[cost] Add model: NSFileManager.contentsOfDirectoryAtURL

Summary:
This diff adds a model for NSFileManager.contentsOfDirectoryAtURL as returning a constant-length
collection.

The analyzer cannot know files in a directory.  We have some options to handle such unknown data.

1. Use `Unknown` value, ie `top`
2. Use a symbolic value
3. Use a constant value

We had been used the first option.  An upside of this is that the analyzer can remain as sound.
However, a downside of this is the top value can be propagated to other procedures, making their
costs top, thus we may miss some cost changes of them.

The second option is to introduce a symbolic value, ie. that for the number of files.  A problem is
that the symbolic value will never be concretized.  As a result, the symbol can be propagated to
other procedures, increasing the coefficient of the complexity or making top costs.  Note that handling multiple
symbols is somewhat limited in Inferbo's interval domain.

The last option is to introduce a constant value.  I think this is the best approach we can take among above.
Even though we may have FNs when there are a lot of files in a directory, we cannot reason or expect about
that at the analysis time anyway.

Reviewed By: ezgicicek

Differential Revision: D24418099

fbshipit-source-id: bf8cf3538

infer/src/bufferoverrun/bufferOverrunModels.ml

@@ -1713,6 +1713,9 @@ module Call = struct
         $--> NSCollection.new_collection_by_add_all
       ; +PatternMatch.ObjectiveC.implements "NSEnumerator"
         &:: "nextObject" <>$ capt_exp $--> NSCollection.next_object
+      ; +PatternMatch.ObjectiveC.implements "NSFileManager"
+        &:: "contentsOfDirectoryAtURL:includingPropertiesForKeys:options:error:"
+        &--> NSCollection.new_collection
       ; +PatternMatch.ObjectiveC.implements "NSKeyedUnarchiver"
         &:: "decodeObjectForKey:" $ capt_var_exn $+...$--> NSCollection.get_any_index
       ; +PatternMatch.ObjectiveC.implements "NSMutableArray"

infer/tests/codetoanalyze/objc/performance/NSFileManager.m

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+#import <Foundation/Foundation.h>
+
+/* We model the file lookup as returning a constant-length collection
+ * (T77961166). */
+void contents_of_directory_at_url_constant(NSFileManager* file_manager,
+                                           NSURL* url,
+                                           NSArray* keys) {
+  NSArray* array = [file_manager contentsOfDirectoryAtURL:url
+                               includingPropertiesForKeys:keys
+                                                  options:0
+                                                    error:nil];
+  for (int i = 0; i < array.count; i++) {
+  }
+}

infer/tests/codetoanalyze/objc/performance/cost-issues.exp

@@ -43,6 +43,7 @@ codetoanalyze/objc/performance/NSDictionary.m, nsdictionary_find_key_constant, 1
 codetoanalyze/objc/performance/NSDictionary.m, nsdictionary_init_dictionary_constant, 3,  OnUIThread:false, []
 codetoanalyze/objc/performance/NSDictionary.m, nsdictionary_init_literal_constant, 45,  OnUIThread:false, []
 codetoanalyze/objc/performance/NSDictionary.m, nsdictionary_init_with_dictionary_linear, 7 + 3 ⋅ dict->elements.length.ub + 4 ⋅ (dict->elements.length.ub + 1),  OnUIThread:false, [{dict->elements.length.ub + 1},Loop,{dict->elements.length.ub},Loop]
+codetoanalyze/objc/performance/NSFileManager.m, contents_of_directory_at_url_constant, 11,  OnUIThread:false, []
 codetoanalyze/objc/performance/NSInteger.m, nsinteger_value_linear, 3 + 3 ⋅ integer + 2 ⋅ (1+max(0, integer)),  OnUIThread:false, [{1+max(0, integer)},Loop,{integer},Loop]
 codetoanalyze/objc/performance/NSInteger.m, nsnumber_number_with_int_integer_value_constant, 34,  OnUIThread:false, []
 codetoanalyze/objc/performance/NSKeyedUnarchiver.m, decode_object_for_key_linear, 9 + 3 ⋅ keyed_unarchiver->elements[*].ub + 2 ⋅ (1+max(0, keyed_unarchiver->elements[*].ub)),  OnUIThread:false, [{1+max(0, keyed_unarchiver->elements[*].ub)},Loop,{keyed_unarchiver->elements[*].ub},Loop]

infer/tests/codetoanalyze/objc/performance/issues.exp

@@ -1,5 +1,6 @@
 codetoanalyze/objc/performance/NSArray.m, nsarray_empty_array_constant, 3, CONDITION_ALWAYS_FALSE, no_bucket, WARNING, [Here]
 codetoanalyze/objc/performance/NSArray.m, nsarray_init_constant, 3, CONDITION_ALWAYS_FALSE, no_bucket, WARNING, [Here]
+codetoanalyze/objc/performance/NSFileManager.m, contents_of_directory_at_url_constant, 7, CONDITION_ALWAYS_FALSE, no_bucket, WARNING, [Here]
 codetoanalyze/objc/performance/NSMutableArray.m, nsarray_new_constant, 2, CONDITION_ALWAYS_FALSE, no_bucket, WARNING, [Here]
 codetoanalyze/objc/performance/NSMutableArray.m, nsmarray_remove_constant, 6, BUFFER_OVERRUN_L1, no_bucket, ERROR, [<Length trace>,Array declaration,Through,Through,Through,Through,Array access: Offset: 0 Size: 0]
 codetoanalyze/objc/performance/NSMutableArray.m, nsmarray_remove_in_loop_constant, 6, BUFFER_OVERRUN_L3, no_bucket, ERROR, [<Offset trace>,Set array size,<Length trace>,Assignment,Set array size,Array access: Offset: [0, 9] Size: [1, 10]]