Add a debugging option to stress-test outfuncs.c and readfuncs.c.

In the normal course of operation, query trees will be serialized only if
they are stored as views or rules; and plan trees will be serialized only
if they get passed to parallel-query workers.  This leaves an awful lot of
opportunity for bugs/oversights to not get detected, as indeed we've just
been reminded of the hard way.

To improve matters, this patch adds a new compile option
WRITE_READ_PARSE_PLAN_TREES, which is modeled on the longstanding option
COPY_PARSE_PLAN_TREES; but instead of passing all parse and plan trees
through copyObject, it passes them through nodeToString + stringToNode.
Enabling this option in a buildfarm animal or two will catch problems
at least for cases that are exercised by the regression tests.

A small problem with this idea is that readfuncs.c historically has
discarded location fields, on the reasonable grounds that parse
locations in a retrieved view are not relevant to the current query.
But doing that in WRITE_READ_PARSE_PLAN_TREES breaks pg_stat_statements,
and it could cause problems for future improvements that might try to
report error locations at runtime.  To fix that, provide a variant
behavior in readfuncs.c that makes it restore location fields when
told to.

In passing, const-ify the string arguments of stringToNode and its
subsidiary functions, just because it annoyed me that they weren't
const already.

Discussion: https://postgr.es/m/17114.1537138992@sss.pgh.pa.us

src/backend/nodes/read.c

@@ -28,18 +28,30 @@
 
 
 /* Static state for pg_strtok */
-static char *pg_strtok_ptr = NULL;
+static const char *pg_strtok_ptr = NULL;
+
+/* State flag that determines how readfuncs.c should treat location fields */
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+bool		restore_location_fields = false;
+#endif
 
 
 /*
  * stringToNode -
- *	  returns a Node with a given legal ASCII representation
+ *	  builds a Node tree from its string representation (assumed valid)
+ *
+ * restore_loc_fields instructs readfuncs.c whether to restore location
+ * fields rather than set them to -1.  This is currently only supported
+ * in builds with the WRITE_READ_PARSE_PLAN_TREES debugging flag set.
  */
-void *
-stringToNode(char *str)
+static void *
+stringToNodeInternal(const char *str, bool restore_loc_fields)
 {
-	char	   *save_strtok;
 	void	   *retval;
+	const char *save_strtok;
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+	bool		save_restore_location_fields;
+#endif
 
 	/*
 	 * We save and restore the pre-existing state of pg_strtok. This makes the
@@ -51,13 +63,45 @@ stringToNode(char *str)
 
 	pg_strtok_ptr = str;		/* point pg_strtok at the string to read */
 
+	/*
+	 * If enabled, likewise save/restore the location field handling flag.
+	 */
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+	save_restore_location_fields = restore_location_fields;
+	restore_location_fields = restore_loc_fields;
+#endif
+
 	retval = nodeRead(NULL, 0); /* do the reading */
 
 	pg_strtok_ptr = save_strtok;
 
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+	restore_location_fields = save_restore_location_fields;
+#endif
+
 	return retval;
 }
 
+/*
+ * Externally visible entry points
+ */
+void *
+stringToNode(const char *str)
+{
+	return stringToNodeInternal(str, false);
+}
+
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+
+void *
+stringToNodeWithLocations(const char *str)
+{
+	return stringToNodeInternal(str, true);
+}
+
+#endif
+
+
 /*****************************************************************************
  *
  * the lisp token parser
@@ -104,11 +148,11 @@ stringToNode(char *str)
  * code should add backslashes to a string constant to ensure it is treated
  * as a single token.
  */
-char *
+const char *
 pg_strtok(int *length)
 {
-	char	   *local_str;		/* working pointer to string */
-	char	   *ret_str;		/* start of token to return */
+	const char *local_str;		/* working pointer to string */
+	const char *ret_str;		/* start of token to return */
 
 	local_str = pg_strtok_ptr;
 
@@ -166,7 +210,7 @@ pg_strtok(int *length)
  *	  any protective backslashes in the token are removed.
  */
 char *
-debackslash(char *token, int length)
+debackslash(const char *token, int length)
 {
 	char	   *result = palloc(length + 1);
 	char	   *ptr = result;
@@ -198,10 +242,10 @@ debackslash(char *token, int length)
  *	  Assumption: the ascii representation is legal
  */
 static NodeTag
-nodeTokenType(char *token, int length)
+nodeTokenType(const char *token, int length)
 {
 	NodeTag		retval;
-	char	   *numptr;
+	const char *numptr;
 	int			numlen;
 
