Guard against race conditions

When working with concurrent operations, always implement proper guards to prevent race conditions between processes. This includes:

1. Adding explicit checks before operations that might be affected by concurrent processes:

// Before performing an operation that assumes file existence
if (mdnblocks(reln, forknum) >= blocknum)
{
    // Operation is safe to proceed
    // Comment explaining the race condition being prevented
}

1. Using precise mechanisms to detect resource availability in multi-process environments. For shared memory access, prefer UsedShmemSegAddr over IsUnderPostmaster since some processes may have detached shared memory:

// Correct check for shared memory availability
if (newval && *newval != '\0' && UsedShmemSegAddr && shared_resource_available)

1. Implementing robust interrupt handling for operations that may be suspended. Always recalculate timing after potential interruptions:

// Loop to handle interrupted sleeps
while (elapsed_time < required_delay)
{
    pg_usleep(required_delay - elapsed_time);
    
    // Handle potential interrupts
    CHECK_FOR_INTERRUPTS();
    
    // Recalculate elapsed time with fresh timestamp
    current_time = GetCurrentTimestamp();
    elapsed_time = current_time - start_time;
}

Always add clear comments explaining the race condition being prevented to help future developers understand the reasoning behind these guards.