[SanderMuller]

With Xdebug, pcov or tideways loaded, PHPStan now restarts once, before the console application starts. This is the follow-up that #6757 (comment) describes.

Before this change, a run with Xdebug loaded did this:

1. TurboProcessRestarter exec'd the process for Turbo and OPcache. Xdebug was still loaded.
2. The new process booted up to CommandHelper::begin() and started a child process without Xdebug through xdebug-handler.
3. That child had lost the -d entries, so it exec'd again for Turbo and OPcache.

Now TurboProcessRestarter makes the decision at the top of bin/phpstan, without the autoloader:

• If Xdebug, pcov or tideways has to go, it loads the autoloader and passes its own -d entries to ProcessRestarter. The process is replaced anyway, so the late autoloader does not matter.
• xdebug-handler writes the ini without those extensions. Then one pcntl_exec replaces the process with the Turbo, OPcache and marker entries on the command line. The command-line opcache.enable_cli=1 overrides the opcache.enable_cli=0 that xdebug-handler writes into its ini.
• Nobody waits for the exec, so a small forked process does the job of the waiting parent. It ignores Ctrl-C, waits until the main process ends, and removes the ini.
• On the exec path, ProcessRestarter prints the same "Process restarting" lines as xdebug-handler under XDEBUG_HANDLER_DEBUG. The pcov e2e jobs check for them.
• CommandHelper still calls check(). In the restarted process it reads the restart settings, so the --debug note about the restart still shows. Without pcntl, it is still the fallback that restarts in a child process.

restartIfSuitable() gets two optional parameters, $autoloadPath and $appName. Rector can then do the same restart with one call: restartIfSuitable($argv, $phpstanPhar, null, 'rector'). Rector's own XdebugHandler then sees the restart and does nothing.

I tested the restart paths with a 2.3.x phar (807d030f6) that has these files patched in, on macOS with Homebrew PHP 8.5 and Xdebug 3.5:

• With Xdebug: one exec with the same pid. The final process has Turbo and OPcache on and Xdebug off. This holds for the phar, for bin/phpstan and for a vendor/bin/phpstan proxy.
• --xdebug, PHPSTAN_ALLOW_XDEBUG=1 and xdebug.mode=off keep Xdebug, as before.
• With pcntl_exec or pcntl_fork in disable_functions, the old CommandHelper restart runs, and the worker gets Turbo without Xdebug.
• After Ctrl-C to the process group, SIGINT or SIGKILL to the main process, or a normal exit, the ini is gone. Without the ignored signals in the watchdog, the ini stays after Ctrl-C.

CPU time (user + sys) of a small run with Xdebug loaded, median of 10 interleaved runs: 1.14 s before, 0.69 s after. The saving is in the startup, so I expect about the same absolute saving on a larger run, but I measured only small runs.

I could not test pcov and tideways locally. The pcov e2e jobs cover pcov.

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