fix: print "Listening on" only after the socket is bound (#4400)#4413
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Emit the "Listening on ..." startup message after startServer()/ startPipeServer() returns rather than before, so the announced URL is guaranteed to be accepting connections. httpuv binds synchronously, so by the time the handle is returned the port is live. Previously the message could be printed one statement before the bind, leaving a window (widened under load) where anything scraping the line as a readiness signal (shinytest2, log watchers) could reach a not-yet-bound port.
schloerke
marked this pull request as ready for review
July 15, 2026 18:26
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Fixes #4400
Summary
The
Listening on http://…startup message was emitted one statement before thestartServer()call that binds the listening socket. Because httpuv binds synchronously, moving the message to afterstartServer()/startPipeServer()returns guarantees the announced URL is already accepting connections. The domain-socket branch also validates themaskattribute before binding so an invalid socket errors without printing a misleading line. This matters because theListening online is used as a readiness signal (shinytest2, process supervisors, log watchers); under load the freshly-forked process could be descheduled between the two statements, leaving a window where the advertised port refused connections.Verification
Unit tests in
tests/testthat/test-server.Rmock the synchronous bind and assert the message is emitted only after it returns (and not at all underquiet = TRUE). Reverting the reorder makes the ordering test fail; restoring it passes. Manual reproduction using the issue reporter's tracer now shows the port accepting connections by the time the line prints.