1 files changed, 31 insertions, 4 deletions

diff --git a/src/bin/psql/variables.h b/src/bin/psql/variables.h
index 274b4af5537..84be7805098 100644
--- a/src/bin/psql/variables.h
+++ b/src/bin/psql/variables.h
@@ -18,12 +18,12 @@
  * 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
+ * An assign 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.
  *
- * When a hook function is installed with SetVariableAssignHook(), it is
+ * When an assign hook function is installed with SetVariableHooks(), 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.
@@ -31,15 +31,39 @@
 typedef bool (*VariableAssignHook) (const char *newval);
 
 /*
+ * Variables can also be given "substitute hook" functions.  The substitute
+ * hook can replace values (including NULL) with other values, allowing
+ * normalization of variable contents.  For example, for a boolean variable,
+ * we wish to interpret "\unset FOO" as "\set FOO off", and we can do that
+ * by installing a substitute hook.  (We can use the same substitute hook
+ * for all bool or nearly-bool variables, which is why this responsibility
+ * isn't part of the assign hook.)
+ *
+ * The substitute hook is called before any attempted assignment, and before
+ * the assign hook if any, passing the proposed new value of the variable as a
+ * malloc'd string (or NULL, if an \unset is being attempted).  It can return
+ * the same value, or a different malloc'd string, or modify the string
+ * in-place.  It should free the passed-in value if it's not returning it.
+ * The substitute hook generally should not complain about erroneous values;
+ * that's a job for the assign hook.
+ *
+ * When a substitute hook is installed with SetVariableHooks(), it is applied
+ * to the variable's current value (typically NULL, if it wasn't set yet).
+ * That also happens before applying the assign hook.
+ */
+typedef char *(*VariableSubstituteHook) (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.
+ * keeping the struct around so as not to forget about its hook function(s).
  */
 struct _variable
 {
 	char	   *name;
 	char	   *value;
+	VariableSubstituteHook substitute_hook;
 	VariableAssignHook assign_hook;
 	struct _variable *next;
 };
@@ -65,10 +89,13 @@ int GetVariableNum(VariableSpace space,
 void		PrintVariables(VariableSpace space);
 
 bool		SetVariable(VariableSpace space, const char *name, const char *value);
-void		SetVariableAssignHook(VariableSpace space, const char *name, VariableAssignHook hook);
 bool		SetVariableBool(VariableSpace space, const char *name);
 bool		DeleteVariable(VariableSpace space, const char *name);
 
+void SetVariableHooks(VariableSpace space, const char *name,
+				 VariableSubstituteHook shook,
+				 VariableAssignHook ahook);
+
 void		PsqlVarEnumError(const char *name, const char *value, const char *suggestions);
 
 #endif   /* VARIABLES_H */