json_selection: fix pretty print duplicate $

[nicholascioli]

Fixes an issue where a path with a variable would duplicate the $.

---

Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Note any exceptions here

Notes

Footnotes

1. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. ↩

2. Configuration is an important part of many changes. Where applicable please try to document configuration examples. ↩

3. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

[router-perf]

CI performance tests

• events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• reload - Reload test over a long period of time at a constant rate of users
• large-request - Stress test with a 1 MB request payload
• events - Stress test for events with a lot of users and deduplication ENABLED
• const - Basic stress test that runs with a constant number of users
• step - Basic stress test that steps up the number of users over time
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• no-graphos - Basic stress test, no GraphOS.
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled

[benjamn]

Please add more tests that actually exercise this code, as well as other aspects of JSONSelection syntax. Making the one-line change I suggested does not give me any reassurance the rest of the pretty_print code is correct, or will remain correct in the future.

[benjamn]

Thanks for going the extra mile and moving this trait into its own file, in addition to adding all the tests.

I realize I may have already asked for more of your time than you wanted to spend on this, so if you don't have time to try the Result simplification I mentioned below, I can get to it when I put up my next PR (which has a use for pretty_print).

[benjamn]

It looks like the only reason pretty_print needs to return a Result<String, std::fmt::Error> instead of String is because it uses the write! and writeln! macros, which I suppose can be fallible.

Since this code is only building up a string, I suspect we could use result.push_str (more than once in some cases) instead of the write[ln]! macros, and then the pretty_print signature could be just fn pretty_print(&self) -> String.

Was there another reason for returning Result here?

[nicholascioli]

The main motivation was the fact that using write! allows for directly using formatted strings vs having to use the nested .push_str(&format!(...)). If you get the chance to simplify this in your next PR, please do!

[goto-bus-stop]

you should definitely use write!() over push_str(&format!()). If the output is known to be a String, write!() is infallible, so you can do write!().unwrap() or even _ = write!() to silence the warning.

[goto-bus-stop]

That said, even better would be if the pretty printing trait used an f: &mut fmt::Formatter parameter for output, to avoid allocating many intermediate strings. That is not something that needs to be done in this PR of course :) See the query_plan::display module for inspiration.

[benjamn]

To clarify, I wasn't suggesting push_str(&format!(...)) but just push_str(&str) (and sometimes push(char)), as in the following diff:

@@ -378,26 +389,29 @@ impl PathSelection {
         }
     }
 
-    pub fn pretty_print_with_indentation(&self, indentation: usize) -> Result<String, std::fmt::Error> {
+    pub fn pretty_print_with_indentation(&self, indentation: usize) -> String {
         let mut result = String::new();
 
         match self {
             PathSelection::Var(var, path) => {
-                let rest = path.pretty_print(indentation)?;
-                write!(result, "${var}{rest}")?;
+                let path_string = path.pretty_print(indentation);
+                result.push_str(var.as_str());
+                result.push_str(path_string.as_str());
             }
             PathSelection::Key(key, path) => {
-                let rest = path.pretty_print(indentation)?;
-                write!(result, "{key}{rest}")?;
+                let path_string = path.pretty_print(indentation);
+                result.push_str(key.dotted().as_str());
+                result.push_str(path_string.as_str());
             }
             PathSelection::Selection(sub) => {
-                let sub = sub.pretty_print(indentation)?;
-                write!(result, " {sub}")?;
+                let sub_string = sub.pretty_print(indentation);
+                result.push(' ');
+                result.push_str(sub_string.as_str());
             }
             PathSelection::Empty => {}
-        }
+        };
 
-        Ok(result)
+        result
     }
 }