fix(NODE-6829): incorrect negative bigint handling (#774)

Author: W-A-James

src/utils/number_utils.ts

@@ -79,14 +79,23 @@ export const NumberUtils: NumberUtils = {
 
   /** Reads a little-endian 64-bit integer from source */
   getBigInt64LE(source: Uint8Array, offset: number): bigint {
-    const lo = NumberUtils.getUint32LE(source, offset);
-    const hi = NumberUtils.getUint32LE(source, offset + 4);
-
-    /*
-      eslint-disable-next-line no-restricted-globals
-      -- This is allowed since this helper should not be called unless bigint features are enabled
-     */
-    return (BigInt(hi) << BigInt(32)) + BigInt(lo);
+    // eslint-disable-next-line no-restricted-globals
+    const hi = BigInt(
+      source[offset + 4] +
+        source[offset + 5] * 256 +
+        source[offset + 6] * 65536 +
+        (source[offset + 7] << 24)
+    ); // Overflow
+
+    // eslint-disable-next-line no-restricted-globals
+    const lo = BigInt(
+      source[offset] +
+        source[offset + 1] * 256 +
+        source[offset + 2] * 65536 +
+        source[offset + 3] * 16777216
+    );
+    // eslint-disable-next-line no-restricted-globals
+    return (hi << BigInt(32)) + lo;
   },
 
   /** Reads a little-endian 64-bit float from source */

test/node/bigint.test.ts

@@ -105,6 +105,46 @@ describe('BSON BigInt support', function () {
 
       it(description, test);
     }
+
+    it('correctly deserializes min 64 bit int (-2n**63n)', function () {
+      expect(
+        BSON.deserialize(Buffer.from('10000000126100000000000000008000', 'hex'), {
+          useBigInt64: true
+        })
+      ).to.deep.equal({ a: -(2n ** 63n) });
+    });
+
+    it('correctly deserializes -1n', function () {
+      expect(
+        BSON.deserialize(Buffer.from('10000000126100FFFFFFFFFFFFFFFF00', 'hex'), {
+          useBigInt64: true
+        })
+      ).to.deep.equal({ a: -1n });
+    });
+
+    it('correctly deserializes 0n', function () {
+      expect(
+        BSON.deserialize(Buffer.from('10000000126100000000000000000000', 'hex'), {
+          useBigInt64: true
+        })
+      ).to.deep.equal({ a: 0n });
+    });
+
+    it('correctly deserializes 1n', function () {
+      expect(
+        BSON.deserialize(Buffer.from('10000000126100010000000000000000', 'hex'), {
+          useBigInt64: true
+        })
+      ).to.deep.equal({ a: 1n });
+    });
+
+    it('correctly deserializes max 64 bit int (2n**63n -1n)', function () {
+      expect(
+        BSON.deserialize(Buffer.from('10000000126100FFFFFFFFFFFFFF7F00', 'hex'), {
+          useBigInt64: true
+        })
+      ).to.deep.equal({ a: 2n ** 63n - 1n });
+    });
   });
 
   describe('BSON.serialize()', function () {