While doing some regression testing, I noticed that the systemtap.base/flightrec4.exp test case is failing intermittently. The test is supposed to start up stap in the background, writing to a file called flightrec4.out.0. The test case then sends 3 USR2 signals to stapio, to make stapio switch output to flightrec4.out.1, flightrec4.out.2, flightrec4.out.3. When the test fails, the output gets sent instead to: flightrec4.out.0, flightrec4.out.1, flightrec4.out.NUM, flightrec4.out.NUM+1, where NUM is a large number like 54681. Sometimes stapio seems to exit before creating the last output file. At first I thought the problem might be with the test case itself, but after updating it I've found that isn't the case. The failures happen quite randomly, I get about 1 failure per 10 test case runs.
Fixed in commit dc6bf1e. Sometimes the thread handling the signals would also be the reader thread. So, the code ended up sending a signal to itself.