Provide database object names as separate fields in error messages.

This patch addresses the problem that applications currently have to
extract object names from possibly-localized textual error messages,
if they want to know for example which index caused a UNIQUE_VIOLATION
failure.  It adds new error message fields to the wire protocol, which
can carry the name of a table, table column, data type, or constraint
associated with the error.  (Since the protocol spec has always instructed
clients to ignore unrecognized field types, this should not create any
compatibility problem.)

Support for providing these new fields has been added to just a limited set
of error reports (mainly, those in the "integrity constraint violation"
SQLSTATE class), but we will doubtless add them to more calls in future.

Pavel Stehule, reviewed and extensively revised by Peter Geoghegan, with
additional hacking by Tom Lane.

src/backend/utils/cache/relcache.c

@@ -3998,6 +3998,82 @@ RelationGetExclusionInfo(Relation indexRelation,
 }
 
 
+/*
+ * Routines to support ereport() reports of relation-related errors
+ *
+ * These could have been put into elog.c, but it seems like a module layering
+ * violation to have elog.c calling relcache or syscache stuff --- and we
+ * definitely don't want elog.h including rel.h.  So we put them here.
+ */
+
+/*
+ * errtable --- stores schema_name and table_name of a table
+ * within the current errordata.
+ */
+int
+errtable(Relation rel)
+{
+	err_generic_string(PG_DIAG_SCHEMA_NAME,
+					   get_namespace_name(RelationGetNamespace(rel)));
+	err_generic_string(PG_DIAG_TABLE_NAME, RelationGetRelationName(rel));
+
+	return 0;			/* return value does not matter */
+}
+
+/*
+ * errtablecol --- stores schema_name, table_name and column_name
+ * of a table column within the current errordata.
+ *
+ * The column is specified by attribute number --- for most callers, this is
+ * easier and less error-prone than getting the column name for themselves.
+ */
+int
+errtablecol(Relation rel, int attnum)
+{
+	TupleDesc	reldesc = RelationGetDescr(rel);
+	const char *colname;
+
+	/* Use reldesc if it's a user attribute, else consult the catalogs */
+	if (attnum > 0 && attnum <= reldesc->natts)
+		colname = NameStr(reldesc->attrs[attnum - 1]->attname);
+	else
+		colname = get_relid_attribute_name(RelationGetRelid(rel), attnum);
+
+	return errtablecolname(rel, colname);
+}
+
+/*
+ * errtablecolname --- stores schema_name, table_name and column_name
+ * of a table column within the current errordata, where the column name is
+ * given directly rather than extracted from the relation's catalog data.
+ *
+ * Don't use this directly unless errtablecol() is inconvenient for some
+ * reason.  This might possibly be needed during intermediate states in ALTER
+ * TABLE, for instance.
+ */
+int
+errtablecolname(Relation rel, const char *colname)
+{
+	errtable(rel);
+	err_generic_string(PG_DIAG_COLUMN_NAME, colname);
+
+	return 0;			/* return value does not matter */
+}
+
+/*
+ * errtableconstraint --- stores schema_name, table_name and constraint_name
+ * of a table-related constraint within the current errordata.
+ */
+int
+errtableconstraint(Relation rel, const char *conname)
+{
+	errtable(rel);
+	err_generic_string(PG_DIAG_CONSTRAINT_NAME, conname);
+
+	return 0;			/* return value does not matter */
+}
+
+
 /*
  *	load_relcache_init_file, write_relcache_init_file
  *