diag: instrument CLI server stdin writes for EPIPE origin

Query-server diagnostics showed no query-server exit/error and no
sendRequest failure, so the EPIPE is not the query server. codeQL.
restartQueryServer also restarts the CLI server (cliServer.restartCliServer),
so instrument the CLI server: log restart/kill boundaries, launched pids,
and attach a stdin 'error' listener + guard around the command write to
capture the exact write site and restart interleaving.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 90105d62-02ab-405e-9c95-10466c971814

Author: cklin

extensions/ql-vscode/src/codeql-cli/cli.ts

@@ -313,6 +313,10 @@ export class CodeQLCliServer implements Disposable {
 
   killProcessIfRunning(): void {
     if (this.process) {
+      // DIAG-EPIPE: CLI server teardown boundary.
+      console.error(
+        `DIAG-CLI killProcessIfRunning pid=${this.process.pid} commandInProcess=${this.commandInProcess} at=${new Date().toISOString()}`,
+      );
       // Tell the Java CLI server process to shut down.
       void this.logger.log("Sending shutdown request");
       try {
@@ -353,8 +357,16 @@ export class CodeQLCliServer implements Disposable {
     // If the server is not running a command run this immediately
     // otherwise add to the front of the queue (as we want to run this after the next command()).
     if (this.commandInProcess) {
+      // DIAG-EPIPE: restart deferred behind an in-flight command.
+      console.error(
+        `DIAG-CLI restartCliServer deferred (commandInProcess) at=${new Date().toISOString()}`,
+      );
       this.commandQueue.unshift(callback);
     } else {
+      // DIAG-EPIPE: restart executed immediately.
+      console.error(
+        `DIAG-CLI restartCliServer immediate at=${new Date().toISOString()}`,
+      );
       callback();
     }
   }
@@ -382,7 +394,7 @@ export class CodeQLCliServer implements Disposable {
         ]
       : [];
 
-    return spawnServer(
+    const child = spawnServer(
       codeQlPath,
       "CodeQL CLI Server",
       ["execute", "cli-server"],
@@ -392,6 +404,11 @@ export class CodeQLCliServer implements Disposable {
         /**/
       },
     );
+    // DIAG-EPIPE: record CLI server pid to correlate with stdin errors.
+    console.error(
+      `DIAG-CLI launched CLI server pid=${child.pid} at=${new Date().toISOString()}`,
+    );
+    return child;
   }
 
   private async runCodeQlCliInternal(
@@ -422,9 +439,27 @@ export class CodeQLCliServer implements Disposable {
         return await this.handleProcessOutput(this.process, {
           handleNullTerminator: true,
           onListenStart: (process) => {
+            // DIAG-EPIPE: capture the async EPIPE (emitted on stdin) with a
+            // stack, and the pid we are writing to, to confirm the CLI server
+            // is the source and observe the restart interleaving.
+            process.stdin.on("error", (e: Error) => {
+              console.error(
+                `DIAG-CLI stdin 'error' pid=${process.pid} isCurrent=${this.process === process} ` +
+                  `commandInProcess=${this.commandInProcess} err=${e.name}:${e.message} at=${new Date().toISOString()}\n` +
+                  `${e.stack}`,
+              );
+            });
             // Write the command followed by a null terminator.
-            process.stdin.write(JSON.stringify(args), "utf8");
-            process.stdin.write(this.nullBuffer);
+            try {
+              process.stdin.write(JSON.stringify(args), "utf8");
+              process.stdin.write(this.nullBuffer);
+            } catch (e) {
+              console.error(
+                `DIAG-CLI stdin write threw pid=${process.pid} isCurrent=${this.process === process} ` +
+                  `err=${e instanceof Error ? `${e.name}:${e.message}` : String(e)} at=${new Date().toISOString()}`,
+              );
+              throw e;
+            }
           },
           description,
           args,