test(api): add parallel-session stress test (I2)

Wires up Track I2 from the v1.6.5->1.7 readiness taskboard. Drives 32 independent DocumentSession instances on a fixed-size thread pool through 4 iterations to catch race conditions in process-wide shared state (font registry, glyph cache, built-in node definitions, shape outline cache).

Two assertions:

1. identicalSessionsInParallelProduceIdenticalLayoutGraphs: every parallel render produces the same layout-graph toString() as a sequential baseline. Catches any shared mutable state racing in the prepare/measure pipeline.

2. independentSessionsInParallelProduceValidPdfBytes: every PDF carries the %PDF magic, is at least 256 bytes, and size variance across threads is <256 bytes (loose enough not to lock against legitimate timestamp drift, tight enough to catch corruption).

Each DocumentSession is single-threaded by contract; this test exercises a fleet of INDEPENDENT sessions, not concurrent access to a single session. ~1.6s locally; does not bloat CI.

Author: DemchaAV

CHANGELOG.md

@@ -34,6 +34,20 @@ JitPack continue to resolve through the existing coordinates.
   excluded by convention (`InternalAnnotationCoverageTest` covers those).
   Method-level `@since` backfill for the ~380 public methods in these
   packages is intentionally out of scope here and tracked separately.
+- **Parallel-session stress test** (Track I2). New
+  `DocumentSessionParallelStressTest` drives 32 independent
+  `DocumentSession` instances on a fixed-size thread pool through 4
+  iterations and asserts (a) all parallel renders produce a layout-graph
+  signature byte-equal to the sequential baseline — exercising the
+  shared font registry, glyph cache, built-in node definitions, and
+  shape-outline cache for race conditions; (b) every PDF output starts
+  with the `%PDF` magic, is at least 256 bytes, and has size variance
+  under 256 bytes across threads (catching corruption or rare
+  non-determinism without locking exact byte counts that timestamps
+  could drift). 128 + 128 = 256 renders complete in ~1.6 s locally, so
+  the test does not bloat CI. The contract is that each
+  `DocumentSession` is single-threaded but the process-wide machinery
+  handles concurrent _independent_ sessions safely; this test pins that.
 - **`no-poi` Maven profile + CI job** (Track I1). The `poi-ooxml`
   dependency is declared `<optional>true</optional>` so callers that
   render only PDFs don't pay the ~10 MB POI footprint; this PR adds a

src/test/java/com/demcha/compose/document/api/DocumentSessionParallelStressTest.java

