Make psql's \set display variables in alphabetical order.

"\set" with no arguments displays all defined variables, but it does so
in the order that they appear in variables.c's list, which previously
was mostly creation order.  That makes the list ugly and hard to find
things in, and it exposes some psql implementation details to users.
(For instance, ordinary variables will move to the bottom of the list
if unset and set again, but variables that have hooks won't.)

Fix that by keeping the list in alphabetical order at all times, which
isn't much more complicated than breaking out of the insertion search
loops once we reach an entry that should be after the one to be inserted.

Discussion: https://postgr.es/m/31785.1485900786@sss.pgh.pa.us

src/bin/psql/variables.c

@@ -43,6 +43,9 @@ valid_variable_name(const char *name)
 /*
  * A "variable space" is represented by an otherwise-unused struct _variable
  * that serves as list header.
+ *
+ * The list entries are kept in name order (according to strcmp).  This
+ * is mainly to make the results of PrintVariables() more pleasing.
  */
 VariableSpace
 CreateVariableSpace(void)
@@ -74,11 +77,15 @@ GetVariable(VariableSpace space, const char *name)
 
 	for (current = space->next; current; current = current->next)
 	{
-		if (strcmp(current->name, name) == 0)
+		int			cmp = strcmp(current->name, name);
+
+		if (cmp == 0)
 		{
 			/* this is correct answer when value is NULL, too */
 			return current->value;
 		}
+		if (cmp > 0)
+			break;				/* it's not there */
 	}
 
 	return NULL;
@@ -247,7 +254,9 @@ SetVariable(VariableSpace space, const char *name, const char *value)
 		 current;
 		 previous = current, current = current->next)
 	{
-		if (strcmp(current->name, name) == 0)
+		int			cmp = strcmp(current->name, name);
+
+		if (cmp == 0)
 		{
 			/*
 			 * Found entry, so update, unless assign hook returns false.
@@ -293,6 +302,8 @@ SetVariable(VariableSpace space, const char *name, const char *value)
 
 			return confirmed;
 		}
+		if (cmp > 0)
+			break;				/* it's not there */
 	}
 
 	/* not present, make new entry ... unless we were asked to delete */
@@ -303,7 +314,7 @@ SetVariable(VariableSpace space, const char *name, const char *value)
 		current->value = pg_strdup(value);
 		current->substitute_hook = NULL;
 		current->assign_hook = NULL;
-		current->next = NULL;
+		current->next = previous->next;
 		previous->next = current;
 	}
 	return true;
@@ -343,7 +354,9 @@ SetVariableHooks(VariableSpace space, const char *name,
 		 current;
 		 previous = current, current = current->next)
 	{
-		if (strcmp(current->name, name) == 0)
+		int			cmp = strcmp(current->name, name);
+
+		if (cmp == 0)
 		{
 			/* found entry, so update */
 			current->substitute_hook = shook;
@@ -354,6 +367,8 @@ SetVariableHooks(VariableSpace space, const char *name,
 				(void) (*ahook) (current->value);
 			return;
 		}
+		if (cmp > 0)
+			break;				/* it's not there */
 	}
 
 	/* not present, make new entry */
@@ -362,7 +377,7 @@ SetVariableHooks(VariableSpace space, const char *name,
 	current->value = NULL;
 	current->substitute_hook = shook;
 	current->assign_hook = ahook;
-	current->next = NULL;
+	current->next = previous->next;
 	previous->next = current;
 	if (shook)
 		current->value = (*shook) (current->value);