Fix POSIX timer thread naming.

Spencer Low points out that we never actually set a name because the constant
part of the string was longer than the kernel's maximum, and the kernel
rejects long names rather than truncate.

Shorten the fixed part of the string while still keeping it meaningful. 9999
POSIX timers should be enough for any process...

Bug: https://code.google.com/p/android/issues/detail?id=170089
Change-Id: Ic05f07584c1eac160743519091a540ebbf8d7eb1

libc/bionic/posix_timers.cpp

@@ -174,10 +174,10 @@ int timer_create(clockid_t clock_id, sigevent* evp, timer_t* timer_id) {
     return -1;
   }
 
-  // Give the thread a meaningful name.
+  // Give the thread a specific meaningful name.
   // It can't do this itself because the kernel timer isn't created until after it's running.
-  char name[32];
-  snprintf(name, sizeof(name), "POSIX interval timer %d", to_kernel_timer_id(timer));
+  char name[16]; // 16 is the kernel-imposed limit.
+  snprintf(name, sizeof(name), "POSIX timer %d", to_kernel_timer_id(timer));
   pthread_setname_np(timer->callback_thread, name);
 
   *timer_id = timer;

tests/pthread_test.cpp

@@ -403,7 +403,9 @@ TEST(pthread, pthread_sigmask) {
 }
 
 TEST(pthread, pthread_setname_np__too_long) {
-  ASSERT_EQ(ERANGE, pthread_setname_np(pthread_self(), "this name is far too long for linux"));
+  // The limit is 15 characters --- the kernel's buffer is 16, but includes a NUL.
+  ASSERT_EQ(0, pthread_setname_np(pthread_self(), "123456789012345"));
+  ASSERT_EQ(ERANGE, pthread_setname_np(pthread_self(), "1234567890123456"));
 }
 
 TEST(pthread, pthread_setname_np__self) {