Protect Session Map with a Recursive Mutex

(This is a merge of http://go/wvgerrit/72764)

Netflix has identified a calling pattern that causes this mutex to be
taken recursively. This is not guaranteed to be safe for Widevine's
old custom Lock implementation nor std::mutex. However, it is guaranteed
to be safe for std::recursive_mutex. This patch updates the mutex in use
accordingly.

In the long-term, this lock needs to be reconsidered, as already noted
by comments in the code. It would be great if the reconsidered locking
did not require a recursive-safe lock. The TODO for this has been spun
off into its own bug and the comment has been updated to point to this.

Bug: 120471929
Test: CE CDM Unit Tests
Test: Android Unit Tests
Change-Id: I34df64456de4b469b75caf25a33f0bc53a5da330

libwvdrmengine/cdm/core/include/cdm_engine.h

@@ -403,7 +403,7 @@ class CdmEngine {
   // Protect release_key_sets_ from non-thread-safe operations.
   std::mutex release_key_sets_lock_;
 
-  // TODO(rfrias): Replace with two sets of locks, one to protect
+  // TODO(b/124471172): Replace with two sets of locks, one to protect
   // the CdmSessionMap and a per-session lock to control access to
   // session usage/destruction.
   // Locks the session map |session_map_| and session usage/destruction
@@ -412,7 +412,9 @@ class CdmEngine {
   // The layer above the CDM implementation is expected to handle thread
   // synchronization to make sure other functions that access sessions do not
   // occur simultaneously with OpenSession or CloseSession.
-  std::mutex session_map_lock_;
+  // This mutex must be recursive because it is sometimes held while callbacks
+  // occur that may subsequently call back into CdmEngine.
+  std::recursive_mutex session_map_lock_;
 
   CORE_DISALLOW_COPY_AND_ASSIGN(CdmEngine);
 };