[srgg]

On a peripheral, a service hidden with removeService(svc, false) can't be brought back while a central is connected: addService(svc) only flags it for the next GATT reset, and start() only resets when no central is connected.

The connected central also never hears about the hide or the show. The only Service Changed indication is sent after that reset, so its attribute cache goes stale.

With this change a hidden service is shown in place, and each hide or show is announced:

• resetGATT() keeps hidden services registered, so they always have a handle. start() hides them again after ble_gatts_start().
• addService() on a hidden service makes it visible right away with ble_gatts_svc_set_visibility(handle, 1), unless a reset is already pending, in which case it waits for that reset as before.
• Hide and show call sendServiceChangedIndication(). It covers the full range because ble_svc_gatt_changed() keeps a single pending range, and a narrower one could be overwritten before it is indicated.
• Deleting a hidden service marks it for deletion; resetGATT() frees it, since the stack still points to its definition until then.

Testing:

• ESP32-S3 peripheral that hides one service in some of its states, macOS (CoreBluetooth) central. The patch ran on top of a 3.0.0-dev fork branch, not this base commit.
• One 600 s run: the central disconnected and reconnected, which triggered a reset; then, with the central connected, the peripheral changed state 14 times, hiding and showing the service twice each. Every ble_gatts_svc_set_visibility() call on the service's handle succeeded. Without the patch the reset cleared that handle and every later call failed with BLE_HS_ENOENT.
• Not tested: whether the central actually re-reads the shown service, centrals other than CoreBluetooth, and the hide-then-delete path in removeService().

Summary by CodeRabbit

• Bug Fixes
  • Removing a service without deleting it now hides it from clients and sends a Service Changed indication. Hidden services can be restored without a reset when no service change is pending.
  • Services marked for deletion are removed during the next successful GATT reset, after connections close. Hidden services are registered during resets and hidden again when the server starts.
  • GATT reset failures are detected and reported before registered services are modified. Enabling Service Changed indications after an indication has been sent also triggers an indication to the client.

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Walkthrough

Arr, NimBLEServer tracks Service Changed indications and distinguishes hidden services from services marked for deletion. Hidden services can be restored in place. GATT resets handle service registration and visibility, and report reset or service-start failures.

Changes: Service Visibility and Change Indications

Layer / File(s)	Summary
Track Service Changed indications src/NimBLEServer.cpp, src/NimBLEServer.h	Arr, the server records when it sends a Service Changed indication. If a client later enables indications for that characteristic, the server sends the client an indication.
Hide, delete, and restore services src/NimBLEServer.cpp	Removal hides a service or marks it for deletion. A deletion request for an already hidden service schedules a database change. addService() can restore a hidden service in place when no reset is pending and visibility can be enabled.
Register services during GATT resets src/NimBLEServer.cpp	Registration excludes services marked for deletion. Startup hides services that remain hidden. resetGATT() reports reset failures and returns false if a service fails to start.

Priority: Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant NimBLEServer
  participant GATTDatabase
  NimBLEServer->>GATTDatabase: Send Service Changed indication
  GATTDatabase-->>NimBLEServer: Indication sent
  Client->>NimBLEServer: Enable Service Changed indications
  NimBLEServer->>GATTDatabase: Send indication to subscribed client

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Possibly related PRs

• h2zero/NimBLE-Arduino#1054: PR #1054 processes registered handles during NimBLEServer::start(), which also handles hidden-service registration in this change.
• h2zero/NimBLE-Arduino#1205: PR #1205 changes how service visibility updates run, relating to the hide, show, and delete operations in this change.

Suggested reviewers: h2zero

Merge Risk: Low · up to 4a11d

This change shows hidden services to connected centrals and announces the change. In a rare resource-exhaustion case, a central that subscribes to Service Changed can miss the indication on that connection and keep its cached attributes until it reconnects. The earlier deletion and restore-order concerns are addressed, so the change is mergeable with that known limitation.

