Add GET DIAGNOSTICS ... PG_CONTEXT in PL/PgSQL

This adds the ability to get the call stack as a string from within a
PL/PgSQL function, which can be handy for logging to a table, or to
include in a useful message to an end-user.

Pavel Stehule, reviewed by Rushabh Lathia and rather heavily whacked
around by Stephen Frost.

src/pl/plpgsql/src/pl_exec.c

@@ -1599,6 +1599,16 @@ exec_stmt_getdiag(PLpgSQL_execstate *estate, PLpgSQL_stmt_getdiag *stmt)
 									 estate->cur_error->schema_name);
 				break;
 
+			case PLPGSQL_GETDIAG_CONTEXT:
+				{
+					char *contextstackstr = GetErrorContextStack();
+
+					exec_assign_c_string(estate, var, contextstackstr);
+
+					pfree(contextstackstr);
+				}
+				break;
+
 			default:
 				elog(ERROR, "unrecognized diagnostic item kind: %d",
 					 diag_item->kind);

src/pl/plpgsql/src/pl_funcs.c

@@ -277,6 +277,8 @@ plpgsql_getdiag_kindname(int kind)
 			return "ROW_COUNT";
 		case PLPGSQL_GETDIAG_RESULT_OID:
 			return "RESULT_OID";
+		case PLPGSQL_GETDIAG_CONTEXT:
+			return "PG_CONTEXT";
 		case PLPGSQL_GETDIAG_ERROR_CONTEXT:
 			return "PG_EXCEPTION_CONTEXT";
 		case PLPGSQL_GETDIAG_ERROR_DETAIL:

src/pl/plpgsql/src/pl_gram.y

@@ -303,6 +303,7 @@ static	List			*read_raise_options(void);
 %token <keyword>	K_OPTION
 %token <keyword>	K_OR
 %token <keyword>	K_PERFORM
+%token <keyword>	K_PG_CONTEXT
 %token <keyword>	K_PG_DATATYPE_NAME
 %token <keyword>	K_PG_EXCEPTION_CONTEXT
 %token <keyword>	K_PG_EXCEPTION_DETAIL
@@ -894,6 +895,7 @@ stmt_getdiag	: K_GET getdiag_area_opt K_DIAGNOSTICS getdiag_list ';'
 								/* these fields are disallowed in stacked case */
 								case PLPGSQL_GETDIAG_ROW_COUNT:
 								case PLPGSQL_GETDIAG_RESULT_OID:
+								case PLPGSQL_GETDIAG_CONTEXT:
 									if (new->is_stacked)
 										ereport(ERROR,
 												(errcode(ERRCODE_SYNTAX_ERROR),
@@ -976,6 +978,9 @@ getdiag_item :
 						else if (tok_is_keyword(tok, &yylval,
 												K_RESULT_OID, "result_oid"))
 							$$ = PLPGSQL_GETDIAG_RESULT_OID;
+						else if (tok_is_keyword(tok, &yylval,
+												K_PG_CONTEXT, "pg_context"))
+							$$ = PLPGSQL_GETDIAG_CONTEXT;
 						else if (tok_is_keyword(tok, &yylval,
 												K_PG_EXCEPTION_DETAIL, "pg_exception_detail"))
 							$$ = PLPGSQL_GETDIAG_ERROR_DETAIL;
@@ -2287,6 +2292,7 @@ unreserved_keyword	:
 				| K_NO
 				| K_NOTICE
 				| K_OPTION
+				| K_PG_CONTEXT
 				| K_PG_DATATYPE_NAME
 				| K_PG_EXCEPTION_CONTEXT
 				| K_PG_EXCEPTION_DETAIL

src/pl/plpgsql/src/pl_scanner.c

@@ -135,6 +135,7 @@ static const ScanKeyword unreserved_keywords[] = {
 	PG_KEYWORD("no", K_NO, UNRESERVED_KEYWORD)
 	PG_KEYWORD("notice", K_NOTICE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("option", K_OPTION, UNRESERVED_KEYWORD)
+	PG_KEYWORD("pg_context", K_PG_CONTEXT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("pg_datatype_name", K_PG_DATATYPE_NAME, UNRESERVED_KEYWORD)
 	PG_KEYWORD("pg_exception_context", K_PG_EXCEPTION_CONTEXT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("pg_exception_detail", K_PG_EXCEPTION_DETAIL, UNRESERVED_KEYWORD)

src/pl/plpgsql/src/plpgsql.h

@@ -124,6 +124,7 @@ enum
 {
 	PLPGSQL_GETDIAG_ROW_COUNT,
 	PLPGSQL_GETDIAG_RESULT_OID,
+	PLPGSQL_GETDIAG_CONTEXT,
 	PLPGSQL_GETDIAG_ERROR_CONTEXT,
 	PLPGSQL_GETDIAG_ERROR_DETAIL,
 	PLPGSQL_GETDIAG_ERROR_HINT,