[Issue 22346] New: TaskPool blocked when execute another task with TaskPool

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22346

          Issue ID: 22346
           Summary: TaskPool blocked when execute another task with
                    TaskPool
           Product: D
           Version: D2
          Hardware: x86
                OS: Windows
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: bitworld at qq.com

OS: Debian 4.9.144-3.1 (2019-02-19) x86_64 GNU/Linux
Compiler: DMD64 D Compiler v2.097.2

Here is the test code:

```d
import std.stdio;

import core.thread;
import core.time;

import std.parallelism;
import std.experimental.logger;

void main() {
        testTaskPool();
}

void testTaskPool() {

    // enum Total = 15;  // It's Ok when Total <= 15
    enum Total = 16;  // It's blocked by Thread.sleep(dur) when Total >= 16;

    for(size_t group = 0; group<Total; group++) {

                auto testTask = task(() {
                                tracef("testing...");

                                try {
                                        useTaskPool();  // bug test
                                        // useThread();                 // It's
always ok
                                } catch(Exception ex) {
                                        warning(ex);
                                }

                                Duration dur = 10.seconds;
                                infof("Sleeping %s", dur);
                                Thread.sleep(dur);
                                infof("awake now");
                                tracef("testing done");
                });

                taskPool.put(testTask);
    }

    warning("press any key to close");
    getchar();  
}


void useThread() {
        Thread th = new Thread(&doSomething);
        th.start();
        // th.join();
}

void useTaskPool() {
        auto testTask = task(&doSomething);
        taskPool.put(testTask);
}

```

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