Fix preparer output routing to preserve stdout/stderr distinction#144356
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mark-vieira merged 3 commits intoelastic:mainfrom Mar 17, 2026
Merged
Fix preparer output routing to preserve stdout/stderr distinction#144356mark-vieira merged 3 commits intoelastic:mainfrom
mark-vieira merged 3 commits intoelastic:mainfrom
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The ServerLauncher routes all preparer text output to stderr, breaking tools that expect --version/--help on stdout. Tag stderr-destined lines at the stream level so the launcher can dispatch them correctly. Untagged output defaults to stdout. Made-with: Cursor
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Replace line-based stderr tagging with a byte-level mode-switching protocol so that stdout/stderr routing is preserved even for partial-line output (e.g. print() without a trailing newline). Made-with: Cursor
rjernst
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Mar 16, 2026
| stdout.flush(); | ||
| stderr.flush(); | ||
| } catch (IOException e) { | ||
| // stream closed, nothing to do |
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can we avoid swallowing ioexceptions? here and below
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Good catch. We now rethrow UncheckedIOException.
| * | ||
| * <p> The preparer uses an {@code OutputStreamMux} (in the cli-launcher module) to | ||
| * multiplex its stdout and stderr onto a single pipe. Mode byte {@link #STDOUT_MODE} | ||
| * ({@code 0x01}) switches to stdout; {@link #STDERR_MODE} ({@code 0x02}) switches to |
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so this assumes that 0x01 and 0x02 never appear in the actual streams, right? can we guarantee that? (I think we can, unicode shouldn't have any dangling high bit 0 bytes and these are unprintable codes, but just want to be sure).
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Yeah. Assuming we aren't writing any non-unicode to the terminal here (which we shouldn't be).
Made-with: Cursor
michalborek
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Mar 23, 2026
…astic#144356) * Fix preparer output routing to preserve stdout/stderr The ServerLauncher routes all preparer text output to stderr, breaking tools that expect --version/--help on stdout. Tag stderr-destined lines at the stream level so the launcher can dispatch them correctly. Untagged output defaults to stdout.
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Summary
The recent ServerLauncher changes (#143712) route all preparer (server-cli) text output to stderr, breaking downstream tools that expect output like
--versionand--helpon stdout.This PR fixes the issue by multiplexing stdout and stderr onto a single pipe using byte-level mode switching, then demultiplexing in the launcher:
OutputStreamMuxin the preparer that replaces bothSystem.outandSystem.errwith mux channels writing to a single pipe. Mode byte0x01marks stdout,0x02marks stderr. Mode markers are only emitted on transitions, making same-channel writes zero-overhead.PreparerOutputPumpthat reads raw bytes, tracks the current mode, and routes data to the correct stream.This byte-level approach (as opposed to the earlier line-based tagging) correctly preserves stdout/stderr distinction even for partial-line output like
print()without a trailing newline (e.g. password prompts).Test plan
OutputStreamMux(mode emission, lazy switching, interleaved partial lines, UTF-8)PreparerOutputPump(mode switching, default mode, marker stripping)CliToolLauncherTestsverifying mode markers on stdout/stderr outputPackagingTestCase.assertElasticsearchFailureworkaround (no longer needed)