diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 22652f1..9d4a05e 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -80,13 +80,15 @@ jobs:
   # E00-S03-T04: the static assertions of tests/database-postgres-lock.test.mjs
   # gate every PR — the suite locks in the migration advisory lock (session-
   # scoped pg_advisory_lock/pg_try_advisory_lock over a stable keyed hash on a
-  # dedicated connection, driver-boundary re-export) with mutation probes, and
-  # the docker-gated real-stack concurrent probe (a second runner waits or
-  # fails while the first holds the lock; the lock releases when the holding
-  # session ends) runs where a Docker daemon is available and skips cleanly
-  # otherwise. The job installs the frozen workspace because the real-stack
-  # probe executes the committed lock module from the host (it imports `pg`
-  # through the package's own links).
+  # dedicated connection, re-entrant-safe in-flight acquire so concurrent
+  # acquire() calls share one connection, driver-boundary re-export) with
+  # mutation probes, and the docker-gated real-stack concurrent probe (a
+  # second runner waits or fails while the first holds the lock; concurrent
+  # acquire() checks out exactly one connection; the lock releases when the
+  # holding session ends) runs where a Docker daemon is available and skips
+  # cleanly otherwise. The job installs the frozen workspace because the
+  # real-stack probe executes the committed lock module from the host (it
+  # imports `pg` through the package's own links).
   database-postgres-lock:
     name: Migration advisory lock (E00-S03-T04)
     runs-on: ubuntu-latest
diff --git a/packages/database-postgres/src/lock.ts b/packages/database-postgres/src/lock.ts
index 88a3def..413b352 100644
--- a/packages/database-postgres/src/lock.ts
+++ b/packages/database-postgres/src/lock.ts
@@ -47,6 +47,8 @@ export class MigrationLock {
   private readonly pool: Pool;
   private client: PoolClient | null = null;
   private held = false;
+  /** In-flight acquire, memoized so concurrent acquire() calls share one connection (re-entrant-safe). */
+  private acquireInFlight: Promise<void> | null = null;
 
   /** @param pool The package-owned PostgreSQL pool (`pg.Pool`). */
   constructor(pool: Pool) {
@@ -61,12 +63,30 @@ export class MigrationLock {
   /**
    * Acquires the migration advisory lock, blocking until it is free — a
    * second runner waits here while the first holds the lock. Idempotent:
-   * acquiring an already-held instance is a no-op. The lock is held on a
-   * dedicated connection until `release()` (or until the session ends — e.g.
-   * the runner exits and the pool closes its connections).
+   * acquiring an already-held instance is a no-op. Re-entrant-safe:
+   * concurrent `acquire()` calls on the same instance share the single
+   * in-flight acquire (memoized in `acquireInFlight`), so exactly one
+   * connection is checked out and no locked connection leaks. The lock is
+   * held on a dedicated connection until `release()` (or until the session
+   * ends — e.g. the runner exits and the pool closes its connections).
    */
   async acquire(): Promise<void> {
     if (this.held) return;
+    // A second concurrent acquire() on this instance returns the in-flight
+    // acquire instead of checking out another connection: exactly one
+    // connection is checked out and no locked connection leaks.
+    if (this.acquireInFlight !== null) return this.acquireInFlight;
+    const inFlight = this.doAcquire();
+    this.acquireInFlight = inFlight;
+    try {
+      await inFlight;
+    } finally {
+      this.acquireInFlight = null;
+    }
+  }
+
+  /** The memoized acquire body: checks out one dedicated connection and takes the session-scoped lock on it. */
+  private async doAcquire(): Promise<void> {
     const client = await this.pool.connect();
     try {
       await client.query(
diff --git a/tests/database-postgres-lock.test.mjs b/tests/database-postgres-lock.test.mjs
index 8170a7b..1b0dabe 100644
--- a/tests/database-postgres-lock.test.mjs
+++ b/tests/database-postgres-lock.test.mjs
@@ -20,6 +20,14 @@
  *     session that takes the lock and then closes (the runner exits)
  *     releases it — the issue's rollback note ("the lock releases when the
  *     runner exits").
+ *   - "concurrent acquire() calls on the same lock instance are
+ *     re-entrant-safe" → `acquire()` memoizes the in-flight acquire
+ *     (`acquireInFlight`): a second concurrent `acquire()` on the same
+ *     instance returns the in-flight acquire instead of checking out another
+ *     connection, so exactly one connection is checked out and no locked
+ *     connection leaks; the real-stack probe proves it behaviorally (two
+ *     concurrent acquire() calls → `pool.totalCount` stays 1, one granted
+ *     advisory lock, none left after release).
  *   - the lock is part of the driver boundary: `src/index.ts` re-exports
  *     `MigrationLock` + `MIGRATION_LOCK_KEY`, so no other package needs the
  *     `pg` driver to lock migration state.
@@ -113,6 +121,33 @@ function assertNonBlockingAcquire(src) {
   assert.match(src, /return false;/, 'tryAcquire() must resolve false when another runner holds the lock');
 }
 
