qa: Reduce `-maxconnections` in the functional test framework #36078

hebasto commented at 1:19 PM on August 25, 2026: member

This PR follows up on bitcoin/bitcoin#35730 and fixes a regression on NetBSD.

Since bitcoin/bitcoin#35730 the HTTP server reserves file descriptors for its listen sockets and for -rpcmaxconnections connected clients (16 by default), so min_required_fds in init.cpp grew.

On select()-based platforms available_fds is capped at FD_SETSIZE, which is 256 on NetBSD. The previous value of 94 no longer fits and every node in the test suite started up with a warning, which the framework treats as unexpected stderr and fails on.

Recompute the value with the new accounting (256 - 179 = 77) and update the comment to match the current variable names in init.cpp.

hebasto added the label Tests on Aug 25, 2026

DrahtBot commented at 1:19 PM on August 25, 2026: contributor

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36078.

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ACK	hodlinator, winterrdog, achow101
Concept ACK	pinheadmz

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hebasto commented at 1:21 PM on August 25, 2026: member

See the CI run on NetBSD here: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/32852803381.

hebasto added this to the milestone 32.0 on Aug 25, 2026

hodlinator approved

hodlinator commented at 1:28 PM on August 25, 2026: contributor

utcrACK 4669defc0f3495060938818f94d2669fec5b1a68

in test/functional/test_framework/util.py:571 in 4669defc0f outdated

 574 | -        #  nMaxConnections = available_fds - min_required_fds = 256 - 161 = 94;
 575 | -        f.write("maxconnections=94\n")
 576 | +        #  min_required_fds = MIN_CORE_FDS + MAX_ADDNODE_CONNECTIONS + num_p2p_bind + num_rpc_bind + DEFAULT_MAX_HTTP_CONNECTIONS =
 577 | +        #    = 151 + 8 + 2 + 2 + 16 = 179;
 578 | +        #  num_p2p_max_connections = available_fds - min_required_fds = 256 - 179 = 77;
 579 | +        f.write("maxconnections=77\n")

winterrdog commented at 2:04 PM on August 25, 2026:

nit / just curious, since this value has had to change a few times (across years) would it make sense to encode the maxconnections's calculation in the code? something along the lines of deriving it from the FD budget constants & using the resulting value, rather than hardcoding a value like 77

just wondering if there is a reason to prefer the static value

hebasto commented at 3:54 PM on August 25, 2026:

nit / just curious, since this value has had to change a few times (across years) would it make sense to encode the maxconnections's calculation in the code?

Wouldn't this require the test framework to duplicate the constants from the C++ code? Also, some of the inputs, such as "maximum number of bound P2P interfaces", depend on how particular tests are written rather than on anything bitcoind exposes.

winterrdog commented at 2:17 PM on August 25, 2026: contributor

crACK 4669defc0f3495060938818f94d2669fec5b1a68

hebasto commented at 2:41 PM on August 25, 2026: member

See the CI run on NetBSD here: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/32852803381.

Unfortunately, some test fail.

interface_http.py:

test_framework.test_node.FailedToStartError: [node 0] bitcoind exited with status 1 during initialization. Error: Not enough file descriptors available. 256 available, 290 required.

p2p_getaddr_caching.py:

AssertionError: Unexpected stderr Warning: Reducing -maxconnections from 77 to 76, because of system limitations. !=

janb84 commented at 2:43 PM on August 25, 2026: contributor

Code looks good but the CI still fails :(

hebasto commented at 3:00 PM on August 25, 2026: member

p2p_getaddr_caching.py:

AssertionError: Unexpected stderr Warning: Reducing -maxconnections from 77 to 76, because of system limitations. !=

This test uses 2 extra -bind options, so num_p2p_bind should be 3.

hebasto force-pushed on Aug 25, 2026

pinheadmz commented at 3:43 PM on August 25, 2026: member

concept ACK, setting up a BSD VM to test locally...

hebasto commented at 3:44 PM on August 25, 2026: member

Reworked.

The CI on NetBSD is available here: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/32867589050.

qa: Reduce `-maxconnections` in the functional test framework

Since bitcoin/bitcoin#35730 the HTTP server reserves file descriptors
for its listen sockets and for `-rpcmaxconnections` connected clients
(16 by default), so `min_required_fds` in init.cpp grew.

On select()-based platforms `available_fds` is capped at FD_SETSIZE,
which is 256 on NetBSD. The previous value of 94 no longer fits and
every node in the test suite started up with a warning, which the
framework treats as unexpected stderr and fails on.

Recompute the value with the new accounting (256 - 179 = 77) and
update the comment to match the current variable names in init.cpp.

6f4109b448

qa: Lower `-rpcmaxconnections` in `interface_http.py` test

On some systems, such as NetBSD, the non-default
`-rpcmaxconnections=128` is too high, so bitcoind refuses to start:
```
Error: Not enough file descriptors available. 256 available, 290 required.
```

The test only needs a value above the default of 16. Use 64 and lower
`-maxconnections` in that case so the total fits in 256.

b8a8893bf2

in test/functional/test_framework/util.py:571 in 6c7358f102

 574 | -        #  nMaxConnections = available_fds - min_required_fds = 256 - 161 = 94;
 575 | -        f.write("maxconnections=94\n")
 576 | +        #  min_required_fds = MIN_CORE_FDS + MAX_ADDNODE_CONNECTIONS + num_p2p_bind + num_rpc_bind + DEFAULT_MAX_HTTP_CONNECTIONS =
 577 | +        #    = 151 + 8 + 3 + 2 + 16 = 180;
 578 | +        #  num_p2p_max_connections = available_fds - min_required_fds = 256 - 180 = 76;
 579 | +        f.write("maxconnections=77\n")

hodlinator commented at 3:46 PM on August 25, 2026:

76 != 77 ?

        #  num_p2p_max_connections = available_fds - min_required_fds = 256 - 180 = 76;
        f.write("maxconnections=76\n")

hebasto commented at 3:55 PM on August 25, 2026:

Thanks! Fixed.

hebasto force-pushed on Aug 25, 2026

DrahtBot added the label CI failed on Aug 25, 2026

hebasto commented at 3:56 PM on August 25, 2026: member

Fixed a typo.

CI: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/32868897305.

hodlinator approved

hodlinator commented at 5:17 PM on August 25, 2026: contributor

re-ACK b8a8893bf2a101e24f63902097c59ec0baddabb5

DrahtBot requested review from winterrdog on Aug 25, 2026

DrahtBot requested review from pinheadmz on Aug 25, 2026

winterrdog commented at 5:18 PM on August 25, 2026: contributor

re-ACK b8a8893bf2a101e24f63902097c59ec0baddabb5

DrahtBot removed the label CI failed on Aug 25, 2026

achow101 commented at 6:25 PM on August 25, 2026: member

ACK b8a8893bf2a101e24f63902097c59ec0baddabb5

achow101 merged this on Aug 25, 2026

achow101 closed this on Aug 25, 2026

hebasto deleted the branch on Aug 25, 2026

pinheadmz commented at 7:50 PM on August 25, 2026: member

post-merge ACK b8a8893bf2a101e24f63902097c59ec0baddabb5

Confirmed the regression locally in NetBSD VM (macos arm64 qemu) and verified the fix.