fix: Close underlying Writers in RecordingSession to prevent resource leaks

The fastjson JSONWriter.close() method does not close the underlying
Writer passed to its constructor - it only closes the internal
SerializeWriter, which has no effect. This caused FileOutputStream and
OutputStreamWriter instances to leak in RecordingSession.

Changes:
- Make AppMapSerializer implement AutoCloseable and track the underlying
  Writer to close it explicitly
- Add try-with-resources for AppMapSerializer in checkpoint() to ensure
  proper cleanup on exceptions
- Add try-finally in stop() to ensure serializer is closed even if
  write() fails
- Consolidate JSON finalization and closing logic into close() method,
  removing the redundant finish() method

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: dividedmind

agent/src/main/java/com/appland/appmap/record/AppMapSerializer.java

@@ -17,7 +17,7 @@
 /**
  * Writes AppMap data to JSON.
  */
-public class AppMapSerializer {
+public class AppMapSerializer implements AutoCloseable {
   public static class FileSections {
     public static final String Version = "version";
     public static final String Metadata = "metadata";
@@ -37,10 +37,13 @@ private class SectionInfo {
   }
 
   private final JSONWriter json;
+  private final Writer underlyingWriter;
   private SectionInfo currentSection = null;
   private final HashSet<String> sectionsWritten = new HashSet<String>();
+  private boolean closed = false;
 
   private AppMapSerializer(Writer writer) {
+    this.underlyingWriter = writer;
     this.json = new JSONWriter(writer);
     // The eventUpdates object contains Event objects that are also in the
     // events array. Setting DisableCircularReferenceDetect tells fastjson that
@@ -73,7 +76,7 @@ public void write(CodeObjectTree classMap, Metadata metadata, Map<Integer, Event
       writeMetadata(git, metadata);
     }
     writeEventUpdates(eventUpdates);
-    finish();
+    close();
   }
 
   private void setCurrentSection(String section, String type) throws IOException {
@@ -280,12 +283,22 @@ private void writeEventUpdates(Map<Integer, Event> updates) throws IOException {
   }
 
   /**
-   * Closes outstanding JSON objects and closes the writer.
+   * Closes outstanding JSON objects and closes the underlying writer. Safe to call multiple times.
    * @throws IOException If a writer error occurs
    */
-  private void finish() throws IOException {
-    this.setCurrentSection("EOF", "");
-    this.json.endObject();
-    this.json.close();
+  @Override
+  public void close() throws IOException {
+    if (!this.closed) {
+      try {
+        this.setCurrentSection("EOF", "");
+        this.json.endObject();
+        this.json.close();
+      } finally {
+        // JSONWriter.close() does not close the underlying writer, so we must do it explicitly
+        // Always close the underlying writer, even if JSON finalization fails
+        this.underlyingWriter.close();
+        this.closed = true;
+      }
+    }
   }
 }

agent/src/main/java/com/appland/appmap/record/RecordingSession.java

@@ -93,22 +93,24 @@ public synchronized Recording checkpoint() {
       // By using RandomAccessFile we can erase that character.
       // If we don't let the JSON writer write the "begin object" token, it refuses
       // to do anything else properly either.
-      RandomAccessFile raf = new RandomAccessFile(targetPath.toFile(), "rw");
-      Writer fw = new OutputStreamWriter(new OutputStream() {
-        @Override
-        public void write(int b) throws IOException {
-          raf.write(b);
+      try (RandomAccessFile raf = new RandomAccessFile(targetPath.toFile(), "rw")) {
+        Writer fw = new OutputStreamWriter(new OutputStream() {
+          @Override
+          public void write(int b) throws IOException {
+            raf.write(b);
+          }
+        }, StandardCharsets.UTF_8);
+        raf.seek(targetPath.toFile().length());
+
+        if (eventReceived) {
+          fw.write("],");
         }
-      }, StandardCharsets.UTF_8);
-      raf.seek(targetPath.toFile().length());
+        fw.flush();
 
-      if (  eventReceived ) {
-        fw.write("],");
+        try (AppMapSerializer serializer = AppMapSerializer.reopen(fw, raf)) {
+          serializer.write(this.getClassMap(), this.metadata, this.eventUpdates);
+        }
       }
-      fw.flush();
-
-      AppMapSerializer serializer = AppMapSerializer.reopen(fw, raf);
-      serializer.write(this.getClassMap(), this.metadata, this.eventUpdates);
     } catch (IOException e) {
       throw new RuntimeException(e);
     }
@@ -124,16 +126,24 @@ public synchronized Recording stop() {
       throw new IllegalStateException("AppMap: Unable to stop the recording because no recording is in progress.");
     }
 
+    File file = this.tmpPath.toFile();
     try {
       this.serializer.write(this.getClassMap(), this.metadata, this.eventUpdates);
     } catch (IOException e) {
       throw new RuntimeException(e);
+    } finally {
+      // Ensure serializer is closed even if write() throws an exception
+      try {
+        if (this.serializer != null) {
+          this.serializer.close();
+        }
+      } catch (IOException e) {
+        logger.error("Failed to close serializer", e);
+      }
+      this.serializer = null;
+      this.tmpPath = null;
     }
 
-    File file = this.tmpPath.toFile();
-    this.serializer = null;
-    this.tmpPath = null;
-
     logger.debug("Recording finished");
     logger.debug("Wrote recording to file {}", file.getPath());
 
@@ -163,7 +173,9 @@ void start() {
     try {
       this.tmpPath = Files.createTempFile(null, ".appmap.json");
       this.tmpPath.toFile().deleteOnExit();
-      this.serializer = AppMapSerializer.open(new OutputStreamWriter(new FileOutputStream(this.tmpPath.toFile()), StandardCharsets.UTF_8));
+      FileOutputStream fileOutputStream = new FileOutputStream(this.tmpPath.toFile());
+      OutputStreamWriter writer = new OutputStreamWriter(fileOutputStream, StandardCharsets.UTF_8);
+      this.serializer = AppMapSerializer.open(writer);
     } catch (IOException e) {
       this.tmpPath = null;
       this.serializer = null;