Improve exception handling in cache.asMap().computeIfAbsent etc.

Don't store an exception thrown by the compute function. It doesn't appear that the stored exception is ever used. Tests still pass after this deletion.

Also move the evaluation of the compute function to before the possible creation of a new entry, so if it throws an exception then no entry is created.

Fixes #5438, fixes #7625, fixes #7975.

RELNOTES=Handling of exceptions from compute functions in `Cache.asMap().computeIfAbsent` and the like has been improved. (We do still recommend using Caffeine rather than `com.google.common.cache`.)
PiperOrigin-RevId: 805944057

Author: eamonnmcmanus

guava-tests/test/com/google/common/cache/LocalCacheMapComputeTest.java

@@ -139,7 +139,7 @@ public void testComputeIfPresent_error() {
                       throw new Error();
                     }));
     assertThat(cache.getIfPresent(key)).isEqualTo("1");
-    assertThat(cache.asMap().computeIfPresent(key, (k, v) -> "2")).isEqualTo(null);
+    assertThat(cache.asMap().computeIfPresent(key, (k, v) -> "2")).isEqualTo("2");
   }
 
   public void testUpdates() {

guava/src/com/google/common/cache/LocalCache.java

@@ -2259,6 +2259,9 @@ V waitForLoadingValue(ReferenceEntry<K, V> e, K key, ValueReference<K, V> valueR
         // the value for the key is already being computed.
         computingValueReference = new ComputingValueReference<>(valueReference);
 
+        // Compute the value now, so if it throws an exception we don't change anything.
+        newValue = computingValueReference.compute(key, function);
+
         if (e == null) {
           createNewEntry = true;
           e = newEntry(key, hash, first);
@@ -2268,7 +2271,6 @@ V waitForLoadingValue(ReferenceEntry<K, V> e, K key, ValueReference<K, V> valueR
           e.setValueReference(computingValueReference);
         }
 
-        newValue = computingValueReference.compute(key, function);
         if (newValue != null) {
           if (valueReference != null && newValue == valueReference.get()) {
             computingValueReference.set(newValue);
@@ -3585,13 +3587,7 @@ public ListenableFuture<V> loadFuture(K key, CacheLoader<? super K, V> loader) {
       } catch (ExecutionException e) {
         previousValue = null;
       }
-      V newValue;
-      try {
-        newValue = function.apply(key, previousValue);
-      } catch (Throwable th) {
-        this.setException(th);
-        throw th;
-      }
+      V newValue = function.apply(key, previousValue);
       this.set(newValue);
       return newValue;
     }