20 as test02, 'Hello' as test03 \gset pref01_
 -- should fail: bad variable name
 select 10 as "bad name"
 \gset
-could not set variable "bad name"
+invalid variable name: "bad name"
 -- multiple backslash commands in one line
 select 1 as x, 2 as y \gset pref01_ \\ \echo :pref01_x
 1

src/test/regress/sql/psql.sql

@@ -3,6 +3,14 @@
 -- specific server features
 --
 
+-- \set
+
+-- fail: invalid name
+\set invalid/name foo
+-- fail: invalid value for special variable
+\set AUTOCOMMIT foo
+\set FETCH_COUNT foo
+
 -- \gset
 
 select 10 as test01, 20 as test02, 'Hello' as test03 \gset pref01_