+/**
+ * Asserts the re-entrancy guard: `acquire()` memoizes the in-flight acquire
+ * (`acquireInFlight`) so a concurrent `acquire()` on the same instance
+ * returns the same in-flight promise instead of checking out another
+ * connection — exactly one connection is checked out and no locked
+ * connection leaks. The memo is cleared once the acquire settles, so later
+ * acquire() calls behave normally.
+ */
+function assertReentrantAcquire(src) {
+  assert.match(
+    src,
+    /private acquireInFlight: Promise<void> \| null = null;/,
+    'the lock module must memoize the in-flight acquire (acquireInFlight field)',
+  );
+  const acquireBody = src.slice(src.indexOf('async acquire()'), src.indexOf('async tryAcquire()'));
+  assert.match(
+    acquireBody,
+    /if \(this\.acquireInFlight !== null\) return this\.acquireInFlight;/,
+    'a concurrent acquire() on the same instance must return the in-flight acquire (memoized) instead of checking out another connection',
+  );
+  assert.match(
+    acquireBody,
+    /this\.acquireInFlight = null;/,
+    'the in-flight acquire memo must be cleared once the acquire settles',
+  );
+}
+
 /**
  * Asserts the session-scoped release: `release()` unlocks the holding session
  * (`SELECT pg_advisory_unlock(hashtextextended($1, 0))`) BEFORE returning the
@@ -307,16 +342,47 @@ try {
     await sleep(100);
   }
 
-  console.log('LOCK_PROBE_RESULT ' + JSON.stringify({
-    baseline,
-    heldCount,
-    secondFailsFast,
-    observedWait,
-    secondAcquiredAfterRelease,
-    countAfterRelease,
-    heldWhileSessionOpen,
-    thirdAcquiresAfterSessionEnd,
-  }));
+  // Re-entrant-safe acquire (acceptance criterion: concurrent acquire() on
+  // the same lock instance is re-entrant-safe — the in-flight acquire is
+  // memoized so exactly one connection is checked out and no locked
+  // connection leaks). A fresh pool so the connection counts are exact: two
+  // concurrent acquire() calls must share one in-flight acquire — one
+  // connection checked out (pool.totalCount stays 1), the lock granted once,
+  // and nothing left held after release.
+  let reentrantPool = null;
+  try {
+    reentrantPool = new Pool({ connectionString: process.env.DATABASE_URL });
+    const lockR = new MigrationLock(reentrantPool);
+    await Promise.all([lockR.acquire(), lockR.acquire()]);
+    const reentrantPoolTotal = reentrantPool.totalCount;
+    const reentrantPoolIdle = reentrantPool.idleCount;
+    const reentrantHeld = lockR.isHeld;
+    const reentrantHeldCount = await advisoryCount();
+    await lockR.release();
+    const reentrantCountAfterRelease = await advisoryCount();
+    await reentrantPool.end();
+    reentrantPool = null;
+
+    console.log('LOCK_PROBE_RESULT ' + JSON.stringify({
+      baseline,
+      heldCount,
+      secondFailsFast,
+      observedWait,
+      secondAcquiredAfterRelease,
+      countAfterRelease,
+      heldWhileSessionOpen,
+      thirdAcquiresAfterSessionEnd,
+      reentrantPoolTotal,
+      reentrantPoolIdle,
+      reentrantHeld,
+      reentrantHeldCount,
+      reentrantCountAfterRelease,
+    }));
+  } finally {
+    if (reentrantPool !== null) {
+      await reentrantPool.end().catch(() => {});
+    }
+  }
 } finally {
   await pool.end();
 }
@@ -340,6 +406,10 @@ test('tryAcquire() fails fast while another runner holds the lock (a second runn
   assertNonBlockingAcquire(read(LOCK_SRC));
 });
 