Pre-merge checks: 5

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title accurately summarizes the main change: making hidden services visible to connected centrals and announcing the service change.
Docstring Coverage		Docstring coverage is 87.50% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentati…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Explanation

Docstring coverage is 87.50% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 8 functions across 2 files.

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_{Arr, indications mark the way,
Hidden services wait their day.
Deletion joins the reset tide,
Restored services come inside.
GATT checks before it sails.
The server logs reset failures.}

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Actionable comments posted: 3

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⚠️ Outside diff range comments (1)

Major · Arr, defer deletion until the GATT reset. · NimBLEServer.cpp:858

src/NimBLEServer.cpp:858
🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Arr, defer deletion until the GATT reset.

After a hidden service survives a reset, this deletion frees its service definition while NimBLE still retains a pointer to it. A later ble_gatts_find_svc() can dereference that pointer before the next reset. Mark the service for deletion and keep it alive until resetGATT() unregisters it. (raw.githubusercontent.com)

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Review comment at @src/NimBLEServer.cpp at line 858:
Update the service-deletion path containing delete *it to defer freeing hidden
services while NimBLE may still retain their pointers. Keep each service alive
until resetGATT() unregisters it, then delete it.

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

Prompt to fix review comments

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Inline comments:
Review comments at @src/NimBLEServer.cpp:
- Line 218: Update the ble_svc_gatt_changed call to cover the full GATT database
range rather than only the latest changed range, so multiple service changes
remain discoverable by bonded peers after reconnecting.
- Around line 907-908: Update the service-visibility restoration condition in
this branch so it restores the service only when it has no pending database
changes; preserve the existing removal-state, handle, and visibility checks.
- Line 908: Update the visibility-change paths in addService() and
removeService() to synchronize calls to ble_gatts_svc_set_visibility() with
host-task access, preventing concurrent modification of the shared service lists
during traversal.

---

Outside diff comments:
Review comments at @src/NimBLEServer.cpp:
- Line 858: Update the service-deletion path containing delete *it to defer
freeing hidden services while NimBLE may still retain their pointers. Keep each
service alive until resetGATT() unregisters it, then delete it.

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[srgg]

@coderabbitai review
Thanks, addressed in the latest commit:

• Single-service range (line 218): agreed. ble_svc_gatt_changed() keeps one pending range, so a second change before the indication goes out overwrote the first. Hide and show now call sendServiceChangedIndication() (full range).
• Show while a reset is pending (907-908): agreed. addService() shows in place only when m_svcChanged is clear; otherwise it waits for the reset as before.
• Deleting a hidden service (858): agreed, and this PR made it reachable after a reset. The service is now marked NIMBLE_ATT_REMOVE_DELETE and freed in resetGATT(), like the regular delete path.
• Visibility vs. the host task (908): not changed here. removeService() already calls ble_gatts_svc_set_visibility() from the application task, and the host side (ble_att_svr_find_by_handle(), ble_att_svr_move_entries()) walks the attribute list without ble_hs_lock, so a lock in NimBLEServer would not close the race. That needs a fix in the host itself; happy to look at it separately if wanted.

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[srgg]

On the two risks in the latest summary:

• Failed reset, then delete: agreed, and it predates this PR — resetGATT() ignored ble_gatts_reset()'s result. That call refuses with BLE_HS_EBUSY while an advertising, discovery or connection procedure is active (ble_gatts_mutable()), leaving every attribute registered, and resetGATT() then freed the services marked for deletion. It now returns false before freeing anything; m_svcChanged stays set, so the next start() retries. This PR routes the delete of a hidden service through that path too, so the check lands here.
• Showing a service during central discovery: not changed. A central that was mid-discovery gets the Service Changed indication sent right after the show, and rediscovers the indicated range (Core Spec Vol 3 Part G, 2.5.2).

[srgg]

Testing this PR on the hardware turned up a regression, fixed by a fourth commit, 6d0d93d "Re-send Service Changed to a client as it subscribes".

