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diff --git a/src/bin/psql/variables.h b/src/bin/psql/variables.h
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+/*
+ * psql - the PostgreSQL interactive terminal
+ *
+ * Copyright (c) 2000-2021, 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
+
+/*
+ * 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.
+ *
+ * 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 pg_log_error() to tell the user why.
+ *
+ * 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.
+ */
+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(s).
+ */
+struct _variable
+{
+	char	   *name;
+	char	   *value;
+	VariableSubstituteHook substitute_hook;
+	VariableAssignHook assign_hook;
+	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,
+							  bool *result);
+
+bool		ParseVariableNum(const char *value, const char *name,
+							 int *result);
+
+void		PrintVariables(VariableSpace space);
+
+bool		SetVariable(VariableSpace space, const char *name, const char *value);
+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);
+bool		VariableHasHook(VariableSpace space, const char *name);
+
+void		PsqlVarEnumError(const char *name, const char *value, const char *suggestions);
+
+#endif							/* VARIABLES_H */