Add new error code for "file name too long"

This new error code, named file_name_too_long, maps internally to the
errno ENAMETOOLONG to produce a proper error code rather than an
internal code under errcode_for_file_access().  This error code can be
reached with some SQL command patterns, like a snapshot file name.

Reported-by: Alexander Lakhin
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/Zo4ROR9mgy8bowMo@paquier.xyz

src/backend/utils/errcodes.txt

@@ -439,6 +439,7 @@ Section: Class 58 - System Error (errors external to PostgreSQL itself)
 58030    E    ERRCODE_IO_ERROR                                               io_error
 58P01    E    ERRCODE_UNDEFINED_FILE                                         undefined_file
 58P02    E    ERRCODE_DUPLICATE_FILE                                         duplicate_file
+58P03    E    ERRCODE_FILE_NAME_TOO_LONG                                     file_name_too_long
 
 Section: Class F0 - Configuration File Error
 

src/backend/utils/error/elog.c

@@ -929,6 +929,10 @@ errcode_for_file_access(void)
 			edata->sqlerrcode = ERRCODE_IO_ERROR;
 			break;
 
+		case ENAMETOOLONG:		/* File name too long */
+			edata->sqlerrcode = ERRCODE_FILE_NAME_TOO_LONG;
+			break;
+
 			/* All else is classified as internal errors */
 		default:
 			edata->sqlerrcode = ERRCODE_INTERNAL_ERROR;