Add additional log messages and javadoc around reads (apache#29640)

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/BatchSpannerRead.java

@@ -246,7 +246,8 @@ public void processElement(ProcessContext c) throws Exception {
         }
       } catch (SpannerException e) {
         serviceCallMetric.call(e.getErrorCode().getGrpcStatusCode().toString());
-        LOG.error("Error while processing element", e);
+        LOG.error(
+            "Error while reading partition for operation: " + op.getReadOperation().toString(), e);
         throw (e);
       }
       serviceCallMetric.call("ok");

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/NaiveSpannerRead.java

@@ -34,6 +34,8 @@
 import org.apache.beam.sdk.values.PCollectionView;
 import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.annotations.VisibleForTesting;
 import org.checkerframework.checker.nullness.qual.Nullable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /** A naive version of Spanner read that doesn't use the Batch API. */
 @VisibleForTesting
@@ -44,6 +46,8 @@
 abstract class NaiveSpannerRead
     extends PTransform<PCollection<ReadOperation>, PCollection<Struct>> {
 
+  private static final Logger LOG = LoggerFactory.getLogger(NaiveSpannerRead.class);
+
   public static NaiveSpannerRead create(
       SpannerConfig spannerConfig,
       PCollectionView<Transaction> txView,
@@ -109,6 +113,7 @@ public void processElement(ProcessContext c) throws Exception {
         }
       } catch (SpannerException e) {
         serviceCallMetric.call(e.getErrorCode().getGrpcStatusCode().toString());
+        LOG.error("Error while reading operation: " + op.toString(), e);
         throw (e);
       }
       serviceCallMetric.call("ok");

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/ReadOperation.java

@@ -69,6 +69,11 @@ abstract static class Builder {
 
     abstract Builder setKeySet(KeySet keySet);
 
+    /**
+     * Note: {@link PartitionOptions} are currently ignored. See <a
+     * href="https://cloud.google.com/spanner/docs/reference/rpc/google.spanner.v1#google.spanner.v1.PartitionOptions">
+     * PartitionOptions in RPC documents</a>
+     */
     abstract Builder setPartitionOptions(PartitionOptions partitionOptions);
 
     abstract ReadOperation build();

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/SpannerIO.java

@@ -852,6 +852,11 @@ public Read withIndex(String index) {
       return withReadOperation(getReadOperation().withIndex(index));
     }
 
+    /**
+     * Note that {@link PartitionOptions} are currently ignored. See <a
+     * href="https://cloud.google.com/spanner/docs/reference/rpc/google.spanner.v1#google.spanner.v1.PartitionOptions">
+     * PartitionOptions in RPC documents</a>
+     */
     public Read withPartitionOptions(PartitionOptions partitionOptions) {
       return withReadOperation(getReadOperation().withPartitionOptions(partitionOptions));
     }
@@ -2165,6 +2170,7 @@ public void processElement(ProcessContext c) throws Exception {
           // fall through and retry individual mutationGroups.
         } else if (failureMode == FailureMode.FAIL_FAST) {
           mutationGroupsWriteFail.inc(mutations.size());
+          LOG.error("Failed to write a batch of mutation groups", e);
           throw e;
         } else {
           throw new IllegalArgumentException("Unknown failure mode " + failureMode);