fix(pg-protocol): reset Writer state when error throws from valueMapper in bind()

When valueMapper throws during serialize.bind(), the module-level
singleton Writer instances (writer and paramWriter) are left with
partial data from the interrupted operation. This corrupts all
subsequent serializer calls since they share the same Writer instances.

Add Writer.clear() method that resets the write cursor without
allocating a new buffer (zero overhead on the happy path). Wrap
writeValues() in bind() with try-catch to clear both writers on error.

Author: APshenkin

packages/pg-protocol/src/buffer-writer.ts

@@ -82,4 +82,9 @@ export class Writer {
     this.buffer = Buffer.allocUnsafe(this.size)
     return result
   }
+
+  public clear(): void {
+    this.offset = 5
+    this.headerPosition = 0
+  }
 }

packages/pg-protocol/src/outbound-serializer.test.ts

@@ -273,4 +273,62 @@ describe('serializer', () => {
     const expected = new BufferList().addInt16(1234).addInt16(5678).addInt32(3).addInt32(4).join(true)
     assert.deepEqual(actual, expected)
   })
+
+  describe('bind error recovery', () => {
+    const throwingMapper = () => {
+      throw new Error('valueMapper error')
+    }
+
+    it('produces correct bind output after a valueMapper exception', () => {
+      assert.throws(() => {
+        serialize.bind({
+          values: ['fail'],
+          valueMapper: throwingMapper,
+        })
+      }, /valueMapper error/)
+
+      const actual = serialize.bind({
+        portal: 'bang',
+        statement: 'woo',
+        values: ['1', 'hi', null, 'zing'],
+      })
+      const expectedBuffer = new BufferList()
+        .addCString('bang')
+        .addCString('woo')
+        .addInt16(4)
+        .addInt16(0)
+        .addInt16(0)
+        .addInt16(0)
+        .addInt16(0)
+        .addInt16(4)
+        .addInt32(1)
+        .add(Buffer.from('1'))
+        .addInt32(2)
+        .add(Buffer.from('hi'))
+        .addInt32(-1)
+        .addInt32(4)
+        .add(Buffer.from('zing'))
+        .addInt16(1)
+        .addInt16(0)
+        .join(true, 'B')
+      assert.deepEqual(actual, expectedBuffer)
+    })
+
+    it('produces correct output from other serializer methods after a failed bind', () => {
+      assert.throws(() => {
+        serialize.bind({
+          values: ['fail'],
+          valueMapper: throwingMapper,
+        })
+      }, /valueMapper error/)
+
+      const parseActual = serialize.parse({ text: '!' })
+      const parseExpected = new BufferList().addCString('').addCString('!').addInt16(0).join(true, 'P')
+      assert.deepEqual(parseActual, parseExpected)
+
+      const queryActual = serialize.query('select 1')
+      const queryExpected = new BufferList().addCString('select 1').join(true, 'Q')
+      assert.deepEqual(queryActual, queryExpected)
+    })
+  })
 })

packages/pg-protocol/src/serializer.ts

@@ -152,7 +152,13 @@ const bind = (config: BindOpts = {}): Buffer => {
   writer.addCString(portal).addCString(statement)
   writer.addInt16(len)
 
-  writeValues(values, config.valueMapper)
+  try {
+    writeValues(values, config.valueMapper)
+  } catch (err) {
+    writer.clear()
+    paramWriter.clear()
+    throw err
+  }
 
   writer.addInt16(len)
   writer.add(paramWriter.flush())