fix: stop silently truncating oversized canonical pty input on macos - #3402
fix: stop silently truncating oversized canonical pty input on macos#3402caner-akca wants to merge 2 commits into
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Included review availability: Your plan provides up to 10 included reviews per hour; 2 remain after this review. 📝 WalkthroughWalkthroughThe PTY actor checks potentially truncated canonical-mode input before enqueueing it. On macOS, the check uses the 1024-byte per-line limit and active termios terminators. Timeout or unavailable verdicts return Merge Risk: 🟠 High · up to The change can still lose injected input during handoff, falsely report completion after data loss, or bypass protection after a partial write and terminal-mode transition, causing commands to be truncated or misreported as delivered. These are high-impact correctness risks that should be fixed before merging. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 51.02% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 49 functions across 6 files. (1 skipped: 1 unsupported.)
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Greptile SummaryThe replacement design removes the prior hold-and-discard mechanism and instead rejects macOS input before queueing when the active canonical line discipline would truncate it.
Confidence Score: 5/5The PR appears safe to merge. No blocking failure remains; the two previously reported hold-based failures are eliminated because oversized input is now refused before entering the write queue.
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| Filename | Overview |
|---|---|
| src/pty/actor/unix.rs | Replaces accepted-input holding with a bounded pre-queue discipline query and explicit refusal, leaving neither prior stall nor timed discard path. |
| src/platform/macos.rs | Models Darwin canonical queue limits, line terminators, and relevant input translations for oversized-submission checks. |
| src/pty/actor.rs | Introduces cross-runtime PTY write errors and keeps Windows behavior aligned through explicit unavailable errors. |
| src/pane.rs | Propagates PTY refusal errors through pane input and paste APIs. |
| src/terminal/runtime.rs | Updates terminal runtime input methods to expose the new PTY write-error contract. |
| src/platform/mod.rs | Provides no-op canonical-limit behavior for non-macOS Unix targets. |
| docs/next/website/src/content/docs/troubleshooting.mdx | Documents macOS refusal behavior, retry guidance, affected commands, and the known existing-line limitation. |
Sequence Diagram
sequenceDiagram
participant Caller
participant Handle as PTY handle
participant Runner as PTY runner
participant Kernel as Darwin PTY
Caller->>Handle: Submit input
alt Payload is at most 1024 bytes
Handle->>Runner: Queue input
Runner->>Kernel: Write bytes
Handle-->>Caller: Accepted
else Payload may exceed the queue
Handle->>Runner: Request discipline verdict
Runner->>Kernel: Read active termios
alt Canonical line would exceed limit
Runner-->>Handle: Limit exceeded
Handle-->>Caller: Explicit refusal
else Input survives intact
Runner-->>Handle: Safe
Handle->>Runner: Queue input
Runner->>Kernel: Write bytes
Handle-->>Caller: Accepted
end
end
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🧹 Nitpick comments (2)
src/pty/actor/unix.rs (2)
835-838: 🎯 Functional Correctness | 🔵 Trivial | 💤 Low valueThe undeliverable check restarts the line count at
current_write_offset.
canonical_submission_is_undeliverablescansfront[self.current_write_offset..]. After a partial write, the slice can start in the middle of a line.canonical_line_exceeds_input_queuethen counts that remainder as a new line, so an over-long line that was partly written can pass the check.The kernel already accepted the earlier bytes of that line in this case, so the outcome matches the documented limitation for writes into an already-full queue. Record the assumption in the doc comment so a later reader does not treat the offset slice as a full-line view.
926-926: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low valueRename
master_input_discipline_is_canonicalto reflect its return type.Its only caller handles
Option<CanonicalDiscipline>, and both the function and struct use the same macOS gate. The current name describes a boolean predicate;master_canonical_disciplinebetter matches the returned contract.
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src/pty/actor/unix.rs (3)
771-772: 🗄️ Data Integrity & Integration | 🟠 Major | 🏗️ Heavy liftPreserve the held submission when a handoff fails.
begin_handoff()callsflush_pending_writes_once(). If canonical mode stays active, this branch drops the frontByteswhen the hold expires. The actor then services the queuedRollbackHandoffonly after that drop. A failed handoff therefore reopens the gate after the original submission is lost. A handoff timeout of at least 750 ms can also returnOk(())after silently discarding input.Keep the held submission across rollback, or make the handoff abort before this generic drop path and return a failure. The added test checks only that later input is accepted; it does not verify that the retained submission remains deliverable.
