Fix high memory usage

* Do not load all domain names in a file to memory, use generator instead
* Do not construct async tasks for all domains, slice it by TASK_AMOUNT_MULTIPLIER
    instead

This modification decreases memory usage from ~470 MB down to ~35-40 MB

scripts/resolve-dns-nxdomain.py

@@ -8,6 +8,7 @@ import dns.rdatatype
 import dns.asyncresolver
 import dns.exception
 import dns._asyncio_backend
+import itertools
 
 # DNS timeout (in seconds) for the initial DNS resolving pass
 INITIAL_PASS_TIMEOUT = 3
@@ -19,6 +20,9 @@ FINAL_PASS_TIMEOUT = 10
 # Number of concurrent resolving 'threads' for final pass
 FINAL_PASS_CONCURRENCY = 35
 
+# Multiplication number of asynchronous tasks to build for a single
+# resolver loop
+TASK_AMOUNT_MULTIPLIER = 5
 
 NS_FILTER_SUBSTRINGS = ("parking", "expired", ".afternic.com", "parklogic", ".parktons.com",
                         ".above.com", ".ztomy.com", ".notneiron.com", ".ibspark.com", ".bodis.com",
@@ -49,17 +53,25 @@ class AZResolver(dns.asyncresolver.Resolver):
                 # Do not thread domain as broken if the answer is empty
                 pass
 
-async def runTasksWithProgress(tasks):
+def tasksProvider(domainiter, resolver):
+    for domain in domainiter:
+        yield asyncio.ensure_future(resolver.nxresolve(domain.strip()))
+
+async def runTasksWithProgress(tasks, tasknumber, concurrent_tasks):
     progress = 0
     old_progress = 0
     ret = []
 
-    for task in asyncio.as_completed(tasks):
+    tasklist = list(itertools.islice(tasks, concurrent_tasks))
+    while tasklist:
+        for task in asyncio.as_completed(tasklist):
             ret.append(await task)
-        progress = int(len(ret) / len(tasks) * 100)
+            progress = int(len(ret) / tasknumber * 100)
             if old_progress < progress:
                 print("{}%...".format(progress), end='\r', file=sys.stderr, flush=True)
                 old_progress = progress
+        tasklist = list(itertools.islice(tasks, concurrent_tasks))
+
     print(file=sys.stderr)
     return ret
 
@@ -74,44 +86,50 @@ async def main():
     r.lifetime = INITIAL_PASS_TIMEOUT
 
     # Load domain file list and schedule resolving
-    tasks = []
+    domaincount = 0
     try:
-        with open(sys.argv[1], 'rb') as domainlist:
+        with open(sys.argv[1], 'r') as domainlist:
             for domain in domainlist:
-                tasks.append(asyncio.ensure_future(r.nxresolve(domain.decode().strip())))
+                domaincount += 1
     except OSError as e:
         print("Can't open file", sys.argv[1], e, file=sys.stderr)
         sys.exit(2)
 
-    print("Loaded list of {} elements, resolving NXDOMAINS".format(len(tasks)), file=sys.stderr)
+    print("Loaded list of {} elements, resolving NXDOMAINS".format(domaincount), file=sys.stderr)
     #sys.exit(0)
 
     try:
+        domainfile = open(sys.argv[1], 'r')
+
         # Resolve domains, first try
-        nxresolved_first = await runTasksWithProgress(tasks)
+        nxresolved_first = await runTasksWithProgress(
+            tasksProvider(domainfile, r), domaincount,
+            INITIAL_PASS_CONCURRENCY * TASK_AMOUNT_MULTIPLIER
+        )
         nxresolved_first = list(filter(None, nxresolved_first))
 
         print("Got {} broken domains, trying to resolve them again "
               "to make sure".format(len(nxresolved_first)), file=sys.stderr)
 
         # Second try
-        tasks = []
         r.limitConcurrency(FINAL_PASS_CONCURRENCY)
         r.timeout = FINAL_PASS_TIMEOUT
         r.lifetime = FINAL_PASS_TIMEOUT
 
-        for domain in nxresolved_first:
-            tasks.append(asyncio.ensure_future(r.nxresolve(domain)))
-        nxresolved_second = await runTasksWithProgress(tasks)
+        nxresolved_second = await runTasksWithProgress(
+            tasksProvider(nxresolved_first, r), len(nxresolved_first),
+            FINAL_PASS_CONCURRENCY * TASK_AMOUNT_MULTIPLIER
+        )
         nxresolved_second = list(filter(None, nxresolved_second))
 
         print("Finally, got {} broken domains".format(len(nxresolved_second)), file=sys.stderr)
         for domain in nxresolved_second:
             print(domain)
 
+        domainfile.close()
+
     except (SystemExit, KeyboardInterrupt):
-        for task in tasks:
-            task.cancel()
+        print("Exiting")
 
 
 if __name__ == '__main__':