ZiyaZhang
9a299f9827
Descriptor-phase errnos in readTextForDiff fold to before: null so a file deleted or made unreadable after the caller's preflight cannot fail the committed write; cancellation and non-errno faults still propagate. Drops the now-covered isFile v8 ignore, extends llm-replay with catalog capability parity (defaultMaxTokens/reasoningEfforts), and records the fs-write-overwrite-bounded keyless snapshot pinning the over-limit whole-file fallback through the real acp-agent composition.
4.2 KiB
Agent Note: Bound overwrite contextual-diff bases at the provider
Status: implemented
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Problem
dsh-fs-local returned the complete prior file in FsWriteOutcome.before so consumers could build a contextual overwrite diff. That presentation-only pre-read was unbounded: a large overwrite could allocate the entire prior file, and checking an earlier path stat alone could not enforce a limit because an external process could replace or grow the file between the stat and the read. A large replacement also made the contextual hunk approach the replacement size even when the prior file was small. This closes the deferred bound recorded by result-time applied-hunk diffs.
Decision
LocalFileSystem.Config.diffBasisMaxBytes is a positive safe-integer deployment setting no greater than the runtime's Buffer-allocation and string-decoding limits, with a 10 MiB default. An overwrite supplies before only when the UTF-8 replacement is strictly below that limit and the prior file opened for the basis also ends below it. The prior read opens a descriptor, checks that descriptor, and reads at most the configured byte count in cancellation-aware chunks; reaching the boundary returns null. A size change after descriptor stat also returns null, even if the final size remains below the limit, because a partial prefix would be an incorrect diff basis. Binary or invalid UTF-8 prior content likewise returns null, as does any descriptor-phase errno — a prior file deleted or made unreadable between the caller's preflight and the basis open cannot fail a write the caller already committed to; only cancellation and non-errno faults propagate. These outcomes do not block the atomic write.
The local provider owns this decision because before is its optional, best-effort basis: it can avoid acquiring prior content that the configured pair limit has already made ineligible. tool-fs continues to own diff computation, retention, and presentation. The setting is independent of tool-fs.readStreamMinSize; read routing and overwrite presentation are different policies and need not share a value.
before: null asks consumers to use their existing whole-file fallback. The limit bounds only the extra prior-content acquisition and eligibility for a contextual pair. It does not bound the caller-owned replacement, the returned after value, or a consumer's fallback rendering.
Alternatives considered
Keep a hardcoded threshold equal to the read tool's streaming threshold. Rejected because the read threshold is deployment-configurable and consumer-owned. Two same-valued constants would create an unenforced cross-package coupling, while the overwrite basis is itself a deployment memory/presentation choice.
Gate only the prior side in the provider and cap new-content diffing in tool-fs. Rejected because it would acquire prior text even when the provider's configured pair limit already excludes the replacement, and it would split one before eligibility rule across two plugins. Consumers remain free to impose additional output limits.
Trust the initial probe() size before using an ordinary whole-file read. Rejected because that size can become stale before the read. The descriptor reader must enforce the bound on the object it actually reads.
Stream a contextual diff for arbitrarily large pairs. Rejected for this bug fix because the current filesystem seam returns complete before/after strings and the current diff implementation consumes them. A streaming diff would require a separate cross-package protocol and presentation design.
Consequences
Deployments can tune the extra overwrite-basis cost without changing read routing. At or above the exclusive limit, overwrites still succeed and remain visible through the whole-file fallback, but lose contextual hunks. Below the limit, the provider can still hold almost diffBasisMaxBytes of prior text in addition to the caller's replacement. The bounded descriptor read adds an open/stat/read sequence for eligible overwrites, while preventing a stale path probe from turning that sequence into an unbounded allocation.