x/build: linux/mipsle (soft-float) builder blocked by CIPD hard-float binary

[AaronGrand]

This is a follow-up to #77417

The linux/mipsle builder (linux-mipsle-hooc-ag) is currently blocked due to an incompatibility between the Swarming-provided CIPD client and the target hardware.

Current status

• Builder hardware: MediaTek MT7621 (MIPS32, soft-float, OpenWrt)
• Swarming bot:
  • Connects successfully
  • Receives tasks
• Failure occurs when executing CIPD:
  [Errno 8] Exec format error: '.../cipd_cache/bin/cipd'

Root cause

• Swarming downloads a hard-float MIPS CIPD binary
• The target system is soft-float (no FPU)
• Hard-float binaries cannot execute → immediate failure

Verified on device:

• Downloaded CIPD binary:
  ELF 32-bit LSB executable, MIPS32 rel2, hard-float
• System requires:
  GOMIPS=softfloat

Workaround tested

• Built a local CIPD client:
  `GOOS=linux GOARCH=mipsle GOMIPS=softfloat``

• This binary works correctly

• However:

• Swarming still replaces it with the incompatible CIPD client

Blocking issue

There currently appears to be no supported way to:

1. Provide a soft-float CIPD client for linux/mipsle, or
2. Override the CIPD binary used by Swarming tasks

Request

What would be the preferred path forward?

• Add soft-float CIPD support for linux/mipsle?
• Allow overriding the CIPD client on builders?
• Alternative approach?

Notes

The builder itself is otherwise stable and ready:

• Swarming bot runs reliably
• Tasks execute correctly up to the CIPD step

However, it is currently kept offline until the CIPD incompatibility is resolved.

Thanks!

CC @dmitshur @prattmic

[dr2chase]

@golang/release