varnishd: truncate thread name on Linux

On Linux, threads can not have name longer than 15 bytes plus a terminating
'\0' byte:
> PR_SET_NAME (since Linux 2.6.9)
>     Set the name of the calling thread, using the value in the loca‐
>     tion  pointed  to  by  (char  *) arg2.  The name can be up to 16
>     bytes long, including the terminating null byte.  (If the length
>     of  the  string, including the terminating null byte, exceeds 16
>     bytes, the string is silently truncated.)  This is the same  at‐
>     tribute  that can be set via pthread_setname_np(3) and retrieved
>     using pthread_getname_np(3).  The attribute is likewise accessi‐
>     ble via /proc/self/task/tid/comm (see proc(5)), where tid is the
>     thread ID of the calling thread, as returned by gettid(2).

We have until now ignored the return value from pthread_setname_np(), this
is not great as the call then becomes a NOP:
> The pthread_setname_np() function can be used to  set  a
>     unique  name  for  a  thread,  which can be useful for debugging multi‐
>     threaded applications.  The thread name  is  a  meaningful  C  language
>     string, whose length is restricted to 16 characters, including the ter‐
>     minating null byte ('\0').
> [...]
>     ERANGE The  length  of  the string specified pointed to by name exceeds
>         the allowed limit.

This patch truncates long names to 14 characters plus a tilde ('~') character.
Signed-off-by: Asad Sajjad Ahmed <asadsa@varnish-software.com>

bin/varnishd/cache/cache_main.c

@@ -130,6 +130,22 @@ THR_GetWorker(void)
 
 static pthread_key_t name_key;
 
+static void
+thr_setname_generic(const char *name)
+{
+	char buf[16];
+
+	/* The Linux kernel enforces a strict limitation of 15 bytes name,
+	 * truncate the name if we would overflow it.
+	 */
+	if (strlen(name) > 15) {
+		bprintf(buf, "%.14s~", name);
+		name = buf;
+	}
+
+	PTOK(pthread_setname_np(pthread_self(), name));
+}
+
 void
 THR_SetName(const char *name)
 {
@@ -140,7 +156,7 @@ THR_SetName(const char *name)
 #elif defined(__NetBSD__)
 	(void)pthread_setname_np(pthread_self(), "%s", (char *)(uintptr_t)name);
 #else
-	(void)pthread_setname_np(pthread_self(), name);
+	thr_setname_generic(name);
 #endif
 }