Fix decoding of very small Decimal values from NUMERIC data (#603)

When generating a string representation of a PostgresNumeric, we were misinterpreting the weight value for values with very small fractional parts. This caused decoding as Decimal to return incorrect values. Thanks to @patchthecode for reporting this issue!

Author: gwynne

.github/workflows/test.yml

@@ -37,7 +37,7 @@ jobs:
       - name: Install curl for Codecov
         run: apt-get update -y -q && apt-get install -y curl
       - name: Check out package
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Run unit tests with Thread Sanitizer
         run: |
           swift test --filter='^(PostgresNIOTests|ConnectionPoolModuleTests)' --sanitize=thread --enable-code-coverage
@@ -92,15 +92,15 @@ jobs:
           [[ -z "${SWIFT_VERSION}" ]] && SWIFT_VERSION="$(cat /.swift_tag 2>/dev/null || true)"
           printf 'OS:  %s\nTag: %s\nVersion:\n' "${SWIFT_PLATFORM}-${RUNNER_ARCH}" "${SWIFT_VERSION}" && swift --version
       - name: Check out package
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with: { path: 'postgres-nio' }
       - name: Run integration tests
         run: swift test --package-path postgres-nio --filter=^IntegrationTests
       - name: Check out postgres-kit dependent
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with: { repository: 'vapor/postgres-kit', path: 'postgres-kit' }
       - name: Check out fluent-postgres-driver dependent
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with: { repository: 'vapor/fluent-postgres-driver', path: 'fluent-postgres-driver' }
       - name: Use local package in dependents
         run: |
@@ -136,7 +136,6 @@ jobs:
       POSTGRES_PASSWORD: 'test_password'
       POSTGRES_DB: 'postgres'
       POSTGRES_AUTH_METHOD: ${{ matrix.postgres-auth }}
-      POSTGRES_SOCKET: '/tmp/.s.PGSQL.5432'
       POSTGRES_FORMULA: ${{ matrix.postgres-formula }}
     steps:
       - name: Select latest available Xcode
@@ -151,7 +150,7 @@ jobs:
           pg_ctl start --wait
         timeout-minutes: 15
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Run all tests
         run: swift test --enable-code-coverage
       - name: Submit code coverage
@@ -165,7 +164,7 @@ jobs:
     container: swift:noble
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
       # https://github.com/actions/checkout/issues/766

Sources/PostgresNIO/Data/PostgresData+Numeric.swift

@@ -4,7 +4,7 @@ import struct Foundation.Decimal
 public struct PostgresNumeric: CustomStringConvertible, CustomDebugStringConvertible, ExpressibleByStringLiteral {
     /// The number of digits after this metadata
     internal var ndigits: Int16
-    /// How many of the digits are before the decimal point (always add 1)
+    /// How many positions before or after the deicmal point the value is offset by
     internal var weight: Int16
     /// If 0x4000, this number is negative. See NUMERIC_NEG in
     /// https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/numeric.c
@@ -159,8 +159,10 @@ public struct PostgresNumeric: CustomStringConvertible, CustomDebugStringConvert
         let offset: Int16
         if self.weight > 0 {
             offset = (self.weight + 1) - self.ndigits
+        } else if self.weight < 0 {
+            offset = abs(self.weight + 1)
         } else {
-            offset = abs(self.weight) - self.ndigits
+            offset = 0
         }
         if offset > 0 {
             for _ in 0..<offset {

Tests/IntegrationTests/PostgresNIOTests.swift

@@ -619,24 +619,52 @@ final class PostgresNIOTests: XCTestCase {
         let b = PostgresNumeric(string: "-123456.789123")!
         let c = PostgresNumeric(string: "3.14159265358979")!
         let d = PostgresNumeric(string: "1234567898765")!
+        let e = PostgresNumeric(string: "0.00000080216390553684")!
+        let f = PostgresNumeric(string: "0.0000080216390553684")!
+        let g = PostgresNumeric(string: "0.000080216390553684")!
+        let h = PostgresNumeric(string: "802163905536840000.0")!
+        let i = PostgresNumeric(string: "8021639055368400000.0")!
+        let j = PostgresNumeric(string: "80216390553684000000.0")!
+        let k = PostgresNumeric(string: "802163905536840000.000080216390553684")!
         var rows: PostgresQueryResult?
         XCTAssertNoThrow(rows = try conn?.query("""
         select
             $1::numeric as a,
             $2::numeric as b,
             $3::numeric as c,
-            $4::numeric as d
+            $4::numeric as d,
+            $5::numeric as e,
+            $6::numeric as f,
+            $7::numeric as g,
+            $8::numeric as h,
+            $9::numeric as i,
+            $10::numeric as j,
+            $11::numeric as k
         """, [
             .init(numeric: a),
             .init(numeric: b),
             .init(numeric: c),
-            .init(numeric: d)
+            .init(numeric: d),
+            .init(numeric: e),
+            .init(numeric: f),
+            .init(numeric: g),
+            .init(numeric: h),
+            .init(numeric: i),
+            .init(numeric: j),
+            .init(numeric: k)
         ]).wait())
         let row = rows?.first?.makeRandomAccess()
         XCTAssertEqual(row?[data: "a"].decimal, Decimal(string: "123456.789123")!)
         XCTAssertEqual(row?[data: "b"].decimal, Decimal(string: "-123456.789123")!)
         XCTAssertEqual(row?[data: "c"].decimal, Decimal(string: "3.14159265358979")!)
         XCTAssertEqual(row?[data: "d"].decimal, Decimal(string: "1234567898765")!)
+        XCTAssertEqual(row?[data: "e"].decimal, Decimal(string: "0.00000080216390553684")!)
+        XCTAssertEqual(row?[data: "f"].decimal, Decimal(string: "0.0000080216390553684")!)
+        XCTAssertEqual(row?[data: "g"].decimal, Decimal(string: "0.000080216390553684")!)
+        XCTAssertEqual(row?[data: "h"].decimal, Decimal(string: "802163905536840000.0")!)
+        XCTAssertEqual(row?[data: "i"].decimal, Decimal(string: "8021639055368400000.0")!)
+        XCTAssertEqual(row?[data: "j"].decimal, Decimal(string: "80216390553684000000.0")!)
+        XCTAssertEqual(row?[data: "k"].decimal, Decimal(string: "802163905536840000.000080216390553684")!)
     }
 
     func testDecimalStringSerialization() {

Tests/PostgresNIOTests/New/Data/Decimal+PSQLCodableTests.swift

@@ -5,7 +5,7 @@ import NIOCore
 class Decimal_PSQLCodableTests: XCTestCase {
 
     func testRoundTrip() {
-        let values: [Decimal] = [1.1, .pi, -5e-12]
+        let values: [Decimal] = [1.1, .pi, -5e-12, 0.00000080216390553684]
 
         for value in values {
             var buffer = ByteBuffer()