Throttle printing repl output to output window

When an excessive amount of output is produced by the repl (for example
as a result of a rogue loop that is printing to stdout) Calva can
sometimes hang while trying to write all the output to the output
window/file.

The only way to resolve is to restart/reload the VSCode window.

This commit introduces a new config entry `replOutputThrottleRate` which
when set to a non-0 number will throttle output from the repl
connection. If more output items are received than the throttle rate in
a 500ms window then they will just be dropped.

Addresses #942
Fixes #2010

CHANGELOG.md

@@ -4,6 +4,7 @@ Changes to Calva.
 
 ## [Unreleased]
 
+- Fix: [Rogue loops that print output to stdout cause Calva to hang](https://github.com/BetterThanTomorrow/calva/issues/2010)
 ## [2.0.323] - 2023-01-07
 
 - Fix: [Provider completions not handling errors gracefully](https://github.com/BetterThanTomorrow/calva/issues/2006)

package.json

@@ -770,6 +770,11 @@
               "lsp"
             ]
           },
+          "calva.replOutputThrottleRate": {
+            "markdownDescription": "If the repl outputs too quickly then results will be dropped from the output window. Setting this to 0 will disable throttling.",
+            "type": "number",
+            "default": 100
+          },
           "calva.depsEdnJackInExecutable": {
             "markdownDescription": "Which executable should Calva Jack-in use for starting a deps.edn project? The default is to let Calva choose. It will choose `clojure` if that is installed and working. Otherwise `deps.clj`, which is bundled with Calva, will be used. (This settings has no effect on Windows, where `deps.clj` will always be used.)",
             "enum": [

src/config.ts

@@ -14,6 +14,8 @@ const REPL_FILE_EXT = 'calva-repl';
 const KEYBINDINGS_ENABLED_CONFIG_KEY = 'calva.keybindingsEnabled';
 const KEYBINDINGS_ENABLED_CONTEXT_KEY = 'calva:keybindingsEnabled';
 
+const REPL_OUTPUT_THROTTLE_RATE_CONFIG_KEY = 'calva.replOutputThrottleRate';
+
 type ReplSessionType = 'clj' | 'cljs';
 
 // include the 'file' and 'untitled' to the
@@ -234,6 +236,7 @@ export {
   REPL_FILE_EXT,
   KEYBINDINGS_ENABLED_CONFIG_KEY,
   KEYBINDINGS_ENABLED_CONTEXT_KEY,
+  REPL_OUTPUT_THROTTLE_RATE_CONFIG_KEY,
   documentSelector,
   ReplSessionType,
   getConfig,

src/results-output/results-doc.ts

@@ -17,6 +17,9 @@ import { formatAsLineComments, splitEditQueueForTextBatching } from './util';
 
 const RESULTS_DOC_NAME = `output.${config.REPL_FILE_EXT}`;
 
+const REPL_OUTPUT_THROTTLE_RATE = vscode.workspace
+  .getConfiguration()
+  .get<number>(config.REPL_OUTPUT_THROTTLE_RATE_CONFIG_KEY);
 const PROMPT_HINT = 'Use `alt+enter` to evaluate';
 
 const START_GREETINGS = [
@@ -331,6 +334,16 @@ export interface OnAppendedCallback {
 
 let resultsBuffer: ResultsBuffer = [];
 
+type BufferThrottleState = {
+  count: number;
+  dropped: number;
+  timeout?: NodeJS.Timeout;
+};
+const throttleState: BufferThrottleState = {
+  count: 0,
+  dropped: 0,
+};
+
 async function writeNextOutputBatch() {
   if (!resultsBuffer[0]) {
     return;
@@ -366,6 +379,30 @@ async function flushOutput() {
 
 /* If something must be done after a particular edit, use the onAppended callback. */
 export function append(text: string, onAppended?: OnAppendedCallback): void {
+  if (REPL_OUTPUT_THROTTLE_RATE > 0) {
+    throttleState.count++;
+
+    if (!throttleState.timeout) {
+      throttleState.timeout = setTimeout(() => {
+        if (throttleState.dropped > 0) {
+          resultsBuffer.push({
+            text: `;; Dropped ${throttleState.dropped} items from output due to throttling\n`,
+          });
+          void flushOutput();
+        }
+
+        throttleState.timeout = undefined;
+        throttleState.count = 0;
+        throttleState.dropped = 0;
+      }, 500);
+    }
+
+    if (throttleState.count > REPL_OUTPUT_THROTTLE_RATE) {
+      throttleState.dropped++;
+      return;
+    }
+  }
+
   resultsBuffer.push({ text, onAppended });
   void flushOutput();
 }