 	/*
@@ -269,7 +313,7 @@ nodeTokenType(char *token, int length)
  * this should only be invoked from within a stringToNode operation).
  */
 void *
-nodeRead(char *token, int tok_len)
+nodeRead(const char *token, int tok_len)
 {
 	Node	   *result;
 	NodeTag		type;

src/backend/nodes/readfuncs.c

@@ -17,10 +17,14 @@
  *	  never read executor state trees, either.
  *
  *	  Parse location fields are written out by outfuncs.c, but only for
- *	  possible debugging use.  When reading a location field, we discard
+ *	  debugging use.  When reading a location field, we normally discard
  *	  the stored value and set the location field to -1 (ie, "unknown").
  *	  This is because nodes coming from a stored rule should not be thought
  *	  to have a known location in the current query's text.
+ *	  However, if restore_location_fields is true, we do restore location
+ *	  fields from the string.  This is currently intended only for use by the
+ *	  WRITE_READ_PARSE_PLAN_TREES test code, which doesn't want to cause
+ *	  any change in the node contents.
  *
  *-------------------------------------------------------------------------
  */
@@ -51,7 +55,7 @@
 
 /* And a few guys need only the pg_strtok support fields */
 #define READ_TEMP_LOCALS()	\
-	char	   *token;		\
+	const char *token;		\
 	int			length
 
 /* ... but most need both */
@@ -120,12 +124,19 @@
 	token = pg_strtok(&length);		/* get field value */ \
 	local_node->fldname = nullable_string(token, length)
 
-/* Read a parse location field (and throw away the value, per notes above) */
+/* Read a parse location field (and possibly throw away the value) */
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+#define READ_LOCATION_FIELD(fldname) \
+	token = pg_strtok(&length);		/* skip :fldname */ \
+	token = pg_strtok(&length);		/* get field value */ \
+	local_node->fldname = restore_location_fields ? atoi(token) : -1
+#else
 #define READ_LOCATION_FIELD(fldname) \
 	token = pg_strtok(&length);		/* skip :fldname */ \
 	token = pg_strtok(&length);		/* get field value */ \
 	(void) token;				/* in case not used elsewhere */ \
 	local_node->fldname = -1	/* set field to "unknown" */
+#endif
 
 /* Read a Node field */
 #define READ_NODE_FIELD(fldname) \
@@ -2804,7 +2815,7 @@ readDatum(bool typbyval)
 	Size		length,
 				i;
 	int			tokenLength;
-	char	   *token;
+	const char *token;
 	Datum		res;
 	char	   *s;
 
@@ -2817,7 +2828,7 @@ readDatum(bool typbyval)
 	token = pg_strtok(&tokenLength);	/* read the '[' */
 	if (token == NULL || token[0] != '[')
 		elog(ERROR, "expected \"[\" to start datum, but got \"%s\"; length = %zu",
-			 token ? (const char *) token : "[NULL]", length);
+			 token ? token : "[NULL]", length);
 
 	if (typbyval)
 	{
@@ -2847,7 +2858,7 @@ readDatum(bool typbyval)
 	token = pg_strtok(&tokenLength);	/* read the ']' */
 	if (token == NULL || token[0] != ']')
 		elog(ERROR, "expected \"]\" to end datum, but got \"%s\"; length = %zu",
-			 token ? (const char *) token : "[NULL]", length);
+			 token ? token : "[NULL]", length);
 
 	return res;
 }
@@ -2860,7 +2871,7 @@ readAttrNumberCols(int numCols)
 {
 	int			tokenLength,
 				i;
-	char	   *token;
+	const char *token;
 	AttrNumber *attr_vals;
 
 	if (numCols <= 0)
@@ -2884,7 +2895,7 @@ readOidCols(int numCols)
 {
 	int			tokenLength,
 				i;
-	char	   *token;
+	const char *token;
 	Oid		   *oid_vals;
 
 	if (numCols <= 0)
@@ -2908,7 +2919,7 @@ readIntCols(int numCols)
 {
 	int			tokenLength,
 				i;
-	char	   *token;
+	const char *token;
 	int		   *int_vals;
 
 	if (numCols <= 0)
@@ -2932,7 +2943,7 @@ readBoolCols(int numCols)
 {
 	int			tokenLength,
 				i;
-	char	   *token;
+	const char *token;
 	bool	   *bool_vals;
 
 	if (numCols <= 0)

src/backend/parser/parse_relation.c

@@ -1335,7 +1335,6 @@ addRangeTableEntryForSubquery(ParseState *pstate,
 	Assert(pstate != NULL);
 
 	rte->rtekind = RTE_SUBQUERY;
-	rte->relid = InvalidOid;
 	rte->subquery = subquery;
 	rte->alias = alias;
 

src/backend/tcop/postgres.c

@@ -633,6 +633,12 @@ pg_parse_query(const char *query_string)
 	}
 #endif
 
+	/*
+	 * Currently, outfuncs/readfuncs support is missing for many raw parse
+	 * tree nodes, so we don't try to implement WRITE_READ_PARSE_PLAN_TREES
+	 * here.
+	 */
+
 	TRACE_POSTGRESQL_QUERY_PARSE_DONE(query_string);
 
 	return raw_parsetree_list;
@@ -763,7 +769,7 @@ pg_rewrite_query(Query *query)
 		ShowUsage("REWRITER STATISTICS");
 
