Add a "void *" passthrough pointer for psqlscan.l's callback functions.

The immediate motivation for this is to provide clean infrastructure
for the proposed \if...\endif patch for psql; but it seems like a good
thing to have even if that patch doesn't get in.  Previously the callback
functions could only make use of application-global state, which is a
pretty severe handicap.

For the moment, the pointer is only passed through to the get_variable
callback function.  I considered also passing it to the write_error
callback, but for now let's not.  Neither psql nor pgbench has a use
for that, and in the case of psql we'd have to invent a separate wrapper
function because we would certainly not want to change the signature of
psql_error().

Discussion: https://postgr.es/m/10108.1489418309@sss.pgh.pa.us

src/bin/psql/common.c

@@ -119,9 +119,13 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * "passthrough" is the pointer previously given to psql_scan_set_passthrough.
+ * psql currently doesn't use this.
  */
 char *
-psql_get_variable(const char *varname, bool escape, bool as_ident)
+psql_get_variable(const char *varname, bool escape, bool as_ident,
+				  void *passthrough)
 {
 	char	   *result;
 	const char *value;

src/bin/psql/common.h

@@ -16,7 +16,8 @@
 extern bool openQueryOutputFile(const char *fname, FILE **fout, bool *is_pipe);
 extern bool setQFout(const char *fname);
 
-extern char *psql_get_variable(const char *varname, bool escape, bool as_ident);
+extern char *psql_get_variable(const char *varname, bool escape, bool as_ident,
+				  void *passthrough);
 
 extern void psql_error(const char *fmt,...) pg_attribute_printf(1, 2);
 

src/bin/psql/psqlscanslash.l

@@ -243,7 +243,8 @@ other			.
 															 yyleng - 1);
 						value = cur_state->callbacks->get_variable(varname,
 																   false,
-																   false);
+																   false,
+																   cur_state->cb_passthrough);
 						free(varname);
 
 						/*

src/fe_utils/psqlscan.l

@@ -700,7 +700,8 @@ other			.
 					if (cur_state->callbacks->get_variable)
 						value = cur_state->callbacks->get_variable(varname,
 																   false,
-																   false);
+																   false,
+																   cur_state->cb_passthrough);
 					else
 						value = NULL;
 
@@ -922,6 +923,19 @@ psql_scan_destroy(PsqlScanState state)
 	free(state);
 }
 
+/*
+ * Set the callback passthrough pointer for the lexer.
+ *
+ * This could have been integrated into psql_scan_create, but keeping it
+ * separate allows the application to change the pointer later, which might
+ * be useful.
+ */
+void
+psql_scan_set_passthrough(PsqlScanState state, void *passthrough)
+{
+	state->cb_passthrough = passthrough;
+}
+
 /*
  * Set up to perform lexing of the given input line.
  *
@@ -1409,7 +1423,8 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 	/* Variable lookup. */
 	varname = psqlscan_extract_substring(state, txt + 2, len - 3);
 	if (state->callbacks->get_variable)
-		value = state->callbacks->get_variable(varname, true, as_ident);
+		value = state->callbacks->get_variable(varname, true, as_ident,
+											   state->cb_passthrough);
 	else
 		value = NULL;
 	free(varname);

src/include/fe_utils/psqlscan.h

@@ -53,7 +53,8 @@ typedef struct PsqlScanCallbacks
 {
 	/* Fetch value of a variable, as a pfree'able string; NULL if unknown */
 	/* This pointer can be NULL if no variable substitution is wanted */
-	char	   *(*get_variable) (const char *varname, bool escape, bool as_ident);
+	char	   *(*get_variable) (const char *varname, bool escape,
+										   bool as_ident, void *passthrough);
 	/* Print an error message someplace appropriate */
 	/* (very old gcc versions don't support attributes on function pointers) */
 #if defined(__GNUC__) && __GNUC__ < 4
@@ -67,6 +68,8 @@ typedef struct PsqlScanCallbacks
 extern PsqlScanState psql_scan_create(const PsqlScanCallbacks *callbacks);
 extern void psql_scan_destroy(PsqlScanState state);
 
+extern void psql_scan_set_passthrough(PsqlScanState state, void *passthrough);
+
 extern void psql_scan_setup(PsqlScanState state,
 				const char *line, int line_len,
 				int encoding, bool std_strings);

src/include/fe_utils/psqlscan_int.h

@@ -115,9 +115,11 @@ typedef struct PsqlScanStateData
 	char	   *dolqstart;		/* current $foo$ quote start string */
 
 	/*
-	 * Callback functions provided by the program making use of the lexer.
+	 * Callback functions provided by the program making use of the lexer,
+	 * plus a void* callback passthrough argument.
 	 */
 	const PsqlScanCallbacks *callbacks;
+	void	   *cb_passthrough;
 } PsqlScanStateData;