Refactor Ports.fetch_port to avoid locking the cache where possible

This is in service of improving support for FROZEN_CACHE and is needed
as part of #13316.

The logic here mirrors the lock in cache.get() where we first attempt to
verify the contents of the cache without holding the lock and early out
in the common case the what we want is already here.

Author: sbc100

tools/system_libs.py

@@ -1696,10 +1696,6 @@ def fetch_project(name, url, subdir, is_tarbz2=False, sha512hash=None):
       logger.debug('    (at ' + fullname + ')')
       Ports.name_cache.add(name)
 
-    class State(object):
-      retrieved = False
-      unpacked = False
-
     def retrieve():
       # retrieve from remote server
       logger.info('retrieving port: ' + name + ' from ' + url)
@@ -1724,7 +1720,6 @@ def retrieve():
           shared.exit_with_error('Unexpected hash: ' + actual_hash + '\n'
                                  'If you are updating the port, please update the hash in the port module.')
       open(fullpath, 'wb').write(data)
-      State.retrieved = True
 
     def check_tag():
       if is_tarbz2:
@@ -1755,28 +1750,29 @@ def unpack():
       with utils.chdir(fullname):
         z.extractall()
 
-      State.unpacked = True
+    # before acquiring the lock we have an early out if the port already exists
+    if os.path.exists(fullpath) and check_tag():
+      return
 
     # main logic. do this under a cache lock, since we don't want multiple jobs to
     # retrieve the same port at once
-
     with shared.Cache.lock():
-      if not os.path.exists(fullpath):
-        retrieve()
-
-      if not os.path.exists(fullname):
-        unpack()
-
-      if not check_tag():
-        logger.warning('local copy of port is not correct, retrieving from remote server')
-        shared.try_delete(fullname)
-        shared.try_delete(fullpath)
-        retrieve()
-        unpack()
-
-      if State.unpacked:
-        # we unpacked a new version, clear the build in the cache
-        Ports.clear_project_build(name)
+      # Another early out in case another process build the library while we were
+      # waiting for the lock
+      if os.path.exists(fullpath):
+        if check_tag():
+          return
+        else:
+          # file exists but tag is bad
+          logger.warning('local copy of port is not correct, retrieving from remote server')
+          shared.try_delete(fullname)
+          shared.try_delete(fullpath)
+
+      retrieve()
+      unpack()
+
+      # we unpacked a new version, clear the build in the cache
+      Ports.clear_project_build(name)
 
   @staticmethod
   def clear_project_build(name):