HADOOP-17181. Handle transient stream read failures in FileSystem contract tests (#2286)

Contributed by Steve Loughran.

* Fixes AbstractContractSeekTest test to use readFully
* Doesn't do this to AbstractContractUnbufferTest test as it changes the test too much.
Instead just notes in the error that this may be transient

The issue is that read(buffer) doesn't guarantee that the buffer is filled, only that it will
read up to a point, and that may be just be the amount of data left in the TCP packet.
readFully corrects for this, but using it in the unbuffer test runs the risk that what
is tested for in terms of unbuffering doesn't actually get validated.

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/fs/contract/AbstractContractSeekTest.java

@@ -317,7 +317,7 @@ public abstract class AbstractContractSeekTest extends AbstractFSContractTestBas
 
     int v = 256;
     byte[] readBuffer = new byte[v];
-    assertEquals(v, instream.read(128, readBuffer, 0, v));
+    instream.readFully(128, readBuffer, 0, v);
     //have gone back
     assertEquals(40000, instream.getPos());
     //content is the same too
@@ -572,8 +572,7 @@ public abstract class AbstractContractSeekTest extends AbstractFSContractTestBas
 
     // now read the entire file in one go
     byte[] fullFile = new byte[TEST_FILE_LEN];
-    assertEquals(TEST_FILE_LEN,
-        instream.read(0, fullFile, 0, fullFile.length));
+    instream.readFully(0, fullFile, 0, fullFile.length);
     assertEquals(0, instream.getPos());
 
     // now read past the end of the file

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/fs/contract/AbstractContractUnbufferTest.java

@@ -137,7 +137,8 @@ public abstract class AbstractContractUnbufferTest extends AbstractFSContractTes
           throws IOException {
     byte[] streamData = new byte[length];
     assertEquals("failed to read expected number of bytes from "
-            + "stream", length, stream.read(streamData));
+            + "stream. This may be transient",
+        length, stream.read(streamData));
     byte[] validateFileBytes;
     if (startIndex == 0 && length == fileBytes.length) {
       validateFileBytes = fileBytes;