PostgreSQL数据库PMsignal——后端进程Postmaster信号通信

src/backend/storage/ipc/pmsignal.c

Postmaster拥有BackendList后端列表，用于存放标识postgres子进程的Backend。AssignPostmasterChildSlot函数为新的postmaster子进程选择未使用的slot，并将其状态设置为ASSIGNED，返回slot number。如果PMSignalState->next_child_flag为0，进入for循环从PMSignalState->num_child_flags数组尾部遍历，–slot为-1小于0，slot更新为PMSignalState->num_child_flags数组元素数量-1。如果该slot未使用则使用PM_CHILD_ASSIGNED，将PMSignalState->next_child_flag赋值为slot，并返回slot+1。
如果PMSignalState->next_child_flag不为0，进入for循环从PMSignalState->num_child_flags数组尾部遍历，–slot不小于0。如果该slot未使用则使用PM_CHILD_ASSIGNED，将PMSignalState->next_child_flag赋值为slot，并返回slot+1。

SendPostmasterSignal函数从子进程向postmaster发送信号，通过kill函数向postmaster发送SIGUSR1信号。支持如下PMSignalReason原由通知postmaster。

void SendPostmasterSignal(PMSignalReason reason) {
	
	if (!IsUnderPostmaster) return;
	
	PMSignalState->PMSignalFlags[reason] = true;
	
	kill(PostmasterPid, SIGUSR1);
}

typedef enum {
	PMSIGNAL_RECOVERY_STARTED,	
	PMSIGNAL_BEGIN_HOT_STANDBY, 
	PMSIGNAL_WAKEN_ARCHIVER,	
	PMSIGNAL_ROTATE_LOGFILE,	
	PMSIGNAL_START_AUTOVAC_LAUNCHER,	
	PMSIGNAL_START_AUTOVAC_WORKER,	
	PMSIGNAL_BACKGROUND_WORKER_CHANGE,	
	PMSIGNAL_START_WALRECEIVER, 
	PMSIGNAL_ADVANCE_STATE_MACHINE, 
	NUM_PMSIGNALS				
} PMSignalReason;

在实现中，后台进程是这样通知Postmaster的：

1. 首先在共享内存中开辟一个数组PMSignalFlags，数组中的每一位对应于一个信号。
2. 然后如果后台进程希望向Postmaster发送一个信号，那么后台首先将信号在数组PMSignalFlags中相应的元素置1（逻辑真），然后调用kill函数向postmaster发送SIGUSR1信号。
3. 当Postmaster收到SIGUSR1信号后首先检测共享存储中PMSingnalFlags，确认具体的信号是什么。同时将信号在数组PMSignalFlags中相应的元素置0（逻辑假）然后作出相应反应。

src/backend/storage/ipc/procsignal.c
通过传入MyBackendId获取ProcSignalSlots数组中的对应的元素槽，将信号槽中的pss_pid设置为MyProcPid，最后将该槽的指针赋值给MyProcSignalSlot。也就是一个后端一个slot，和门铃一样，其他进程可以敲击这个门铃slot来通知信号。

typedef struct {
	pid_t		pss_pid;
	sig_atomic_t pss_signalFlags[NUM_PROCSIGNALS];
} ProcSignalSlot;

static ProcSignalSlot *ProcSignalSlots = NULL;
void ProcSignalInit(int pss_idx) {
	volatile ProcSignalSlot *slot;
	Assert(pss_idx >= 1 && pss_idx <= NumProcSignalSlots);
	slot = &ProcSignalSlots[pss_idx - 1];
	
	if (slot->pss_pid != 0)
		elog(LOG, "process %d taking over ProcSignal slot %d, but it's not empty",MyProcPid, pss_idx);
	
	MemSet(slot->pss_signalFlags, 0, NUM_PROCSIGNALS * sizeof(sig_atomic_t));
	
	slot->pss_pid = MyProcPid;
	
	MyProcSignalSlot = slot;
	
	on_shmem_exit(CleanupProcSignalState, Int32GetDatum(pss_idx));
}

在共享内存中创建ProcSignalSlot数组

Size ProcSignalShmemSize(void) {
	return NumProcSignalSlots * sizeof(ProcSignalSlot);
}

void ProcSignalShmemInit(void) {
	Size		size = ProcSignalShmemSize();
	bool		found;
	ProcSignalSlots = (ProcSignalSlot *)ShmemInitStruct("ProcSignalSlots", size, &found);
	
	if (!found) MemSet(ProcSignalSlots, 0, size);
}

SendProcSignal向一个Postgres进程发送信号，如果信号发送出去了，即成功，返回0；错误返回-1并设置errno.

int SendProcSignal(pid_t pid, ProcSignalReason reason, BackendId backendId){
	volatile ProcSignalSlot *slot;
	if (backendId != InvalidBackendId){
		slot = &ProcSignalSlots[backendId - 1];
		
		if (slot->pss_pid == pid){
			
			slot->pss_signalFlags[reason] = true;
			
			return kill(pid, SIGUSR1);
		}
	}else{
		
		int			i;
		for (i = NumProcSignalSlots - 1; i >= 0; i--){
			slot = &ProcSignalSlots[i];
			if (slot->pss_pid == pid){
				
				
				slot->pss_signalFlags[reason] = true;
				
				return kill(pid, SIGUSR1);
			}
		}
	}
	errno = ESRCH;
	return -1;
}

信号处理函数

void
procsignal_sigusr1_handler(SIGNAL_ARGS)
{
	int			save_errno = errno;

	if (CheckProcSignal(PROCSIG_CATCHUP_INTERRUPT))
		HandleCatchupInterrupt();

	if (CheckProcSignal(PROCSIG_NOTIFY_INTERRUPT))
		HandleNotifyInterrupt();

	if (CheckProcSignal(PROCSIG_PARALLEL_MESSAGE))
		HandleParallelMessageInterrupt();

	if (CheckProcSignal(PROCSIG_WALSND_INIT_STOPPING))
		HandleWalSndInitStopping();

	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_DATAbase))
		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_DATAbase);

	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_TABLESPACE))
		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_TABLESPACE);

	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_LOCK))
		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_LOCK);

	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_SNAPSHOT))
		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_SNAPSHOT);

	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_STARTUP_DEADLOCK))
		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_STARTUP_DEADLOCK);

	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN))
		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN);

	SetLatch(MyLatch);

	latch_sigusr1_handler();

	errno = save_errno;
}