[master] 1a79fde34 varnishd: truncate thread name on Linux

[Nils Goroll]

commit 1a79fde343ebd9f7210ea2489df6e9730940653f
Author: Asad Sajjad Ahmed <asadsa at varnish-software.com>
Date:   Fri Oct 11 13:06:16 2024 +0200

    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 at varnish-software.com>

diff --git a/bin/varnishd/cache/cache_main.c b/bin/varnishd/cache/cache_main.c
index d10a70d31..920980b54 100644
--- a/bin/varnishd/cache/cache_main.c
+++ b/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
 }