Set background thread name with pthread_setname_np()

Summary: Makes it easier to monitor performance with top

Test Plan: ./manual_compaction_test with `top -H` running.  Previously was two `manual_compacti`, now one shows `rocksdb:bg0`.

Reviewers: igor, dhruba

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14367

util/env_posix.cc

@@ -1396,6 +1396,15 @@ class PosixEnv : public Env {
         fprintf(stdout,
                 "Created bg thread 0x%lx\n",
                 (unsigned long)t);
+
+        // Set the thread name to aid debugging
+#if defined(_GNU_SOURCE) && defined(__GLIBC_PREREQ) && (__GLIBC_PREREQ(2, 12))
+        char name_buf[16];
+        snprintf(name_buf, sizeof name_buf, "rocksdb:bg%zu", bgthreads_.size());
+        name_buf[sizeof name_buf - 1] = '\0';
+        pthread_setname_np(t, name_buf);
+#endif
+
         bgthreads_.push_back(t);
       }