 #ifdef COPY_PARSE_PLAN_TREES
-	/* Optional debugging check: pass querytree output through copyObject() */
+	/* Optional debugging check: pass querytree through copyObject() */
 	{
 		List	   *new_list;
 
@@ -776,6 +782,46 @@ pg_rewrite_query(Query *query)
 	}
 #endif
 
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+	/* Optional debugging check: pass querytree through outfuncs/readfuncs */
+	{
+		List	   *new_list = NIL;
+		ListCell   *lc;
+
+		/*
+		 * We currently lack outfuncs/readfuncs support for most utility
+		 * statement types, so only attempt to write/read non-utility queries.
+		 */
+		foreach(lc, querytree_list)
+		{
+			Query	   *query = castNode(Query, lfirst(lc));
+
+			if (query->commandType != CMD_UTILITY)
+			{
+				char	   *str = nodeToString(query);
+				Query	   *new_query = stringToNodeWithLocations(str);
+
+				/*
+				 * queryId is not saved in stored rules, but we must preserve
+				 * it here to avoid breaking pg_stat_statements.
+				 */
+				new_query->queryId = query->queryId;
+
+				new_list = lappend(new_list, new_query);
+				pfree(str);
+			}
+			else
+				new_list = lappend(new_list, query);
+		}
+
+		/* This checks both outfuncs/readfuncs and the equal() routines... */
+		if (!equal(new_list, querytree_list))
+			elog(WARNING, "outfuncs/readfuncs failed to produce equal parse tree");
+		else
+			querytree_list = new_list;
+	}
+#endif
+
 	if (Debug_print_rewritten)
 		elog_node_display(LOG, "rewritten parse tree", querytree_list,
 						  Debug_pretty_print);
@@ -812,7 +858,7 @@ pg_plan_query(Query *querytree, int cursorOptions, ParamListInfo boundParams)
 		ShowUsage("PLANNER STATISTICS");
 
 #ifdef COPY_PARSE_PLAN_TREES
-	/* Optional debugging check: pass plan output through copyObject() */
+	/* Optional debugging check: pass plan tree through copyObject() */
 	{
 		PlannedStmt *new_plan = copyObject(plan);
 
@@ -830,6 +876,30 @@ pg_plan_query(Query *querytree, int cursorOptions, ParamListInfo boundParams)
 	}
 #endif
 
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+	/* Optional debugging check: pass plan tree through outfuncs/readfuncs */
+	{
+		char	   *str;
+		PlannedStmt *new_plan;
+
+		str = nodeToString(plan);
+		new_plan = stringToNodeWithLocations(str);
+		pfree(str);
+
+		/*
+		 * equal() currently does not have routines to compare Plan nodes, so
+		 * don't try to test equality here.  Perhaps fix someday?
+		 */
+#ifdef NOT_USED
+		/* This checks both outfuncs/readfuncs and the equal() routines... */
+		if (!equal(new_plan, plan))
+			elog(WARNING, "outfuncs/readfuncs failed to produce an equal plan tree");
+		else
+#endif
+			plan = new_plan;
+	}
+#endif
+
 	/*
 	 * Print plan if debugging.
 	 */

src/include/nodes/nodes.h

@@ -610,7 +610,10 @@ extern char *bmsToString(const struct Bitmapset *bms);
 /*
  * nodes/{readfuncs.c,read.c}
  */
-extern void *stringToNode(char *str);
+extern void *stringToNode(const char *str);
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+extern void *stringToNodeWithLocations(const char *str);
+#endif
 extern struct Bitmapset *readBitmapset(void);
 extern uintptr_t readDatum(bool typbyval);
 extern bool *readBoolCols(int numCols);

src/include/nodes/readfuncs.h

@@ -16,12 +16,19 @@
 
 #include "nodes/nodes.h"
 
+/*
+ * variable in read.c that needs to be accessible to readfuncs.c
+ */
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+extern bool restore_location_fields;
+#endif
+
 /*
  * prototypes for functions in read.c (the lisp token parser)
  */
-extern char *pg_strtok(int *length);
-extern char *debackslash(char *token, int length);
-extern void *nodeRead(char *token, int tok_len);
+extern const char *pg_strtok(int *length);
+extern char *debackslash(const char *token, int length);
+extern void *nodeRead(const char *token, int tok_len);
 
 /*
  * prototypes for functions in readfuncs.c

src/include/pg_config_manual.h

@@ -286,6 +286,13 @@
  */
 /* #define COPY_PARSE_PLAN_TREES */
 
+/*
+ * Define this to force all parse and plan trees to be passed through
+ * outfuncs.c/readfuncs.c, to facilitate catching errors and omissions in
+ * those modules.
+ */
+/* #define WRITE_READ_PARSE_PLAN_TREES */
+
 /*
  * Define this to force all raw parse trees for DML statements to be scanned
  * by raw_expression_tree_walker(), to facilitate catching errors and