fix(datastreams): correct varint encoding when a group shifts down to 0x80

encodeVarintInto exited its 7-bit loop while the remaining value was still 0x80
and then masked the continuation bit off the final byte, dropping that bit. Every
value whose zig-zag form shifts down to exactly 0x80 encoded as a truncated
number: 64, then a band of 2 ** (exponent - 7) values above every seventh power
of two. Pathway contexts carry millisecond epoch timestamps, which enter the next
affected band on 2039-09-07 and leave it on 2040-03-24.

Author: BridgeAR

packages/dd-trace/src/datastreams/encoding.js

@@ -40,7 +40,7 @@ function encodeVarintInto (target, offset, value) {
   let i = offset
   const limit = offset + maxVarLen64 - 1
   // if first byte is 1, the number is negative in javascript, but we want to interpret it as positive
-  while ((high !== 0 || low < 0 || low > 0x80) && i < limit) {
+  while ((high !== 0 || low < 0 || low > 0x7F) && i < limit) {
     target[i] = (low & 0x7F) | 0x80
     low >>>= 7
     low |= (high & 0x7F) << 25

packages/dd-trace/test/datastreams/encoding.spec.js

@@ -53,6 +53,25 @@ describe('encoding', () => {
       assert.strictEqual(negativeBytes.length, 0)
     })
 
+    it('encoding then decoding should be a no op when a group shifts down to 0x80', () => {
+      // The zig-zag value shifts down to exactly 0x80 for `2 ** exponent` and the
+      // `2 ** (exponent - 7)` values above it. Math.floor(Number.MAX_SAFE_INTEGER / 2)
+      // spans seven 7-bit groups, so there are seven such bands.
+      for (let exponent = 6; exponent <= 48; exponent += 7) {
+        const base = 2 ** exponent
+        const bandTop = base + 2 ** Math.max(exponent - 7, 0) - 1
+
+        for (const n of new Set([base, bandTop, -base, -bandTop])) {
+          const encoded = encodeVarint(n)
+          const [decoded, bytes] = decodeVarint(encoded)
+          assert.strictEqual(decoded, n)
+          assert.strictEqual(bytes.length, 0)
+        }
+      }
+
+      assert.deepStrictEqual([...encodeVarint(2 ** 6)], [128, 1])
+    })
+
     it('encoding a number bigger than Max safe int fails.', () => {
       const n = Number.MAX_SAFE_INTEGER + 10
       const encoded = encodeVarint(n)