fpm: calculate accurate process counts from states during status generation

[sylvesterdamgaard]

Fix PHP-FPM process count discrepancy in status endpoint

Problem

The PHP-FPM status endpoint reports inaccurate active processes and idle processes counts that don't match the actual process states. This causes significant issues for:

• Monitoring tools receiving incorrect metrics
• Autoscaling systems making poor decisions based on false data
• Load balancers routing incorrectly due to inaccurate capacity information
• Operations teams unable to trust FPM status for diagnostics

Example Discrepancy

Replace unreliable scoreboard counters with counts calculated from actual process states. Since status endpoint already locks and reads all processes, this adds zero performance overhead while ensuring 100% accurate metrics.

Resolves monitoring discrepancies where active processes were incorrectly reported as significantly lower than actual non-idle processes under load.

Root Cause Analysis

The issue stems from race conditions in scoreboard counter updates, not the process states themselves:

1. Scoreboard counters are derived data - updated via fpm_scoreboard_update() from multiple locations
2. Process states are source of truth - maintained individually per process under locks
3. Race conditions occur when concurrent processes update shared counters during state transitions
4. Timing windows exist where counters become inconsistent with actual states

Investigation of fpm_scoreboard.c shows that scoreboard counters are not calculated from process states but rather maintained as separate values that can drift due to concurrent updates.

Solution

Calculate active processes and idle processes directly from actual process states during status generation, instead of relying on potentially inconsistent scoreboard counters.

Implementation Details

The fix leverages the existing process acquisition loop in fpm_status_export_to_zval():

int calculated_active = 0, calculated_idle = 0;

for(i=0; i<scoreboard.nprocs; i++) {
    proc_p = fpm_scoreboard_proc_acquire(scoreboard_p, i, 1);
    if (!proc_p) continue;
    
    // Count states while process is locked
    if (proc_p->used) {
        if (proc_p->request_stage == FPM_REQUEST_ACCEPTING) {
            calculated_idle++;
        } else {
            calculated_active++;
        }
    }
    
    procs[i] = *proc_p;
    fpm_scoreboard_proc_release(proc_p);
}

// Use calculated values instead of scoreboard counters
add_assoc_long(status, "idle-processes", calculated_idle);
add_assoc_long(status, "active-processes", calculated_active);
add_assoc_long(status, "total-processes", calculated_idle + calculated_active);

Fixes #18956

[nielsdos]

Please retarget this to branch PHP-8.3. 8.3 is the lowest supported bugfix branch.

[bukka]

This is incomplete solution as it impacts only fpm_status_export_to_zval so not impacting that actual status page where we don't need to have all procs (minimal default status) so we need to address the problem elsewhere (the increment logic should change a bit).

[bukka]

Also there needs to be a test.