broker: set FLUX_ENCLOSING_ID when jobid attribute is set

Problem: It is inconvenient to get the jobid of the closest enclosing
instance (e.g. the ID of a batch or alloc job in the parent instance)
because the broker drops FLUX_JOB_ID in favor of a jobid attribute.
However, a case has been made that an environment variable would be
more convenient since it can be accessed without use of system(3)
or the Flux API.

Introduce FLUX_ENCLOSING_ID, which is set by the broker whenever the
jobid attribute is set (i.e. when the broker is started as a job in
a Flux instance). This will be available in the initial program as
well as being inherited by jobs run within the instance.

Add the variable to the env_blocklist so that it is unset when the
current instance is not a job.

Fixes flux-framework#6474

src/broker/broker.c

@@ -700,9 +700,13 @@ static int create_runat_rc2 (struct runat *r, const char *argz, size_t argz_len)
 
 static int create_runat_phases (broker_ctx_t *ctx)
 {
+    const char *jobid = NULL;
     const char *rc1, *rc3, *local_uri;
     bool rc2_none = false;
 
+    /* jobid may be NULL */
+    (void) attr_get (ctx->attrs, "jobid", &jobid, NULL);
+
     if (attr_get (ctx->attrs, "local-uri", &local_uri, NULL) < 0) {
         log_err ("local-uri is not set");
         return -1;
@@ -718,7 +722,10 @@ static int create_runat_phases (broker_ctx_t *ctx)
     if (attr_get (ctx->attrs, "broker.rc2_none", NULL, NULL) == 0)
         rc2_none = true;
 
-    if (!(ctx->runat = runat_create (ctx->h, local_uri, ctx->sd_notify))) {
+    if (!(ctx->runat = runat_create (ctx->h,
+                                     local_uri,
+                                     jobid,
+                                     ctx->sd_notify))) {
         log_err ("runat_create");
         return -1;
     }

src/broker/runat.c

@@ -62,6 +62,7 @@ struct runat_entry {
 
 struct runat {
     flux_t *h;
+    const char *jobid;
     const char *local_uri;
     zhashx_t *entries;
     flux_msg_handler_t **handlers;
@@ -76,6 +77,7 @@ static const int abort_signal = SIGHUP;
 
 static const char *env_blocklist[] = {
     "FLUX_JOB_ID",
+    "FLUX_ENCLOSING_ID",
     "FLUX_JOB_SIZE",
     "FLUX_JOB_NNODES",
     "FLUX_JOB_TMPDIR",
@@ -363,11 +365,13 @@ static struct runat_command *runat_command_create (char **env, int flags)
 }
 
 /* Unset blocklisted variables in command environment.
+ * Set FLUX_ENCLOSING_ID if "jobid" is non-NULL.
  * Set FLUX_URI if local_uri is non-NULL.
  */
 static int runat_command_modenv (struct runat_command *cmd,
                                  const char **blocklist,
-                                 const char *local_uri)
+                                 const char *local_uri,
+                                 const char *jobid)
 {
     if (blocklist) {
         int i;
@@ -378,6 +382,14 @@ static int runat_command_modenv (struct runat_command *cmd,
         if (flux_cmd_setenvf (cmd->cmd, 1, "FLUX_URI", "%s", local_uri) < 0)
             return -1;
     }
+    if (jobid) {
+        if (flux_cmd_setenvf (cmd->cmd,
+                              1,
+                              "FLUX_ENCLOSING_ID",
+                              "%s",
+                              jobid) < 0)
+            return -1;
+    }
     return 0;
 }
 
@@ -493,7 +505,7 @@ int runat_push_shell_command (struct runat *r,
         return -1;
     if (runat_command_set_cmdline (cmd, NULL, cmdline) < 0)
         goto error;
-    if (runat_command_modenv (cmd, env_blocklist, r->local_uri) < 0)
+    if (runat_command_modenv (cmd, env_blocklist, r->local_uri, r->jobid) < 0)
         goto error;
     if (runat_push (r, name, cmd, false) < 0)
         goto error;
@@ -518,7 +530,7 @@ int runat_push_shell (struct runat *r,
         return -1;
     if (runat_command_set_cmdline (cmd, shell, NULL) < 0)
         goto error;
-    if (runat_command_modenv (cmd, env_blocklist, r->local_uri) < 0)
+    if (runat_command_modenv (cmd, env_blocklist, r->local_uri, r->jobid) < 0)
         goto error;
     if (runat_push (r, name, cmd, true) < 0)
         goto error;
@@ -544,7 +556,7 @@ int runat_push_command (struct runat *r,
         return -1;
     if (runat_command_set_argz (cmd, argz, argz_len) < 0)
         goto error;
-    if (runat_command_modenv (cmd, env_blocklist, r->local_uri) < 0)
+    if (runat_command_modenv (cmd, env_blocklist, r->local_uri, r->jobid) < 0)
         goto error;
     if (runat_push (r, name, cmd, false) < 0)
         goto error;
@@ -693,7 +705,10 @@ static const struct flux_msg_handler_spec htab[] = {
     FLUX_MSGHANDLER_TABLE_END,
 };
 
-struct runat *runat_create (flux_t *h, const char *local_uri, bool sdnotify)
+struct runat *runat_create (flux_t *h,
+                            const char *local_uri,
+                            const char *jobid,
+                            bool sdnotify)
 {
     struct runat *r;
 
@@ -705,6 +720,7 @@ struct runat *runat_create (flux_t *h, const char *local_uri, bool sdnotify)
         goto error;
     zhashx_set_destructor (r->entries, runat_entry_destroy_wrapper);
     r->h = h;
+    r->jobid = jobid;
     r->local_uri = local_uri;
     r->sd_notify = sdnotify;
     if (isatty (STDIN_FILENO)

src/broker/runat.h

@@ -27,7 +27,11 @@ typedef void (*runat_completion_f)(struct runat *r,
                                    const char *name,
                                    void *arg);
 
-struct runat *runat_create (flux_t *h, const char *local_uri, bool sdnotify);
+struct runat *runat_create (flux_t *h,
+                            const char *local_uri,
+                            const char *jobid,
+                            bool sdnotify);
+
 void runat_destroy (struct runat *r);
 
 /* Push command, to be run under shell -c, onto named list.

src/broker/test/runat.c

@@ -68,7 +68,7 @@ void basic (flux_t *h)
 
     ctx.h = h;
 
-    r = runat_create (h, "local://notreally", false);
+    r = runat_create (h, "local://notreally", "f1234", false);
     ok (r != NULL,
         "runat_create works");
 
@@ -268,7 +268,7 @@ void badinput (flux_t *h)
     struct runat *r;
     int rc;
 
-    if (!(r = runat_create (h, NULL, false)))
+    if (!(r = runat_create (h, NULL, NULL, false)))
         BAIL_OUT ("runat_create failed");
 
     ok (runat_is_defined (NULL, "foo") == false,