feat(crashtracking): J9 register parsing

Parse the GPINFO register section (1XHREGISTERS / 2XHREGISTER tags)
from J9/OpenJ9 javacore crash dumps mirroring what was done for HotSpot.

Register format:
  1XHREGISTERS   Registers:
  2XHREGISTER      RIP: 00007F8B7C0B3D7D   (x86-64)
  2XHREGISTER      PC:  0000FFFF98B9FB6C   (aarch64)

Added crash dumps from IBM J9 (Java 8) and IBM Semeru / OpenJ9 (Java 11)
that were obtained using Docker and this crashing program:

```java
import sun.misc.Unsafe;
import java.lang.reflect.Field;
public class Crash {
    public static void main(String[] args) throws Exception {
        Field f = Unsafe.class.getDeclaredField("theUnsafe");
        f.setAccessible(true);
        Unsafe unsafe = (Unsafe) f.get(null);
        unsafe.putInt(0L, 42); // SIGSEGV -> GPF dump
    }
}
```

```
docker run --rm -v /tmp/j9-crash-test:/work ibmjava:8-sdk \
  bash -c "cd /work && javac Crash.java && \
           java -Xdump:java:events=gpf,file=/work/javacore.ibmj9.txt Crash"
```

```
docker run --rm -v /tmp/j9-crash-test:/work ibm-semeru-runtimes:open-11-jdk \
  bash -c "cd /work && javac Crash.java && \
           java -Xdump:java:events=gpf,file=/work/javacore.openj9.txt Crash"
```

Author: bric3

dd-java-agent/agent-crashtracking/src/main/java/datadog/crashtracking/parsers/J9JavacoreParser.java

@@ -7,6 +7,7 @@
 import datadog.crashtracking.buildid.BuildInfo;
 import datadog.crashtracking.dto.CrashLog;
 import datadog.crashtracking.dto.ErrorData;
+import datadog.crashtracking.dto.Experimental;
 import datadog.crashtracking.dto.Metadata;
 import datadog.crashtracking.dto.OSInfo;
 import datadog.crashtracking.dto.ProcInfo;
@@ -21,8 +22,10 @@
 import java.time.format.DateTimeFormatter;
 import java.time.format.DateTimeParseException;
 import java.util.ArrayList;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Locale;
+import java.util.Map;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -80,6 +83,11 @@ public J9JavacoreParser() {
   private static final Pattern NATIVE_STACK_PATTERN = Pattern.compile("4XENATIVESTACK\\s+(.+)");
   private static final Pattern EXCEPTION_DETAIL_PATTERN =
       Pattern.compile("1TISIGINFO.*[Dd]etail\\s+\"(.+?)\".*");
+  // Matches register entries in J9 GPINFO section, e.g.:
+  // 2XHREGISTER      RDI: 0000000000000001  (x86-64)
+  // 2XHREGISTER      R29: 0000FFFF990CDB50  (aarch64)
+  private static final Pattern REGISTER_ENTRY_PARSER =
+      Pattern.compile("([A-Za-z][A-Za-z0-9]*)\\s*:\\s*([0-9a-fA-F]+)");
 
   // Date time formatter for J9 format: YYYY/MM/DD at HH:MM:SS
   private static final DateTimeFormatter J9_DATETIME_FORMATTER =
@@ -108,6 +116,8 @@ public CrashLog parse(String uuid, String javacoreContent) {
     boolean incomplete = false;
     boolean foundThreadSection = false;
 
+    Map<String, String> registers = null;
+
     String[] lines = NEWLINE_SPLITTER.split(javacoreContent);
 
     for (String line : lines) {
@@ -148,6 +158,14 @@ public CrashLog parse(String uuid, String javacoreContent) {
           // OS level and CPU architecture are available in GPINFO section but currently
           // not extracted since OSInfo.current() is used for the crash report.
           // If needed in the future, parse 2XHOSLEVEL and 3XHCPUARCH tags here.
+          if (line.startsWith("1XHREGISTERS")) {
+            registers = new LinkedHashMap<>();
+          } else if (registers != null && line.startsWith("2XHREGISTER")) {
+            final Matcher m = REGISTER_ENTRY_PARSER.matcher(line);
+            while (m.find()) {
+              registers.put(m.group(1), "0x" + m.group(2));
+            }
+          }
           break;
 
         case ENVINFO:
@@ -267,9 +285,12 @@ public CrashLog parse(String uuid, String javacoreContent) {
     Metadata metadata = new Metadata("dd-trace-java", VersionInfo.VERSION, "java", null);
     Integer parsedPid = safelyParseInt(pid);
     ProcInfo procInfo = parsedPid != null ? new ProcInfo(parsedPid) : null;
+    Experimental experimental =
+        (registers != null && !registers.isEmpty()) ? new Experimental(registers) : null;
 
     return new CrashLog(
-        uuid, incomplete, datetime, error, metadata, OSInfo.current(), procInfo, sigInfo, "1.0");
+        uuid, incomplete, datetime, error, metadata, OSInfo.current(), procInfo, sigInfo, "1.0",
+        experimental);
   }
 
   private static Integer safelyParseInt(String value) {

dd-java-agent/agent-crashtracking/src/test/java/datadog/crashtracking/parsers/J9JavacoreParserTest.java

@@ -16,6 +16,7 @@
 import java.util.UUID;
 import java.util.stream.Collectors;
 import org.junit.jupiter.api.Test;
+import org.tabletest.junit.TableTest;
 
 public class J9JavacoreParserTest {
 
@@ -65,6 +66,26 @@ public void testParseGpfCrash() throws Exception {
     assertTrue(crashLog.error.stack.frames.length >= 4); // 3 java + native frames
   }
 
+  @TableTest({
+    "scenario              | resource                           | pcRegister | spRegister",
+    "IBM J9 8 (amd64)     | sample-ibmj9-8-javacore-gpf.txt   | RIP        | RSP       ",
+    "OpenJ9 11 (aarch64)  | sample-openj9-11-javacore-gpf.txt | PC         | SP        ",
+  })
+  public void testParseRealGpfCrash(String resource, String pcRegister, String spRegister)
+      throws Exception {
+    final CrashLog crashLog =
+        new J9JavacoreParser().parse(UUID.randomUUID().toString(), readFileAsString(resource));
+
+    assertFalse(crashLog.incomplete);
+    assertEquals("SIGSEGV", crashLog.sigInfo.name);
+    assertEquals(11, crashLog.sigInfo.number);
+
+    assertNotNull(crashLog.experimental);
+    assertFalse(crashLog.experimental.ucontext.isEmpty());
+    assertTrue(crashLog.experimental.ucontext.containsKey(pcRegister));
+    assertTrue(crashLog.experimental.ucontext.containsKey(spRegister));
+  }
+
   @Test
   public void testParseOomCrash() throws Exception {
     // Given