refactor: improve cbExtension comment and test coverage

- Clarify cbExtension comment to explicitly state it's the size of the
  ibFeatureExtLong pointer (4 bytes), not the FeatureExt data
- Extend test to validate all variable-length fields when checking
  FeatureExt positioning, ensuring complete coverage per MS-TDS spec

Author: claude

src/login7-payload.ts

@@ -256,13 +256,14 @@ class Login7Payload {
     offset = fixedData.writeUInt16LE(dataOffset, offset);
 
     // (cchUnused / cbExtension): 2-byte
-    // For TDS 7.4+, this is the size of the extension block (just the 4-byte offset pointer).
+    // For TDS 7.4+, this is the size of the ibFeatureExtLong offset pointer (4 bytes).
+    // The actual FeatureExt data is appended at the end of the packet, not here.
     // We'll store the FeatureExt data to append at the end after all other variable data.
     let featureExtData: Buffer | undefined;
     let extensionOffsetBuffer: Buffer | undefined;
     if (this.tdsVersion >= versions['7_4']) {
       featureExtData = this.buildFeatureExt();
-      // cbExtension = 4 (just the ibFeatureExtLong pointer size)
+      // cbExtension = 4 (size of the ibFeatureExtLong pointer, not the FeatureExt data)
       offset = fixedData.writeUInt16LE(4, offset);
       // Reserve space for the 4-byte offset pointer; we'll fill in the actual offset later
       extensionOffsetBuffer = Buffer.alloc(4);

test/unit/login7-payload-test.ts

@@ -346,17 +346,39 @@ describe('Login7Payload', function() {
         // 86-89: ibChangePassword/cchChangePassword, 90-93: cbSSPILong
         const ibHostName = data.readUInt16LE(36);
         const cchHostName = data.readUInt16LE(38);
+        const ibUserName = data.readUInt16LE(40);
+        const cchUserName = data.readUInt16LE(42);
+        const ibPassword = data.readUInt16LE(44);
+        const cchPassword = data.readUInt16LE(46);
+        const ibAppName = data.readUInt16LE(48);
+        const cchAppName = data.readUInt16LE(50);
+        const ibServerName = data.readUInt16LE(52);
+        const cchServerName = data.readUInt16LE(54);
+        const ibCltIntName = data.readUInt16LE(60);
+        const cchCltIntName = data.readUInt16LE(62);
+        const ibLanguage = data.readUInt16LE(64);
+        const cchLanguage = data.readUInt16LE(66);
         const ibDatabase = data.readUInt16LE(68);
         const cchDatabase = data.readUInt16LE(70);
+        const ibSSPI = data.readUInt16LE(78);
+        const cbSSPI = data.readUInt16LE(80);
         const ibAttachDBFile = data.readUInt16LE(82);
         const cchAttachDBFile = data.readUInt16LE(84);
         const ibChangePassword = data.readUInt16LE(86);
         const cchChangePassword = data.readUInt16LE(88);
 
         // Calculate the end of all regular variable data (excluding FeatureExt)
+        // Include all variable-length fields to ensure complete coverage
         const variableDataEnd = Math.max(
           ibHostName + cchHostName * 2,
+          ibUserName + cchUserName * 2,
+          ibPassword + cchPassword * 2,
+          ibAppName + cchAppName * 2,
+          ibServerName + cchServerName * 2,
+          ibCltIntName + cchCltIntName * 2,
+          ibLanguage + cchLanguage * 2,
           ibDatabase + cchDatabase * 2,
+          ibSSPI + cbSSPI,
           ibAttachDBFile + cchAttachDBFile * 2,
           ibChangePassword + cchChangePassword * 2
         );