flux-exec: disable stdin flow for sdexec

Problem: sdexec does not yet support flow control

Disable flow control if the service is set to sdexec.
Add a --stdin-flow=on|off hidden option to force it either way.

src/cmd/flux-exec.c

@@ -55,6 +55,9 @@ static struct optparse_option cmdopts[] = {
     { .name = "setopt", .has_arg = 1, .arginfo = "NAME=VALUE",
       .flags = OPTPARSE_OPT_HIDDEN,
       .usage = "Set subprocess option NAME to VALUE (multiple use ok)" },
+    { .name = "stdin-flow", .has_arg = 1, .arginfo = "on|off",
+      .flags = OPTPARSE_OPT_HIDDEN,
+      .usage = "Forcibly enable or disable stdin flow control" },
     { .name = "with-imp", .has_arg = 0,
       .usage = "Run args under 'flux-imp run'" },
     { .name = "jobid", .key = 'j', .has_arg = 1, .arginfo = "JOBID",
@@ -89,6 +92,7 @@ optparse_t *opts = NULL;
 
 int stdin_flags;
 flux_watcher_t *stdin_w;
+bool stdin_enable_flow_control = true;
 
 /* time to wait in between SIGINTs */
 #define INTERRUPT_MILLISECS 1000.0
@@ -210,10 +214,14 @@ void state_cb (flux_subprocess_t *p, flux_subprocess_state_t state)
 
     if (stdin_w) {
         if (started == rank_count) {
-            int min_credits = subprocess_min_credits ();
             /* don't start stdin_w unless all subprocesses have
              * received credits to write to stdin */
-            if (min_credits)
+            if (stdin_enable_flow_control) {
+                int min_credits = subprocess_min_credits ();
+                if (min_credits)
+                    flux_watcher_start (stdin_w);
+            }
+            else
                 flux_watcher_start (stdin_w);
         }
         if (exited == rank_count)
@@ -285,7 +293,10 @@ static void stdin_cb (flux_reactor_t *r,
     flux_subprocess_t *p;
     const char *ptr;
     int len, lenp;
-    int min_credits = subprocess_min_credits ();
+    int min_credits = -1;
+
+    if (stdin_enable_flow_control)
+        min_credits = subprocess_min_credits ();
 
     if (!(ptr = fbuf_read (fb, min_credits, &lenp)))
         log_err_exit ("fbuf_read");
@@ -295,14 +306,15 @@ static void stdin_cb (flux_reactor_t *r,
         while (p) {
             if (flux_subprocess_state (p) == FLUX_SUBPROCESS_INIT
                 || flux_subprocess_state (p) == FLUX_SUBPROCESS_RUNNING) {
-                int updated_credits;
                 if ((len = flux_subprocess_write (p, "stdin", ptr, lenp)) < 0)
                     log_err_exit ("flux_subprocess_write");
-                /* N.B. since we are subtracting the same number of credits
-                 * from all subprocesses, the sorted order in the credits list
-                 * should not change
-                 */
-                subprocess_update_credits (p, -1*len, false);
+                if (stdin_enable_flow_control) {
+                    /* N.B. since we are subtracting the same number
+                     * of credits from all subprocesses, the sorted
+                     * order in the credits list should not change
+                     */
+                    subprocess_update_credits (p, -1*len, false);
+                }
             }
             p = zlistx_next (subprocesses);
         }
@@ -796,6 +808,23 @@ int main (int argc, char *argv[])
     service_name = optparse_get_str (opts,
                                      "service",
                                      job_service ? job_service : "rexec");
+
+    // sdexec stdin flow is disabled by default
+    if (streq (service_name, "sdexec"))
+        stdin_enable_flow_control = false;
+
+    const char *stdin_flow = optparse_get_str (opts, "stdin-flow", NULL);
+    if (stdin_flow) {
+        if (streq (stdin_flow, "off"))
+            stdin_enable_flow_control = false;
+        else if (streq (stdin_flow, "on"))
+            stdin_enable_flow_control = true;
+        else
+            log_msg_exit ("Set --stdin-flow to on or off");
+    }
+    if (!stdin_enable_flow_control)
+        ops.on_credit = NULL;
+
     rank = idset_first (targets);
     while (rank != IDSET_INVALID_ID) {
         flux_subprocess_t *p;