Showing or hiding a service in place sends Service Changed only to the clients connected at that moment, and NimBLE keeps a missed indication for bonded peers only. A macOS client (CoreBluetooth, unbonded) that drops the link as the indication goes out keeps the table it last discovered and serves it to its next connection. In our firmware a service hidden while a client is connected, then shown again as that client disconnects, reads as absent on the next connection: 3 of 3 runs, characteristics resolved from the cache in 24 ms. Before this PR nothing was indicated while connected, CoreBluetooth discovered over the air on every connection, and the same sequence passed 4 of 4.

The commit marks the server once any Service Changed has been sent; from then on each client that enables indications on Service Changed (0x2A05) gets one. That also covers the indication start() sends after a GATT reset, when no client is connected. With it the same sequence passed 4 of 4 on an ESP32-S3, and discovery over the air took 2.2 s as before.

The 0x2A05 subscription is now handled before getCharacteristicByHandle(), so it no longer logs subscribe event; attr_handle=%d, not found.

The branch is also rebased onto master (79ce2c9) to clear the conflict; the three earlier commits are unchanged apart from context.

[coderabbitai]

Actionable comments posted: 2

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Inline comments:
Review comments at @src/NimBLEServer.cpp:
- Around line 1001-1003: When `pSvc->start_internal()` fails in the
service-start loop, clear the service definitions queued by the current attempt
before returning false. Ensure a later `start()` can retry without re-queuing
definitions from the failed attempt.
- Line 586: Update the subscription callback’s ble_gatts_indicate call for
svcChangedHandle to use the host’s per-peer indication scheduling path, so the
Service Changed indication is sent in order with pending indications and does
not conflict with an outstanding indication.

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[h2zero]

I don't think this is right, clients should not cache the database unless bonded, this seems to be fixing a client side issue. Also this will trigger all clients to refresh the database instead of just the current one.

[srgg]

Yes, an unbonded client is expected to rediscover services on each connection (Core 4.2, Vol 3, Part G, 2.5.2). However, macOS CoreBluetooth seems to use a cached table instead, so this is a workaround for CoreBluetooth rather than a regression in the PR. Sorry for the confusion!

Just one small clarification: the indication is sent only to the subscribing connection (ble_gatts_indicate(conn_handle, …)), not to all connected clients. Any client subscribing after a change will still receive it, so a compliant client would simply perform one extra rediscovery.

Would you be open to keeping this in the PR as a CoreBluetooth workaround? I think it would be useful for macOS users (such as I'm). CoreBluetooth(macOS) gives a client app very little control here: there is no API to clear its attribute cache or force a fresh discovery, so the only lever I found is a Service Changed indication from the server.

UPD In case extra rediscovery for compliant clients is a concern, it can be opt-in: off by default, and enabled explicitly for peripheral devices that need macOS clients, e.g. NimBLEServer::setServiceChangedOnSubscribe(true).

[h2zero]

Sorry I read the call differently and thought you were calling ble_svc_gatt_changed which would indicate to all peers.

The other issue when looking at this is how to deal with multiple clients that are not bonded? This will only send the indication to the first one. Would be nice if everyone just followed the spec lol.

[srgg]

As I understand it, it should reach every unbonded client, not just the first. Each connection has its own CCCD state (conn->bhc_gatt_svr.clt_cfgs), and ble_gatts_clt_cfg_access() raises BLE_GAP_SUBSCRIBE_REASON_WRITE with that client's conn_handle, so the indication goes only to that connection. Also, unlike m_svcChanged, m_svcChangedSent is never cleared, so each client subscribing after the first Service Changed gets an indication.

The downside is that clients with an already up-to-date table will get it too, even if they reconnect hours later, and may end up rediscovering unnecessarily. I didn't find a way to recognize a previously connected, unbonded client.

I tested this on my bench, and every new connection after a change received the indication. I couldn't properly test multiple simultaneous connections, though. CoreBluetooth combines multiple apps on the same Mac into a single BLE connection, so the board saw only one connection and received one indication, even with two clients running.