@@ -0,0 +1,160 @@
+package com.demcha.compose.document.api;
+
+import com.demcha.compose.GraphCompose;
+import com.demcha.compose.document.dsl.DocumentDsl;
+import com.demcha.compose.document.style.DocumentInsets;
+import com.demcha.compose.document.style.DocumentTextStyle;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Stress test for concurrent {@link DocumentSession} usage. Drives N
+ * independent sessions in parallel and asserts each produces a
+ * well-formed PDF (validates no race conditions in shared font /
+ * registry / cache state) and that sessions seeded with identical
+ * content produce identical layout graphs (validates layout
+ * determinism under concurrency).
+ *
+ * <p>Each {@link DocumentSession} is single-threaded by contract — the
+ * stress test exercises a fleet of <strong>independent</strong>
+ * sessions, each owned by exactly one thread, not concurrent access to
+ * a single session. The guarantee under test is that the
+ * <em>process-wide</em> machinery (default font registry, glyph cache,
+ * built-in node definitions, shape outline cache) handles concurrent
+ * lookup safely.</p>
+ *
+ * <p>Tracked as Track I1 / I2 in the v1.6.6 readiness taskboard.</p>
+ */
+class DocumentSessionParallelStressTest {
+
+    private static final int THREAD_COUNT = 32;
+    private static final int ITERATIONS = 4;
+    private static final long TIMEOUT_SECONDS = 60;
+
+    @Test
+    void identicalSessionsInParallelProduceIdenticalLayoutGraphs() throws Exception {
+        // The layout graph is the canonical deterministic snapshot — PDF
+        // bytes may differ across runs (xref hashes, resource-stream
+        // ordering) but the layout structure must be bit-stable. If
+        // concurrent runs ever surface a different layout-graph
+        // toString() than the sequential baseline, we have shared
+        // mutable state racing somewhere in the prepare/measure pipeline.
+        for (int iteration = 0; iteration < ITERATIONS; iteration++) {
+            String baseline = renderLayoutSignature();
+            assertThat(baseline)
+                    .as("sequential baseline must be non-empty")
+                    .isNotBlank();
+
+            Set<String> signatures = runParallel(this::renderLayoutSignature);
+            assertThat(signatures)
+                    .as("parallel iteration %d — every thread should produce the baseline layout", iteration)
+                    .containsExactly(baseline);
+        }
+    }
+
+    @Test
+    void independentSessionsInParallelProduceValidPdfBytes() throws Exception {
+        // Each thread builds its own document and writes a PDF. We don't
+        // assert byte-identity — that would over-specify (PDF timestamps,
+        // resource ordering). We assert each output starts with the PDF
+        // magic %PDF and has plausible size, which is enough to catch
+        // any thread that errored out or produced corrupted bytes.
+        for (int iteration = 0; iteration < ITERATIONS; iteration++) {
+            Set<Integer> sizes = runParallel(() -> {
+                byte[] pdf = renderPdfBytes();
+                assertThat(pdf)
+                        .as("each PDF must be present and non-empty")
+                        .isNotEmpty();
+                assertThat(pdf.length)
+                        .as("each PDF should be at least 256 bytes")
+                        .isGreaterThan(256);
+                assertThat(new String(pdf, 0, 4))
+                        .as("each PDF must carry the %%PDF magic")
+                        .isEqualTo("%PDF");
+                return pdf.length;
+            });
+            // All identical content → byte sizes should be within a tight
+            // range. We don't lock the exact size (timestamps can drift)
+            // but they shouldn't vary wildly — anything beyond a few
+            // percent points at content corruption.
+            int min = sizes.stream().mapToInt(Integer::intValue).min().orElseThrow();
+            int max = sizes.stream().mapToInt(Integer::intValue).max().orElseThrow();
+            assertThat(max - min)
+                    .as("parallel iteration %d — PDF size variance suggests non-deterministic content (min=%d max=%d)",
+                            iteration, min, max)
+                    .isLessThan(256);
+        }
+    }
+
+    private <T> Set<T> runParallel(Callable<T> task) throws Exception {
+        ExecutorService executor = Executors.newFixedThreadPool(THREAD_COUNT);
+        try {
+            List<Callable<T>> tasks = new ArrayList<>(THREAD_COUNT);
+            for (int i = 0; i < THREAD_COUNT; i++) {
+                tasks.add(task);
+            }
+            List<Future<T>> futures = executor.invokeAll(tasks, TIMEOUT_SECONDS, TimeUnit.SECONDS);
+            Set<T> results = new HashSet<>();
+            for (Future<T> future : futures) {
+                results.add(future.get());
+            }
+            return results;
+        } finally {
+            executor.shutdownNow();
+        }
+    }
+
+    /** Sequential render — returns the layout-graph toString for parity checks. */
+    private String renderLayoutSignature() throws Exception {
+        try (DocumentSession session = GraphCompose.document()
+                .pageSize(400, 400)
+                .margin(DocumentInsets.of(20))
+                .create()) {
+
+            DocumentDsl dsl = session.dsl();
+            dsl.pageFlow()
+                    .name("StressFlow")
+                    .module("Header", module -> module
+                            .paragraph(p -> p.text("Concurrent stress: header")
+                                    .textStyle(DocumentTextStyle.DEFAULT)))
+                    .module("Body", module -> module
+                            .paragraph(p -> p.text("Lorem ipsum dolor sit amet.")
+                                    .textStyle(DocumentTextStyle.DEFAULT))
+                            .paragraph(p -> p.text("Consectetur adipiscing elit.")
+                                    .textStyle(DocumentTextStyle.DEFAULT)))
+                    .build();
+
+            return session.layoutGraph().toString();
+        }
+    }
+
+    /** Sequential render — returns the bytes of a small in-memory PDF. */
+    private byte[] renderPdfBytes() throws Exception {
+        try (DocumentSession session = GraphCompose.document()
+                .pageSize(400, 400)
+                .margin(DocumentInsets.of(20))
+                .create()) {
+
+            DocumentDsl dsl = session.dsl();
+            dsl.pageFlow()
+                    .name("StressFlow")
+                    .module("Body", module -> module
+                            .paragraph(p -> p.text("Concurrent stress: body")
+                                    .textStyle(DocumentTextStyle.DEFAULT)))
+                    .build();
+
+            return session.toPdfBytes();
+        }
+    }
+}