Patch from Michael Cassaniti, posted here:
https://bugs.alpinelinux.org/issues/10156#note-5
Until this issue is fixed upstream this patch is needed.

diff --git a/src/lib/jcr.c b/src/lib/jcr.c
index 00bfe8c87..338f90e59 100644
--- a/src/lib/jcr.c
+++ b/src/lib/jcr.c
@@ -77,6 +77,7 @@ static pthread_mutex_t jcr_lock = PTHREAD_MUTEX_INITIALIZER;
 static pthread_mutex_t job_start_mutex = PTHREAD_MUTEX_INITIALIZER;
 static pthread_mutex_t last_jobs_mutex = PTHREAD_MUTEX_INITIALIZER;

+static bool jcr_initialized = false;
 #ifdef HAVE_WIN32
 static bool tsd_initialized = false;
 static pthread_key_t jcr_key;         /* Pointer to jcr for each thread */
@@ -351,6 +352,8 @@ static void create_jcr_key()
       berrno be;
       Jmsg1(NULL, M_ABORT, 0, _("pthread key create failed: ERR=%s\n"),
             be.bstrerror(status));
+   } else {
+     jcr_initialized = true;
    }
 }

@@ -719,7 +722,10 @@ void set_jcr_in_tsd(JCR *jcr)
  */
 JCR *get_jcr_from_tsd()
 {
-   JCR *jcr = (JCR *)pthread_getspecific(jcr_key);
+   JCR *jcr = (JCR *)INVALID_JCR;
+   if (jcr_initialized){
+     jcr = (JCR *)pthread_getspecific(jcr_key);
+   }

    /*
     * Set any INVALID_JCR to NULL which the rest of BAREOS understands
@@ -736,7 +742,7 @@ JCR *get_jcr_from_tsd()
  */
 uint32_t get_jobid_from_tsd()
 {
-   JCR *jcr = (JCR *)pthread_getspecific(jcr_key);
+   JCR *jcr = get_jcr_from_tsd();
    uint32_t JobId = 0;

    if (jcr && jcr != INVALID_JCR) {

This patch fixes a double pthread_detach(), which is undefined behaviour
and leads to a segfault when running with musl libc.
Until this issue is fixed upstream this patch is needed.

--- a/src/dird/ua_server.c
+++ b/src/dird/ua_server.c
@@ -73,7 +73,15 @@
    UAContext *ua;
    JCR *jcr;
 
-   pthread_detach(pthread_self());
+   /* only detach if not yet detached */
+   int _detachstate;
+   pthread_attr_t _gattr;
+   pthread_getattr_np(pthread_self(), &_gattr);
+   pthread_attr_getdetachstate(&_gattr, &_detachstate);
+   pthread_attr_destroy(&_gattr);
+   if(_detachstate != PTHREAD_CREATE_DETACHED) {
+      pthread_detach(pthread_self());
+   }
 
    jcr = new_control_jcr("-Console-", JT_CONSOLE);
 
--- a/src/dird/job.c
+++ b/src/dird/job.c
@@ -420,7 +420,16 @@
 {
    JCR *jcr = (JCR *)arg;
 
-   pthread_detach(pthread_self());
+   /* only detach if not yet detached */
+   int _detachstate;
+   pthread_attr_t _gattr;
+   pthread_getattr_np(pthread_self(), &_gattr);
+   pthread_attr_getdetachstate(&_gattr, &_detachstate);
+   pthread_attr_destroy(&_gattr);
+   if(_detachstate != PTHREAD_CREATE_DETACHED) {
+      pthread_detach(pthread_self());
+   }
+
    Dsm_check(100);
 
    Dmsg0(200, "=====Start Job=========\n");