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tachyon: degrade reads instead of failing the whome read completely#7

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maurycy wants to merge 11 commits into
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tachyon-degrade-gracefully
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tachyon: degrade reads instead of failing the whome read completely#7
maurycy wants to merge 11 commits into
mainfrom
tachyon-degrade-gracefully

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@maurycy

@maurycy maurycy commented Jul 7, 2026 •
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String Field Trigger What is preserved Location field
<unreadable frame> funcname The code object struct itself cannot be read Nothing; filename is set to ~ None
<unknown function> funcname Code object read OK, but the qualname string read failed Filename, if its own read succeeded None
<unknown file> filename Code object read OK, but the filename string read failed Funcname, if its own read succeeded None
(len=N) suffix funcname / filename / task name Longer than the caller's cap: 1024 for qualname and filename, 255 for asyncio task name; first min(len, cap) chars kept, real length appended, eg: very_long_qualnam(len=1100) The truncated prefix; frame is otherwise fully normal (line numbers intact) Normal
(no marker, location dropped) location Linetable read failed, or linetable exceeds MAX_REMOTE_READ (256 KB) Funcname and filename, both real None
~ filename Companion to <unreadable frame>; similar to pstats convention for synthetic entries - None

Fatal v. degrade

Classified by _Py_RemoteDebug_IsFatalReadError():

Exception Meaning Raised by
PermissionError No permissions to read target memory (ptrace scope, KERN_PROTECTION_FAILURE, EPERM/EACCES, ERROR_ACCESS_DENIED) All platforms
ProcessLookupError Target process died (ESRCH; mach task gone / memory space gone) Linux, macOS (it will be a bit different on vm_remap; not sure about Windows
MemoryError Allocation failure? All platforms
Any BaseException not inheriting from Exception KeyboardInterrupt, SystemExit, etc.: All platforms

Everything else (plain OSError such as EFAULT, RuntimeError from corrupted remote data or unknown Mach errors, ValueError from KERN_INVALID_ARGUMENT etc.) is treated as remote read fail

Interaction notes

  • Degraded frames are never cached. Who knows what caused the failure? Maybe it's just transient...
  • Truncated strings are cached, because truncation is deterministic for a given object.
  • If both qualname and filename fail to read, then the frame shows <unknown function> in <unknown file>, and the linetable read is skipped

@maurycy maurycy force-pushed the tachyon-degrade-gracefully branch from 7d98020 to dd0fdc7 Compare July 8, 2026 14:17
@maurycy maurycy changed the title stub degrade Jul 8, 2026
@maurycy maurycy changed the title degrade Tachyon: Do not raise an exception but degrade Jul 8, 2026
@maurycy maurycy changed the title Tachyon: Do not raise an exception but degrade tachyon: degrade Jul 8, 2026
@maurycy maurycy changed the title tachyon: degrade tachyon: degrade reads instead of failing them completely Jul 8, 2026
@maurycy maurycy changed the title tachyon: degrade reads instead of failing them completely tachyon: degrade reads instead of failing the whome read completely Jul 8, 2026
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