Do not use Cargo.lock during cargo build.

* Save Cargo.lock before calling cargo, and restore it after the run.
* Use the --use-cargo-lock flag if the latest dependent crates failed
  to build the current crate. In this case, the published Cargo.lock
  will be used, but the generated .bp might not work with the other
  latest dependent crates in external/rust/crates.
* Fix one gpylint warning.

Bug: 172016745
Test: run through all external/rust/crates/*
Change-Id: I1874a706ac15b74c1dd5a606983b107284d9b28a

scripts/cargo2android.py

@@ -1128,7 +1128,7 @@ class Runner(object):
     out_files = set()
     if list1 or list2:
       os.makedirs('out', exist_ok=True)
-    for path in (list1 + list2):
+    for path in list1 + list2:
       file_name = path.split('/')[-1]
       out_files.add(file_name)
       shutil.copy(path, 'out/' + file_name)
@@ -1223,11 +1223,19 @@ class Runner(object):
       return self
     cargo_toml = './Cargo.toml'
     cargo_out = './cargo.out'
+    # Do not use Cargo.lock, because .bp rules are designed to
+    # run with "latest" crates avaialable on Android.
+    cargo_lock = './Cargo.lock'
+    cargo_lock_saved = './cargo.lock.saved'
+    had_cargo_lock = os.path.exists(cargo_lock)
     if not os.access(cargo_toml, os.R_OK):
       print('ERROR: Cannot find or read', cargo_toml)
       return self
-    if not self.dry_run and os.path.exists(cargo_out):
-      os.remove(cargo_out)
+    if not self.dry_run:
+      if os.path.exists(cargo_out):
+        os.remove(cargo_out)
+      if not self.args.use_cargo_lock and had_cargo_lock:  # save it
+        os.rename(cargo_lock, cargo_lock_saved)
     cmd_tail = ' --target-dir ' + TARGET_TMP + ' >> ' + cargo_out + ' 2>&1'
     # set up search PATH for cargo to find the correct rustc
     saved_path = os.environ['PATH']
@@ -1272,6 +1280,11 @@ class Runner(object):
       if self.args.verbose:
         print('### INFO: restored original Cargo.toml')
     os.environ['PATH'] = saved_path
+    if not self.dry_run:
+      if not had_cargo_lock:  # restore to no Cargo.lock state
+        os.remove(cargo_lock)
+      elif not self.args.use_cargo_lock:  # restore saved Cargo.lock
+        os.rename(cargo_lock_saved, cargo_lock)
     return self
 
   def dump_dependencies(self):
@@ -1534,6 +1547,12 @@ def parse_args():
       action='store_true',
       default=False,
       help='run cargo build --tests after normal build')
+  parser.add_argument(
+      '--use-cargo-lock',
+      action='store_true',
+      default=False,
+      help=('run cargo build with existing Cargo.lock ' +
+            '(used when some latest dependent crates failed)'))
   parser.add_argument(
       '--verbose',
       action='store_true',