HDFS-6912. SharedFileDescriptorFactory should not allocate sparse files (cmccabe)

(cherry picked from commit 8008f0e8191b1c7adbed96ed4c380208e3a37692)

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -378,6 +378,9 @@ Release 2.6.0 - UNRELEASED
     HADOOP-11056. OsSecureRandom.setConf() might leak file descriptors. (yzhang
     via cmccabe)
 
+    HDFS-6912. SharedFileDescriptorFactory should not allocate sparse files
+    (cmccabe)
+
     BREAKDOWN OF HDFS-6134 AND HADOOP-10150 SUBTASKS AND RELATED JIRAS
   
       HADOOP-10734. Implement high-performance secure random number sources.

hadoop-common-project/hadoop-common/src/main/native/src/org/apache/hadoop/io/nativeio/SharedFileDescriptorFactory.c

@@ -37,6 +37,8 @@
 #include <sys/types.h>
 #include <unistd.h>
 
+#define ZERO_FULLY_BUF_SIZE 8192
+
 static pthread_mutex_t g_rand_lock = PTHREAD_MUTEX_INITIALIZER;
 
 JNIEXPORT void JNICALL
@@ -83,6 +85,24 @@ done:
   }
 }
 
+static int zero_fully(int fd, jint length)
+{
+  char buf[ZERO_FULLY_BUF_SIZE];
+  int res;
+
+  memset(buf, 0, sizeof(buf));
+  while (length > 0) {
+    res = write(fd, buf,
+      (length > ZERO_FULLY_BUF_SIZE) ? ZERO_FULLY_BUF_SIZE : length);
+    if (res < 0) {
+      if (errno == EINTR) continue;
+      return errno;
+    }
+    length -= res;
+  }
+  return 0;
+}
+
 JNIEXPORT jobject JNICALL
 Java_org_apache_hadoop_io_nativeio_SharedFileDescriptorFactory_createDescriptor0(
   JNIEnv *env, jclass clazz, jstring jprefix, jstring jpath, jint length)
@@ -136,12 +156,20 @@ Java_org_apache_hadoop_io_nativeio_SharedFileDescriptorFactory_createDescriptor0
     (*env)->Throw(env, jthr);
     goto done;
   }
-  if (ftruncate(fd, length) < 0) {
-    jthr = newIOException(env, "ftruncate(%s, %d) failed: error %d (%s)",
+  ret = zero_fully(fd, length);
+  if (ret) {
+    jthr = newIOException(env, "zero_fully(%s, %d) failed: error %d (%s)",
                           path, length, ret, terror(ret));
     (*env)->Throw(env, jthr);
     goto done;
   }
+  if (lseek(fd, 0, SEEK_SET) < 0) {
+    ret = errno;
+    jthr = newIOException(env, "lseek(%s, 0, SEEK_SET) failed: error %d (%s)",
+                          path, ret, terror(ret));
+    (*env)->Throw(env, jthr);
+    goto done;
+  }
   jret = fd_create(env, fd); // throws exception on error.
 
 done: