S001 ABEND in datasetGetHandler when reading dataset with I/O error

[mgrossmann]

Summary

mvsMF crashes with S001 ABEND when a GET request reads a dataset that is in a bad state (e.g. cataloged but not properly allocated on the volume). Instead of returning HTTP 500, the server ABENDs because crent370's @@AREAD has no SYNAD exit registered.

How to reproduce

# 1. Create a dataset that will end up in a bad state.
#    For example, allocate a PS dataset on a volume, then
#    delete the VTOC entry but leave the catalog entry
#    (or let a failed SVC99 leave a stale catalog entry).
#
#    A simpler way: create a dataset, then try to delete+recreate
#    it while another job still has it allocated. The delete fails,
#    the create fails, and a subsequent read triggers the ABEND.

# 2. Attempt to read the broken dataset via the REST API:
curl -u IBMUSER:SYS1 \
  -H "X-IBM-Data-Type: binary" \
  "http://<host>:<port>/restfiles/ds/IBMUSER.MBT.XMIT.OUT"

The GET triggers read_and_send_dataset → fread → __fread → __fgetc → @@AREAD, which hits an I/O error. Because no SYNAD exit is registered on the DCB, MVS takes the default action: ABEND S001.

ABEND trace

IEC020I 001-1,HTTPD,HTTPD,SYS00008,251,WORK00,IBMUSER.MBT.XMIT.OUT
IEC020I NO ERROR HANDLING, (SYNAD), EXIT SPECIFIED
ABEND S001 detected for module 00000000 epname @@AREAD offset 00B8EEB8

@@AREAD
__fgetc
__fread
fread
read_and_send_dataset
datasetGetHandler
handle_request
main
__start

Root cause

Two layers are missing error handling:

1. crent370 @@AREAD: No SYNAD exit is registered on the DCB. When an I/O error occurs during BSAM/QSAM read, MVS has no error routine to call and issues ABEND S001. A SYNAD exit should be registered so that I/O errors return an error code to the caller instead of crashing.

2. mvsMF datasetGetHandler: Should validate that the dataset is readable before streaming it. If fopen/fread returns an error, the handler should return HTTP 500 gracefully instead of continuing into the read loop.

Expected behavior

• GET /restfiles/ds/BROKEN.DATASET should return HTTP 500 with a JSON error body
• mvsMF should remain running and able to serve subsequent requests

Environment

• MVS 3.8j on Hercules (TK4-)
• mvsMF (latest)
• Triggered during mbt package when XMIT staging dataset was in a bad state