debugger: prevent simultaneous heap snapshots

lib/internal/debugger/inspect_repl.js

@@ -325,6 +325,7 @@ function createRepl(inspector) {
   const history = { control: [], debug: [] };
   const watchedExpressions = [];
   const knownBreakpoints = [];
+  let heapSnapshotPromise = null;
   let pauseOnExceptionState = 'none';
   let lastCommand;
 
@@ -961,7 +962,13 @@ function createRepl(inspector) {
       },
 
       takeHeapSnapshot(filename = 'node.heapsnapshot') {
-        return new Promise((resolve, reject) => {
+        if (heapSnapshotPromise) {
+          print(
+            'Cannot take heap snapshot because another snapshot is in progress.'
+          );
+          return heapSnapshotPromise;
+        }
+        heapSnapshotPromise = new Promise((resolve, reject) => {
           const absoluteFile = Path.resolve(filename);
           const writer = FS.createWriteStream(absoluteFile);
           let sizeWritten = 0;
@@ -983,6 +990,7 @@ function createRepl(inspector) {
             writer.end(() => {
               teardown();
               print(`Wrote snapshot: ${absoluteFile}`);
+              heapSnapshotPromise = null;
               resolve();
             });
           }
@@ -1006,6 +1014,7 @@ function createRepl(inspector) {
             HeapProfiler.takeHeapSnapshot({ reportProgress: true }),
             onResolve, onReject);
         });
+        return heapSnapshotPromise;
       },
 
       get watchers() {

test/sequential/test-debugger-heap-profiler.js

@@ -32,6 +32,10 @@ const filename = 'node.heapsnapshot';
     .then(() => cli.waitForPrompt())
     .then(() => cli.command('takeHeapSnapshot()'))
     .then(() => JSON.parse(readFileSync(filename, 'utf8')))
+    // Check that two simultaneous snapshots don't step all over each other.
+    // Refs: https://github.com/nodejs/node/issues/39555
+    .then(() => cli.command('takeHeapSnapshot(); takeHeapSnapshot()'))
+    .then(() => JSON.parse(readFileSync(filename, 'utf8')))
     .then(() => cli.quit())
     .then(null, onFatal);
 }