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

2 files changed, 6 insertions, 4 deletions

diff --git a/libc/bionic/posix_timers.cpp b/libc/bionic/posix_timers.cpp
index bc3aeb2..c8f71c8 100644
--- a/libc/bionic/posix_timers.cpp
+++ b/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;
diff --git a/tests/pthread_test.cpp b/tests/pthread_test.cpp
index a299f02..a9d1097 100644
--- a/tests/pthread_test.cpp
+++ b/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) {