Rethink the "read/write parameter" mechanism in pl/pgsql.

Performance issues with the preceding patch to re-implement array
element assignment within pl/pgsql led me to realize that the read/write
parameter mechanism is misdesigned.  Instead of requiring the assignment
source expression to be such that *all* its references to the target
variable could be passed as R/W, we really want to identify *one*
reference to the target variable to be passed as R/W, allowing any other
ones to be passed read/only as they would be by default.  As long as the
R/W reference is a direct argument to the top-level (hence last to be
executed) function in the expression, there is no harm in R/O references
being passed to other lower parts of the expression.  Nor is there any
use-case for more than one argument of the top-level function being R/W.

Hence, rewrite that logic to identify one single Param that references
the target variable, and make only that Param pass a read/write
reference, not any other Params referencing the target variable.

Discussion: https://postgr.es/m/4165684.1607707277@sss.pgh.pa.us

src/backend/utils/adt/arrayfuncs.c

@@ -2582,8 +2582,11 @@ array_set_element_expanded(Datum arraydatum,
 
 	/*
 	 * Copy new element into array's context, if needed (we assume it's
-	 * already detoasted, so no junk should be created).  If we fail further
-	 * down, this memory is leaked, but that's reasonably harmless.
+	 * already detoasted, so no junk should be created).  Doing this before
+	 * we've made any significant changes ensures that our behavior is sane
+	 * even when the source is a reference to some element of this same array.
+	 * If we fail further down, this memory is leaked, but that's reasonably
+	 * harmless.
 	 */
 	if (!eah->typbyval && !isNull)
 	{

src/pl/plpgsql/src/pl_gram.y

@@ -2820,7 +2820,7 @@ read_sql_construct(int until,
 	expr->parseMode		= parsemode;
 	expr->plan			= NULL;
 	expr->paramnos		= NULL;
-	expr->rwparam		= -1;
+	expr->target_param	= -1;
 	expr->ns			= plpgsql_ns_top();
 	pfree(ds.data);
 
@@ -3067,7 +3067,7 @@ make_execsql_stmt(int firsttoken, int location)
 	expr->parseMode		= RAW_PARSE_DEFAULT;
 	expr->plan			= NULL;
 	expr->paramnos		= NULL;
-	expr->rwparam		= -1;
+	expr->target_param	= -1;
 	expr->ns			= plpgsql_ns_top();
 	pfree(ds.data);
 
@@ -3949,7 +3949,7 @@ read_cursor_args(PLpgSQL_var *cursor, int until)
 	expr->parseMode		= RAW_PARSE_PLPGSQL_EXPR;
 	expr->plan			= NULL;
 	expr->paramnos		= NULL;
-	expr->rwparam		= -1;
+	expr->target_param	= -1;
 	expr->ns            = plpgsql_ns_top();
 	pfree(ds.data);
 

src/pl/plpgsql/src/plpgsql.h

@@ -221,7 +221,6 @@ typedef struct PLpgSQL_expr
 	RawParseMode parseMode;		/* raw_parser() mode to use */
 	SPIPlanPtr	plan;			/* plan, or NULL if not made yet */
 	Bitmapset  *paramnos;		/* all dnos referenced by this query */
-	int			rwparam;		/* dno of read/write param, or -1 if none */
 
 	/* function containing this expr (not set until we first parse query) */
 	struct PLpgSQL_function *func;
@@ -235,6 +234,17 @@ typedef struct PLpgSQL_expr
 	int32		expr_simple_typmod; /* result typmod, if simple */
 	bool		expr_simple_mutable;	/* true if simple expr is mutable */
 
+	/*
+	 * These fields are used to optimize assignments to expanded-datum
+	 * variables.  If this expression is the source of an assignment to a
+	 * simple variable, target_param holds that variable's dno; else it's -1.
+	 * If we match a Param within expr_simple_expr to such a variable, that
+	 * Param's address is stored in expr_rw_param; then expression code
+	 * generation will allow the value for that Param to be passed read/write.
+	 */
+	int			target_param;	/* dno of assign target, or -1 if none */
+	Param	   *expr_rw_param;	/* read/write Param within expr, if any */
+
 	/*
 	 * If the expression was ever determined to be simple, we remember its
 	 * CachedPlanSource and CachedPlan here.  If expr_simple_plan_lxid matches