+test('concurrent acquire() on the same instance is re-entrant-safe (in-flight acquire memoized, exactly one connection)', () => {
+  assertReentrantAcquire(read(LOCK_SRC));
+});
+
 test('release() unlocks the holding session and returns the connection to the pool', () => {
   assertSessionScopedRelease(read(LOCK_SRC));
 });
@@ -483,6 +553,37 @@ test('concurrent migration runners are serialized by the advisory lock (real sta
       true,
       'the lock must be acquirable again after the holding session closes (the runner exits)',
     );
+
+    // "concurrent acquire() on the same lock instance is re-entrant-safe":
+    // two concurrent acquire() calls share one in-flight acquire — exactly
+    // one connection is checked out (pool.totalCount stays 1), the lock is
+    // granted once, and nothing is left held after release (no locked
+    // connection leaks).
+    assert.equal(
+      result.reentrantPoolTotal,
+      1,
+      `two concurrent acquire() calls must check out exactly one connection (pool.totalCount ${result.reentrantPoolTotal})`,
+    );
+    assert.equal(
+      result.reentrantPoolIdle,
+      0,
+      `the re-entrant acquire must hold its connection checked out while the lock is held (pool.idleCount ${result.reentrantPoolIdle})`,
+    );
+    assert.equal(
+      result.reentrantHeld,
+      true,
+      'the re-entrant acquire must leave the instance holding the lock',
+    );
+    assert.equal(
+      result.reentrantHeldCount,
+      result.baseline + 1,
+      `two concurrent acquire() calls must grant the advisory lock once (baseline ${result.baseline}, got ${result.reentrantHeldCount})`,
+    );
+    assert.equal(
+      result.reentrantCountAfterRelease,
+      result.baseline,
+      `no advisory lock may remain granted after the re-entrant instance releases (got ${result.reentrantCountAfterRelease})`,
+    );
   } finally {
     compose(['down', '-v'], { timeout: 120_000 });
     rmSync(probeFile, { force: true });
@@ -521,6 +622,13 @@ test('removing pg_advisory_unlock makes the session-release criterion fail (muta
   assert.throws(() => assertSessionScopedRelease(withoutUnlock), /pg_advisory_unlock/);
 });
 
+test('removing the in-flight acquire memo makes the re-entrancy criterion fail (mutation probe)', () => {
+  const src = read(LOCK_SRC);
+  const withoutMemo = src.replace(/if \(this\.acquireInFlight !== null\) return this\.acquireInFlight;\n/, '');
+  assert.notEqual(withoutMemo, src, 'the mutation must actually remove the in-flight acquire guard');
+  assert.throws(() => assertReentrantAcquire(withoutMemo), /in-flight acquire/);
+});
+
 test('dropping the lock re-export from the boundary fails the boundary criterion (mutation probe)', () => {
   const src = read(INDEX_SRC);
   const withoutReexport = src.replace(/export \{ MigrationLock, MIGRATION_LOCK_KEY \} from '\.\/lock\.js';\n/, '');