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test: rewrite sysprof test using managed execution Often, tests for sampling profiler require long running loops to be executed, so a certain situation is likely to occur. Thus, the test added in commit 285a1b0a1 ("sysprof: fix crash during FFUNC stream") expects the FFUNC VM state (and even the particular instruction to be executed) at the moment when stacktrace is being collected. Unfortunately, it leads to the test routine hang for several environments, and even if it does not, we cannot guarantee that the desired scenario is tested (only rely on statistics). As a result, the test for the aforementioned patch was disabled for Tarantool CI in the commit fef60a105 ("test: prevent hanging Tarantool CI by sysprof test") until the issue is not resolved. This patch introduces the new approach for testing our sampling profiler via <ptrace>, implementing precise managed execution of the profiled instance mentioned in tarantool/tarantool#9387. Instead of running around <tostring> gazillion times, we accurately step to the exact place where the issue reproduces and manually emit SIGPROF to the Lua VM being profiled. The particular approach implemented in this patch is described below. As it was mentioned, the test makes sysprof collect the particular event type (FFUNC) at the certain instruction in Lua VM (<lj_fff_res1>) to reproduce the issue from tarantool/tarantool#8594. Hence, it's enough to call <tostring> fast function in the profiled instance (i.e., "tracee"). To emit SIGPROF right at <lj_fff_res1> in the scope of <tostring> builtin, the manager (i.e., "tracer") is implemented. Here are the main steps (see comments and `man 2 ptrace' for more info): 1. Poison <int 3> instruction as the first instruction at <lj_ff_tostring> to stop at the beginning of the fast function; 2. Resume the "tracee" from the "tracer"; 3. Hit the emitted interruption, restore the original instruction... (continued)
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