libsubprocess: check that bufsize > 0

Problem: A channel bufsize setting of 0 is technically possible,
which should not be allowed.

Check for bufsize > 0.  Add unit test.

src/common/libsubprocess/test/channel.c

@@ -430,6 +430,12 @@ void test_bufsize_error (flux_reactor_t *r)
     char *av[] = { "/bin/true", NULL };
     flux_cmd_t *cmd;
     flux_subprocess_t *p = NULL;
+    flux_subprocess_ops_t ops = {
+        .on_completion = completion_cb,
+        .on_channel_out = subprocess_standard_output,
+        .on_stdout = subprocess_standard_output,
+        .on_stderr = subprocess_standard_output
+    };
 
     ok ((cmd = flux_cmd_create (1, av, NULL)) != NULL, "flux_cmd_create");
 
@@ -439,18 +445,27 @@ void test_bufsize_error (flux_reactor_t *r)
     ok (flux_cmd_setopt (cmd, "TEST_CHANNEL_BUFSIZE", "ABCD") == 0,
         "flux_cmd_setopt set TEST_CHANNEL_BUFSIZE success");
 
-    flux_subprocess_ops_t ops = {
-        .on_completion = completion_cb,
-        .on_channel_out = subprocess_standard_output,
-        .on_stdout = subprocess_standard_output,
-        .on_stderr = subprocess_standard_output
-    };
     p = flux_local_exec (r, 0, cmd, &ops);
     ok (p == NULL
         && errno == EINVAL,
         "flux_local_exec fails with EINVAL due to bad bufsize input");
 
     flux_cmd_destroy (cmd);
+
+    ok ((cmd = flux_cmd_create (1, av, NULL)) != NULL, "flux_cmd_create");
+
+    ok (flux_cmd_add_channel (cmd, "TEST_CHANNEL") == 0,
+        "flux_cmd_add_channel success adding channel TEST_CHANNEL");
+
+    ok (flux_cmd_setopt (cmd, "TEST_CHANNEL_BUFSIZE", "0") == 0,
+        "flux_cmd_setopt set TEST_CHANNEL_BUFSIZE success");
+
+    p = flux_local_exec (r, 0, cmd, &ops);
+    ok (p == NULL
+        && errno == EINVAL,
+        "flux_local_exec fails with EINVAL due to bufsize zero");
+
+    flux_cmd_destroy (cmd);
 }
 
 int main (int argc, char *argv[])

src/common/libsubprocess/util.c

@@ -55,6 +55,10 @@ int cmd_option_bufsize (flux_subprocess_t *p, const char *name)
         uint64_t size;
         if (parse_size (val, &size) < 0)
             goto cleanup;
+        if (size == 0) {
+            errno = EINVAL;
+            goto cleanup;
+        }
         if (size > INT_MAX) {
             errno = EOVERFLOW;
             goto cleanup;