Workaround bmalloc hang with RT thread priorities.

To enable the usleep you need to set environmental variable WEBKIT_WPE_BMALLOC_MICROSECONDS_SLEEP (microseconds of sleep).

When real time (RT) thread priorities are used for some of the gstreamer pipeline elements, we may run into a situation
where several RT threads start spinning during a mutex acquisition process, leading to a system hang as most other
threads won't be able to run.

Sequence of events leading up to the hang:
1. A web process thread acquires the mutex lock for the heap and is then involuntary descheduled, and does not run again
2. vqueue:src (RT priority) enters the lockSlowCase and starts spinning in the while loop
3. multiqueue0:src (instance 1, RT priority) enters the lockSlowCase and starts spinning in the while loop
4. aqueue:src (RT priority) enters the lockSlowCase and starts spinning in the while loop
5. multiqueue0:src (instance 2, RT priority) enters the lockSlowCase and starts spinning in the while loop

Once stage 5 is hit, the box is hung as the only thing that can run on a CPU core is:
1. one of the above RT threads (aqueue, vqueue, or multiqueue)
2. any other RT thread with a priority equal or greater than the above RT threads
3. any h/w irq

The use of the usleep() will allow the low priority process to run and release the mutex lock, avoiding the hang

Author of issue analysis and fix proposal: Steven Webster.

* Source/bmalloc/bmalloc/Mutex.cpp:
(bmalloc::yield):

Author: pgorszkowski-igalia

Source/bmalloc/bmalloc/Mutex.cpp

@@ -32,6 +32,7 @@
 #include <mach/thread_switch.h>
 #endif
 #include <thread>
+#include <unistd.h>
 
 namespace bmalloc {
 
@@ -41,7 +42,23 @@ static inline void yield()
     constexpr mach_msg_timeout_t timeoutInMS = 1;
     thread_switch(MACH_PORT_NULL, SWITCH_OPTION_DEPRESS, timeoutInMS);
 #else
-    sched_yield();
+    static size_t bmallocMicrosecondsSleep;
+    static std::once_flag onceFlag;
+    std::call_once(onceFlag, [] {
+        const char* env = getenv("WEBKIT_WPE_BMALLOC_MICROSECONDS_SLEEP");
+        if (env) {
+            int value;
+            if (sscanf(env, "%d", &value) == 1 && value > 0)
+                bmallocMicrosecondsSleep = value;
+        }
+    });
+    if (bmallocMicrosecondsSleep) {
+        // The use of sched_yield() can lead to a system hang when real time
+        // thread priorities are used, so use sleep in the absence of a better
+        // alternative.
+        usleep(bmallocMicrosecondsSleep);
+    } else
+        sched_yield();
 #endif
 }