Simplify stderr buffers and EOL splitting

Co-authored-by: Robert <robertbrignull@github.com>

Author: koesie10

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

@@ -362,8 +362,8 @@ export class CodeQLCliServer implements Disposable {
     silent?: boolean,
   ): Promise<string> {
     const stderrBuffers: Buffer[] = [];
-    // The array of fragments of stderr of this line. To be used for logging.
-    let currentLineStderrBuffers: Buffer[] = [];
+    // The current buffer of stderr of a single line. To be used for logging.
+    let currentLineStderrBuffer: Buffer = Buffer.alloc(0);
     if (this.commandInProcess) {
       throw new Error("runCodeQlCliInternal called while cli was running");
     }
@@ -423,28 +423,34 @@ export class CodeQLCliServer implements Disposable {
             stderrBuffers.push(newData);
 
             if (!silent) {
-              currentLineStderrBuffers.push(newData);
+              currentLineStderrBuffer = Buffer.concat([
+                currentLineStderrBuffer,
+                newData,
+              ]);
 
               // Print the stderr to the logger as it comes in. We need to ensure that
               // we don't split messages on the same line, so we buffer the stderr and
               // split it on EOLs.
-              let currentLineBuffer = Buffer.concat(currentLineStderrBuffers);
               const eolBuffer = Buffer.from(EOL);
-              if (currentLineBuffer.includes(eolBuffer)) {
-                while (currentLineBuffer.includes(eolBuffer)) {
-                  const line = currentLineBuffer.subarray(
-                    0,
-                    currentLineBuffer.indexOf(eolBuffer),
-                  );
-
-                  void this.logger.log(line.toString("utf-8"));
-
-                  currentLineBuffer = currentLineBuffer.subarray(
-                    currentLineBuffer.indexOf(eolBuffer) + eolBuffer.length,
-                  );
-                }
 
-                currentLineStderrBuffers = [currentLineBuffer];
+              let hasCreatedSubarray = false;
+
+              let eolIndex;
+              while (
+                (eolIndex = currentLineStderrBuffer.indexOf(eolBuffer)) !== -1
+              ) {
+                const line = currentLineStderrBuffer.subarray(0, eolIndex);
+                void this.logger.log(line.toString("utf-8"));
+                currentLineStderrBuffer = currentLineStderrBuffer.subarray(
+                  eolIndex + eolBuffer.length,
+                );
+                hasCreatedSubarray = true;
+              }
+
+              // We have created a subarray, which means that the complete original buffer is now referenced
+              // by the subarray. We need to create a new buffer to avoid memory leaks.
+              if (hasCreatedSubarray) {
+                currentLineStderrBuffer = Buffer.from(currentLineStderrBuffer);
               }
             }
           });
@@ -461,10 +467,8 @@ export class CodeQLCliServer implements Disposable {
         // Make sure we remove the terminator;
         const data = fullBuffer.toString("utf8", 0, fullBuffer.length - 1);
         if (!silent) {
-          void this.logger.log(
-            Buffer.concat(currentLineStderrBuffers).toString("utf8"),
-          );
-          currentLineStderrBuffers = [];
+          void this.logger.log(currentLineStderrBuffer.toString("utf8"));
+          currentLineStderrBuffer = Buffer.alloc(0);
           void this.logger.log("CLI command succeeded.");
         }
         return data;
@@ -484,10 +488,8 @@ export class CodeQLCliServer implements Disposable {
         cliError.stack += getErrorStack(err);
         throw cliError;
       } finally {
-        if (!silent && currentLineStderrBuffers.length > 0) {
-          void this.logger.log(
-            Buffer.concat(currentLineStderrBuffers).toString("utf8"),
-          );
+        if (!silent && currentLineStderrBuffer.length > 0) {
+          void this.logger.log(currentLineStderrBuffer.toString("utf8"));
         }
         // Remove the listeners we set up.
         process.stdout.removeAllListeners("data");