ipc: support per-address max-connections options on -ipcbind

enirox001 commented at 1:44 pm on April 9, 2026: contributor

This is a draft follow-up to #34978 to make the per-address IPC connection-limit approach concrete.

The branch extends -ipcbind to accept :max-connections=<n> options, for example -ipcbind=unix::max-connections=8 or -ipcbind=unix:/custom/path:max-connections=8, instead of introducing a separate global -ipcmaxconnections option.

It also adds the local listener limit support needed in libmultiprocess and threads the parsed per-address limit through to ListenConnections() so each listener can stop accepting new IPC connections after reaching its local cap and resume accepting after a disconnect.

The draft includes:

parser coverage for -ipcbind max-connections and invalid inputs
functional init coverage for FD reservation logging and init errors
an IPC behavior test verifying that with max_connections=1, a second client only becomes usable after the first disconnects

This is intended to help evaluate the per-address approach downstream against the global-limit direction in #34978.

DrahtBot commented at 1:44 pm on April 9, 2026: contributor

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Concept ACK	ryanofsky

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Conflicts

Reviewers, this pull request conflicts with the following ones:

#34978 (init: Reserve file descriptors for IPC connections by enirox001)
#32427 ((RFC) kernel: Replace leveldb-based BlockTreeDB with flat-file based store by sedited)
#32387 (ipc: add windows support by ryanofsky)
#17783 (common: Disallow calling IsArgSet() on ALLOW_LIST options by ryanofsky)
#17581 (refactor: Remove settings merge reverse precedence code by ryanofsky)
#17580 (refactor: Add ALLOW_LIST flags and enforce usage in CheckArgFlags by ryanofsky)
#17493 (util: Forbid ambiguous multiple assignments in config file by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):

check_bind(“unix::max-connections=8”, “unix:”, 8, “”) in src/ipc/test/ipc_tests.cpp
check_bind(“unix:path.sock:max-connections=8”, “unix:path.sock”, 8, “”) in src/ipc/test/ipc_tests.cpp
check_bind(“unix:path.sock:max-connections=8,unknown=1”, “”, 0, “Unknown socket option ‘unknown’”) in src/ipc/test/ipc_tests.cpp
check_bind(“unix:max-connections=8”, “”, 0, “Missing unix socket path before socket options; use unix:: for the default path”) in src/ipc/test/ipc_tests.cpp
check_bind(“unix::max-connections=-1”, “”, 0, “Invalid max-connections value ‘-1’”) in src/ipc/test/ipc_tests.cpp
check_bind(“unix::max-connections=”, “”, 0, “Missing value for max-connections option”) in src/ipc/test/ipc_tests.cpp

2026-04-09 14:56:02

enirox001 renamed this:
~~04 26 ipcbind max connections draft~~
ipc: support per-address max-connections options on -ipcbind
on Apr 9, 2026

enirox001 force-pushed on Apr 9, 2026

DrahtBot added the label CI failed on Apr 9, 2026

enirox001 force-pushed on Apr 9, 2026

ryanofsky commented at 2:40 pm on April 9, 2026: contributor

Concept ACK. Looks like suggestions from #35036 (comment) apply here too (sorry, saw that PR first in my inbox).

IMO this is a better alternative to adding a new -ipcmaxconnections option like #34978, and I’d prefer this approach but both approaches seem reasonable. Main reason I prefer this approach is IPC is a pretty generic feature, so it seems useful to be able to set different limits for different purposes (like GUIs vs mining clients).

ipc: add per-address max-connections parsing for -ipcbind

Replace the global -ipcmaxconnections option with per-address
max-connections parsing on -ipcbind values. Update init argument
documentation, parse and validate :max-connections=<n> suffixes, and
reserve file descriptors by summing accepted-connection limits across
all configured IPC listeners.

Reject ambiguous syntax like unix:max-connections=8 so option-looking
input is not silently treated as a literal socket path.

Extend IPC parser coverage and functional startup coverage to check the
new -ipcbind forms, FD reservation logging, and init errors for invalid
max-connections values.

aadc51fb5f

ipc: pass per-address connection limits to ListenConnections

Thread optional per-address max-connections values through the Bitcoin
IPC stack and the vendored libmultiprocess listener implementation.

Update interfaces::Ipc and ipc::Protocol listen methods to accept an
optional connection limit, pass parsed -ipcbind limits from init, and
use the vendored ListenConnections(..., max_connections) support so each
listener stops accepting new IPC connections after reaching its local
cap and resumes accepting after a disconnect.

76f53ca4c7

ipc: test per-address connection limiting over unix sockets

Add a Bitcoin-side IPC test covering the local listener limit behavior
through ipc::Process and ipc::Protocol over a real unix socket.

The new test starts a listener with max_connections=1, verifies the
first client is served immediately, checks that a second client remains
blocked while the first connection stays open, and then confirms the
second client becomes usable after the first disconnects. This keeps the
downstream draft from relying only on vendored libmultiprocess tests and
startup log assertions.

57a0a29303

enirox001 force-pushed on Apr 9, 2026

enirox001 commented at 3:50 pm on April 9, 2026: contributor

Thanks for the review @ryanofsky

I pushed updates based on your review here as well

the bind-address type is now shared
parsing moved behind the interfaces::Ipc boundary
the parser handles trailing socket options
listenAddress() now takes the bind-address struct directly

ipc: support per-address max-connections options on -ipcbind #35037

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