compat_pread mapped every non-EOF ReadFile failure to EIO, so the field report
was always the same three words: "Input/output error". #307 then burned three
rounds of guessing across three people — storage? contention? alignment? —
because the actual GetLastError code never appeared anywhere.
compat_pread now stashes the code in a per-thread slot and pread_full appends
it to the failure line:
pread qs: Input/output error (off 780592, 0/8192 bytes, WinErr=1450)
That one number is the difference between "insufficient system resources"
(1450 -> memory pressure), "sharing violation" (32 -> AV interference),
"device not ready" (21 -> the T: drive itself) and a dozen other distinct
diagnoses. Diagnostic only: no behavior change, no retry policy — that
discussion lives in #361 and should be settled AFTER the first report tells
us which error we are actually retrying.
Linux path untouched byte-for-byte; the Windows compile is certified by the
check.yml MSYS2 job.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
JustVugg
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