shell: output stdio output size warning

Problem: If stdout or stderr becomes large, it runs the risk of
hitting the 1G limit.

Solution: If a user job outputs between 100M and 1G of stdout or
stderr, output a message to the user recommending to redirect
to a file instead.

Fixes #6269

src/shell/output.c

@@ -65,6 +65,8 @@
 #define SINGLEUSER_OUTPUT_LIMIT "1G"
 #define MULTIUSER_OUTPUT_LIMIT  "10M"
 #define OUTPUT_LIMIT_MAX        1073741824
+/* 104857600 = 100M */
+#define OUTPUT_LIMIT_WARNING    104857600
 
 enum {
     FLUX_OUTPUT_TYPE_TERM = 1,
@@ -519,6 +521,24 @@ static void output_truncation_warning (struct shell_output *out)
                     out->stdout_bytes);
         warned = true;
     }
+    if (out->stderr_type == FLUX_OUTPUT_TYPE_KVS
+        && (out->stderr_bytes > OUTPUT_LIMIT_WARNING
+            && out->stderr_bytes <= OUTPUT_LIMIT_MAX)) {
+        shell_warn ("high stderr volume (%s), "
+                    "consider redirecting to a file next time "
+                    "(e.g. use --output=FILE)",
+                    encode_size (out->stderr_bytes));
+        warned = true;
+    }
+    if (out->stdout_type == FLUX_OUTPUT_TYPE_KVS
+        && (out->stdout_bytes > OUTPUT_LIMIT_WARNING
+            && out->stdout_bytes <= OUTPUT_LIMIT_MAX)) {
+        shell_warn ("high stdout volume (%s), "
+                    "consider redirecting to a file next time "
+                    "(e.g. use --output=FILE)",
+                    encode_size (out->stdout_bytes));
+        warned = true;
+    }
     /* Ensure KVS output is flushed to eventlogger if a warning was issued:
      */
     if (warned)