add SkTHashTable::LookupOrNull()

This should help avoid confusion from T**.

Change-Id: I1851baa2a55714721fa935d234b6a4a1c6d6504f
Reviewed-on: https://skia-review.googlesource.com/c/182562
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>

include/private/SkTHash.h

@@ -85,6 +85,15 @@ public:
         return nullptr;
     }
 
+    // If there is an entry in the table with this key, return it.  If not, null.
+    // This only works for pointer type T, and cannot be used to find an nullptr entry.
+    T findOrNull(const K& key) const {
+        if (T* p = this->find(key)) {
+            return *p;
+        }
+        return nullptr;
+    }
+
     // Remove the value with this key from the hash table.
     void remove(const K& key) {
         SkASSERT(this->find(key));

tests/HashTest.cpp

@@ -177,3 +177,25 @@ DEF_TEST(HashSetCopyCounter, r) {
     // We allow copies for same-value adds for now.
     REPORTER_ASSERT(r, globalCounter == 5);
 }
+
+
+DEF_TEST(HashFindOrNull, r) {
+    struct Entry {
+        int key = 0;
+        int val = 0;
+    };
+
+    struct HashTraits {
+        static int GetKey(const Entry* e) { return e->key; }
+        static uint32_t Hash(int key) { return key; }
+    };
+
+    SkTHashTable<Entry*, int, HashTraits> table;
+
+    REPORTER_ASSERT(r, nullptr == table.findOrNull(7));
+
+    Entry seven = { 7, 24 };
+    table.set(&seven);
+
+    REPORTER_ASSERT(r, &seven == table.findOrNull(7));
+}