940-941: 🎯 Functional Correctness | 🟡 Minor | ⚡ Quick winModel canonical input after termios preprocessing.
canonical_line_exceeds_input_queue()does not recognize activec_cc[VEOF]orc_cc[VEOL]delimiters. It also treatsCRas a delimiter whenICRNLis set, even whenIGNCRdiscardsCR. Capture these termios values and apply the input-processing order before checking line length. Otherwise, the actor can drop a deliverable submission or miss an overlong line that Darwin discards.Source: MCP tools
835-836: 🗄️ Data Integrity & Integration | 🟠 Major | 🏗️ Heavy liftCarry canonical line state across partial writes.
canonical_submission_is_undeliverable()scans only the suffix aftercurrent_write_offsetand resets its line length to zero. If the slave enablesICANONbefore the next flush, Darwin preserves the queued prefix and enforcesMAX_INPUTacross both parts. The guard can then allow a suffix that causes Darwin to drop or truncate input while the actor reports a successful write. Carry the unfinished line length into the scan and add a macOS regression for this transition.
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macOS caps a canonical input line at 1024 bytes including its terminator. A longer line is not rejected: the kernel keeps a byte-exact 1024-byte prefix, discards the rest, and still reports the whole write() as successful. Herdr trusted that return value, so `agent start` with long arguments answered ok while the composed command sat truncated in the shell's buffer with its closing quote lost, and a launcher built on that ok reported a healthy start for an agent that never launched. Refuse the write instead. Before queueing a submission large enough to be truncated at all, ask the PTY runner -- which holds the pane's only master fd, an invariant handoff depends on -- whether the discipline installed right now would cut it, and return LineExceedsCanonicalQueue when it would. The existing error paths carry it the rest of the way: agent start reports agent_start_input_failed and clears the agent name, so no phantom agent is left behind, and the caller can retry once the program takes over the terminal. The cap is per line, so multi-line input of any total size is unaffected. A discipline that changes between the verdict and the write costs a retryable refusal, never delivered bytes. Writes at or under the cap are answered by a length comparison alone, keeping the check off the path every keystroke takes, and non-Darwin targets compile it to a constant. refs herdrdev#2862
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Inline comments:
In `@docs/next/website/src/content/docs/troubleshooting.mdx`:
- Line 106: Update the multi-line safety statement near the canonical queue
discussion to clarify that each individual line must fit within the supported
limit, while the full-queue exception still applies because input may be
truncated when the queue is already full.
In `@src/pty/actor/unix.rs`:
- Line 155: Update the timeout handling in the canonical write-verdict flow so
Err(_) does not return Ok(()) and allow bytes to be queued as successful. Keep
the submission pending until a verdict arrives, or return an error before
enqueueing the write; preserve normal verdict handling.
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docs/next/website/src/content/docs/troubleshooting.mdxsrc/pane.rssrc/platform/macos.rssrc/platform/mod.rssrc/pty/actor.rssrc/pty/actor/unix.rssrc/terminal/runtime.rs
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Falling back to a plain write when the runner did not answer in time restored exactly the failure this check exists to remove: the runner drops the late reply, writes the bytes unchecked, and the caller is told they landed while the kernel truncates them. Every uncertain answer now refuses, so a caller told its input was accepted can still rely on that. Raise the wait to 250ms, well past a healthy answer, to keep the refusal rare now that expiry costs one, and correct the troubleshooting note: multi-line input is unaffected only while each line fits, and unread text already in the pane counts toward the line being composed. refs herdrdev#2862
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@greptileai review This branch was rebuilt as a replacement commit after your last review; the hold-based design you reviewed no longer exists. Both of your P1s are addressed by removing the mechanism (replies on the threads), and the current head is |
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Closing this — it does not fix the reported failure. The change detects that the canonical line discipline would truncate an Measuring the mechanism properly also showed this approach cannot work in
What removes the failure is to stop typing long lines: when a composed command Replacement PR to follow, carrying a cross-platform reproduction as the refs #2862 |
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Issue
On macOS, injecting a command longer than 1024 bytes into a pane whose foreground process holds the terminal in canonical mode — a fresh pane before the shell finishes starting, or a shell busy running a command — is silently truncated at exactly 1024 bytes. The closing quote is lost, the shell drops into a
quote>continuation, and Herdr reports the injection as delivered.refs #2862
Problem
XNU caps a canonical input line at
MAX_CANON/MAX_INPUT= 1024 bytes, including its terminator. A longer line is not rejected: the kernel keeps a byte-exact 1024-byte prefix, silently discards the rest, and still returns the full count fromwrite(). Standalone probes confirmed there is no reliable kernel signal for how much the discipline actually kept —TIOCOUTQon the master mixes echo and application output with stranded input, andFIONREADreports zero even with readable data — so the "chunk the write and verify delivery" option in the issue is not implementable.PtyIoActorRunner::flush_pending_writes_oncetrusted thewrite()return value, so the actor advanced past bytes the kernel had discarded. The damage is not the truncation itself but the silence:agent startreturnedok, so a launcher built on it reported a healthy start for an agent that never launched.How did we fix it?
