docs(signal-protocol): document per-device session-setup isolation for group SKDM (whatsapp-rust#996)

[jlucaso1]

Summary

Documents the fix from oxidezap/whatsapp-rust#996, which was just merged.

• ensure_sessions_for_devices used to abort the entire SessionPlan (and thus the whole SKDM fan-out) the moment process_prekey_bundle failed for any single target device.
• Since prepare_group_stanza gates the fan-out on session_plan.is_some(), one bad device silently starved every other cohort member's SKDM — and an own companion could get stuck permanently has_key=true because mark_forget_sender_key excludes own-user JIDs.
• The fix logs and skips the failing device instead, matching WhatsApp Web's per-device try/catch in GroupKeyDistributionMsg.

Added a new <Note> in advanced/signal-protocol.mdx right after the existing "Per-device session lock around the SKDM fan-out (v0.6)" note (which documents the related #990 fix), following the same style/versioning convention used for the other v0.6 group-send notes in that file.

Test plan

• Reviewed the rendered note for accuracy against the whatsapp-rust#996 diff and description
• Docs site build/preview (not run in this environment)

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Generated by Claude Code

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Summary by cubic

Adds a v0.6 note to advanced/signal-protocol.mdx documenting per-device isolation of session-setup failures during group SKDM fan-out, so one device’s failure no longer cancels the whole SessionPlan. This reflects the fix in oxidezap/whatsapp-rust#996 and matches WhatsApp Web’s per-device try/catch behavior.

^{Written for commit 1bc40e4. Summary will update on new commits.}

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Review details

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Configuration used: Organization UI

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Run ID: 36de0a40-24f1-4e42-8004-d7747a1ee186

📥 Commits

Reviewing files that changed from the base of the PR and between 3dc9f07 and 1bc40e4.

📒 Files selected for processing (1)

• advanced/signal-protocol.mdx

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[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Auto-approved: Pure documentation change: adds a technical note about a previous bug fix in the signal protocol docs. Only modifies one markdown file with 6 lines added. No source code, config, or logic changes.

Re-trigger cubic</sub>

[greptile-apps]

Greptile Summary

Adds a single <Note> block to advanced/signal-protocol.mdx documenting the whatsapp-rust#996 fix, which changed ensure_sessions_for_devices to log-and-skip a failing device rather than abort the entire SessionPlan.

• The note accurately describes the original bug (one X3DH failure nulling the whole SKDM plan via the session_plan.is_some() gate), the permanent stuck-state for own companions caused by mark_forget_sender_key filtering own-user JIDs, and the fix that matches WhatsApp Web's per-device try/catch in GroupKeyDistributionMsg.
• Placement, title format, (v0.6) version tag, and cross-reference style all match the adjacent #990 note and the other v0.6 notes throughout the file.

Confidence Score: 5/5

Pure documentation addition — no code is changed, no logic is modified, and the new note accurately reflects the whatsapp-rust#996 diff.

The change is a single <Note> block appended to an .mdx file. The described behavior (log-and-skip vs. abort-on-first-failure, the session_plan.is_some() gate, and the mark_forget_sender_key / exclude_own_devices stuck-state) matches the referenced PR description. Style, versioning, and placement are consistent with adjacent notes in the file.

No files require special attention.

Important Files Changed

Filename	Overview
advanced/signal-protocol.mdx	Adds a new <Note> documenting the per-device session-setup failure isolation fix (#996), placed correctly after the sibling #990 note and following established style/versioning conventions.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[prepare_group_stanza called] --> B[ensure_sessions_for_devices]
    B --> C{For each target device: process_prekey_bundle}

    subgraph OLD["Before #996 (old behavior)"]
        C -- "any failure" --> D["Return Err — entire call aborts"]
        D --> E["session_plan = None"]
        E --> F["is_some() gate: SKDM skipped for ALL devices"]
        F --> G["skmsg still ships, phash covers full set"]
        G --> H["External members: retry receipt → recovers"]
        G --> I["Own companion: mark_forget_sender_key\nexclude_own_devices=true → stuck has_key=true forever"]
    end

    subgraph NEW["After #996 (current behavior)"]
        C -- "device fails" --> J["Log error, skip failing device"]
        J --> K["Continue to next device"]
        C -- "device succeeds" --> K
        K --> L["Return Ok(SessionPlan) with successful devices"]
        L --> M["is_some() gate: SKDM proceeds"]
        M --> N["encrypt fan-out skips sessionless device naturally"]
        N --> O["All other devices receive pairwise SKDM ✓"]
    end

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
    A[prepare_group_stanza called] --> B[ensure_sessions_for_devices]
    B --> C{For each target device: process_prekey_bundle}

    subgraph OLD["Before #996 (old behavior)"]
        C -- "any failure" --> D["Return Err — entire call aborts"]
        D --> E["session_plan = None"]
        E --> F["is_some() gate: SKDM skipped for ALL devices"]
        F --> G["skmsg still ships, phash covers full set"]
        G --> H["External members: retry receipt → recovers"]
        G --> I["Own companion: mark_forget_sender_key\nexclude_own_devices=true → stuck has_key=true forever"]
    end

    subgraph NEW["After #996 (current behavior)"]
        C -- "device fails" --> J["Log error, skip failing device"]
        J --> K["Continue to next device"]
        C -- "device succeeds" --> K
        K --> L["Return Ok(SessionPlan) with successful devices"]
        L --> M["is_some() gate: SKDM proceeds"]
        M --> N["encrypt fan-out skips sessionless device naturally"]
        N --> O["All other devices receive pairwise SKDM ✓"]
    end

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