[2.x] fix: Print server stderr on startup failure#8816
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eed3si9n merged 1 commit intosbt:developfrom Feb 26, 2026
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**Problem** When the sbt server fails to start (e.g. wrong JDK version), the client only shows "failed to connect to server" hiding the actual error. The server stderr is redirected to /dev/null to prevent Linux pipe buffer deadlocks (sbt#8442), so diagnostic output is lost. **Solution** Redirect server stderr to a temp file instead of /dev/null. When the server fails to start (portfile never appears), read and print the temp file contents before throwing ServerFailedException. The temp file is cleaned up eagerly on both success and failure paths. Fixes sbt#8812
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**Problem** When the sbt server fails to start (e.g. wrong JDK version), the client only shows "failed to connect to server" hiding the actual error. The server stderr is redirected to /dev/null to prevent Linux pipe buffer deadlocks (sbt#8442), so diagnostic output is lost. **Solution** Redirect server stderr to a temp file instead of /dev/null. When the server fails to start (portfile never appears), read and print the temp file contents before throwing ServerFailedException. The temp file is cleaned up eagerly on both success and failure paths. Fixes sbt#8812
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**Problem** When the sbt server fails to start (e.g. wrong JDK version), the client only shows "failed to connect to server" hiding the actual error. The server stderr is redirected to /dev/null to prevent Linux pipe buffer deadlocks (#8442), so diagnostic output is lost. **Solution** Redirect server stderr to a temp file instead of /dev/null. When the server fails to start (portfile never appears), read and print the temp file contents before throwing ServerFailedException. The temp file is cleaned up eagerly on both success and failure paths. Fixes #8812 Co-authored-by: Dream <[email protected]>
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Fixes #8812
Summary
When the sbt server fails to start, print the actual error instead of only showing "failed to connect to server".
Problem
When the server process fails during auto-start (e.g. wrong JDK version producing
UnsupportedClassVersionError), the client only prints:This hides the real cause. The
--serverflag shows the error because it usesinheritIO(), but the auto-start path redirects stderr to/dev/null(added in #8442 to prevent Linux pipe buffer deadlocks).Solution
Redirect server stderr to a temp file instead of
/dev/null. If the server fails to start (portfile never appears), read and print the temp file contents before throwingServerFailedException. This preserves the #8442 fix (writing to a file avoids pipe buffer deadlocks) while capturing startup errors for diagnostics.The temp file is cleaned up eagerly on both success and failure paths, with
deleteOnExit()as a safety net.Changes
main-command/.../NetworkClient.scala/dev/nullServerFailedException