ProgressTracker: reduce memory usage with long-running work items

ProgressTracker was only freeing memory for work items when they reach
the head of the work tracking queue.  If the first work item takes
hours to complete, and thousands of items are processed every second,
this leads to millions of completed items tracked in memory at a time,
wasting gigabytes of system RAM.

Rewrite ProgressHolderState methods to keep only incomplete work items
in memory, regardless of the order in which they are added or removed.

Also fix the unit tests which were relying on the memory leak to work,
and add test cases for code coverage.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

test/progress.cc

@@ -12,23 +12,49 @@ using namespace std;
 void
 test_progress()
 {
+	// On create, begin == end == constructor argument
 	ProgressTracker<uint64_t> pt(123);
-	auto hold = pt.hold(234);
-	auto hold2 = pt.hold(345);
 	assert(pt.begin() == 123);
-	assert(pt.end() == 345);
-	auto hold3 = pt.hold(456);
-	assert(pt.begin() == 123);
-	assert(pt.end() == 456);
-	hold2.reset();
-	assert(pt.begin() == 123);
-	assert(pt.end() == 456);
-	hold.reset();
+	assert(pt.end() == 123);
+
+	// Holding a position past the end increases the end (and moves begin to match)
+	auto hold345 = pt.hold(345);
 	assert(pt.begin() == 345);
+	assert(pt.end() == 345);
+
+	// Holding a position before begin reduces begin, without changing end
+	auto hold234 = pt.hold(234);
+	assert(pt.begin() == 234);
+	assert(pt.end() == 345);
+
+	// Holding a position past the end increases the end, without affecting begin
+	auto hold456 = pt.hold(456);
+	assert(pt.begin() == 234);
 	assert(pt.end() == 456);
-	hold3.reset();
+
+	// Releasing a position in the middle affects neither begin nor end
+	hold345.reset();
+	assert(pt.begin() == 234);
+	assert(pt.end() == 456);
+
+	// Hold another position in the middle to test begin moving forward
+	auto hold400 = pt.hold(400);
+
+	// Releasing a position at the beginning moves begin forward
+	hold234.reset();
+	assert(pt.begin() == 400);
+	assert(pt.end() == 456);
+
+	// Releasing a position at the end doesn't move end backward
+	hold456.reset();
+	assert(pt.begin() == 400);
+	assert(pt.end() == 456);
+
+	// Releasing a position in the middle doesn't move end backward but does move begin forward
+	hold400.reset();
 	assert(pt.begin() == 456);
 	assert(pt.end() == 456);
+
 }
 
 int