Add diskUnderWarnThreshold check event to prevent gcThread suspended forever

[gaozhangmin]

Motivation

This code snippet implements disk check event trigger if disk usage is above warnThreshold, either marjor is suspend or forcegc is enabled.

There exists a case that if usgae descends to warn threshold, the gc thread is suspend forever. until it goes to full and becomes writable again.

@Override
            public void diskAlmostFull(File disk) {
                if (gcThread.isForceGCAllowWhenNoSpace) {
                    gcThread.enableForceGC();
                } else {
                    gcThread.suspendMajorGC();
                }
            }

Changes

Add diskUnderWarnThreshold check event to prevent gcThread supended forever

[hangc0276]

Would you please add a test to protect the case you described?

[gaozhangmin]

Would you please add a test to protect the case you described?

@gaozhangmin @hangc0276 done, PTAL again

[gaozhangmin]

rerun failure checks