Refuse the write instead of half-delivering it, which is the "reject with a loud error" option the issue asks for.
Before queueing a submission large enough to be truncated at all, the handle asks the PTY runner — which holds the pane's only master fd — whether the line discipline installed right now would cut it, and returns
PtyWriteError::LineExceedsCanonicalQueuewhen it would. The existing error paths carry it the rest of the way with no new plumbing:agent start→agent_start_input_failed, and the same branch already callsclear_agent_name(), so no phantom agent is left behindpane run/ send-text →pane_send_failedagent_send_keys_failed; terminal attach → its existingResult<(), String>Every uncertain answer refuses. A discipline that changes between the verdict and the write, or a runner that does not answer within 250ms (
PtyWriteError::DisciplineUnknown), costs a retryable refusal — never delivered bytes, and never a success report for a write that did not land.Design notes:
live_server_holds_one_pty_master_fd_per_panecorrectly rejected it, since handoff depends on the server holding exactly one ptmx fd per pane.false.ICANON,ICRNL,IGNCR,INLCR, plusVEOL/VEOL2/VEOF). Under a reject design only those can refuse a write the kernel would have accepted; a translation the model misses leaves the pre-existing behaviour untouched rather than losing data. That trade-off is documented on the type.An earlier revision of this branch held oversized submissions and dropped them after a 750ms hold. It was discarded: the drop reached the caller only as a
warn!in the server log, so its end state for the reported case was still a silent loss — the same defect, with the truncated fragment traded for total loss. Both bot findings on that design (Greptile's "canonical programs stall input" and "timed hold discards input") are resolved by removing the mechanism rather than tuning it.Verification
cargo nextest run --no-fail-fast: 3398/3398 pass.just check,just windows-lint,cargo fmt --checkclean. Cross-checked--target x86_64-unknown-linux-gnu.canonical_line_budget_matches_the_kernel_boundary— 1023+\nfits, 1024+\ndoes not, and an unterminated line counts too, matching the boundary measured against XNU.total_size_does_not_decide_the_verdictandline_terminators_follow_the_active_termios— per-line sizing, andICRNL/IGNCR/INLCR/VEOL/VEOL2/VEOFeach changing where a line ends.writes_within_the_queue_skip_the_discipline_read— the fast path never reads termios.oversized_canonical_line_is_refused_instead_of_truncated— a real canonical PTY refuses a 1400-byte line and delivers zero bytes, not a truncated prefix.the_same_input_is_accepted_once_the_program_leaves_canonical_mode— the identical payload arrives whole after the program takes over the terminal, so retry is a real remedy.short_lined_input_larger_than_the_queue_is_accepted_while_canonical— 3000 bytes of short lines into a canonical, unread PTY arrive intact.a_submission_is_refused_when_the_runner_cannot_report_its_discipline— an unanswered verdict refuses and never enqueues.Known limitation
Text already typed into a pane and not yet read counts toward the line the discipline is composing, and the model measures each submission from zero. A short injection appended to a long unread line can therefore still be cut. This is documented in the troubleshooting page; the reported class — a single composed line over the cap — is fully covered.