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Oct 19, 2026
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- execSync, execFileSync and spawnSync throw Node's ENOBUFS past maxBuffer (1 MiB by default), and exec/execFile cut the stream and fail with ERR_CHILD_PROCESS_STDIO_MAXBUFFER.
- An invalid timeout or maxBuffer throws ERR_OUT_OF_RANGE before spawning, synchronously for the promisified forms too.
- Runtime benchmark geomean unchanged (geomean −0.7%; programs without the exec callback forms within 352 B of text).
The sync forms (execSync, execFileSync) and spawnSync apply maxBuffer to both outputs together, with Node's 1 MiB default and 0 meaning no limit. Past it the child gets the kill signal, every stdio pipe is closed so a grandchild holding one cannot keep the call waiting, and the call throws Node's ENOBUFS spawnSync error (spawnSync reports it on result.error). Pipes are also closed on a timeout kill. exec and execFile, in the callback and promisified forms, apply it per stream as lib/child_process.js does: the chunk that passes the limit is cut to the remaining UTF-16 length, both streams are destroyed and the child is killed, and the error is a RangeError with code ERR_CHILD_PROCESS_STDIO_MAXBUFFER and cmd (plus stdout and stderr when promisified). A negative or NaN maxBuffer, or a timeout that is not a non-negative integer, throws ERR_OUT_OF_RANGE before anything spawns. The promisified forms throw it synchronously, as Node's custom promisify calls execFile directly.
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exec compares its output's length with maxBuffer as a number and keeps `maxBuffer - previous total` UTF-16 code units of the chunk that passed it, as Node does. With a fractional limit, that allowance had a fraction left: an ASCII character with less than one unit left took the branch that keeps the high surrogate of an astral character, which decoded and rewrote four bytes from that position, past the end of the capture buffer when the cut fell at its end. The allowance is now truncated to whole units, as Node's slice truncates its end, and the high surrogate is kept only for a complete four-byte character with exactly one unit left. Keeping it now records that a lone surrogate exists, so JSON.stringify and other string operations escape it as Node does.
…bers The merge combined this PR's two-line removal in tests/diagnostics/child-process.ts with main's new piped-stream section, whose snapshot lines kept main's numbering.
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