Improve ExitStatus.description for signals. (#1302)

This PR uses the `sys_signame` array present on many UNIX-like systems
to derive a better description for values of type `ExitStatus` and
`ExitTest.Condition`. (On Linux, the equivalent `sigabbrev_np()` is
used. Windows and WASI don't have an equivalent API.)

Before:

```swift
let s = ExitStatus.signal(SIGABRT)
print(s) // ".signal(12345)"
```

After:

```swift
let s = ExitStatus.signal(SIGABRT)
print(s) // ".signal(SIGABRT)"
```

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/ExitTests/ExitStatus.swift

@@ -96,17 +96,55 @@ public enum ExitStatus: Sendable {
 extension ExitStatus: Equatable {}
 
 // MARK: - CustomStringConvertible
-@_spi(Experimental)
+
+#if os(Linux) && !SWT_NO_DYNAMIC_LINKING
+/// Get the short name of a signal constant.
+///
+/// This symbol is provided because the underlying function was added to glibc
+/// relatively recently and may not be available on all targets. Checking
+/// `__GLIBC_PREREQ()` is insufficient because `_GNU_SOURCE` may not be defined
+/// at the point string.h is first included.
+private let _sigabbrev_np = symbol(named: "sigabbrev_np").map {
+  castCFunction(at: $0, to: (@convention(c) (CInt) -> UnsafePointer<CChar>?).self)
+}
+#endif
+
 #if SWT_NO_PROCESS_SPAWNING
 @available(*, unavailable, message: "Exit tests are not available on this platform.")
 #endif
 extension ExitStatus: CustomStringConvertible {
   public var description: String {
     switch self {
     case let .exitCode(exitCode):
-      ".exitCode(\(exitCode))"
+      return ".exitCode(\(exitCode))"
     case let .signal(signal):
-      ".signal(\(signal))"
+      var signalName: String?
+
+#if SWT_TARGET_OS_APPLE || os(FreeBSD) || os(OpenBSD) || os(Android)
+      // These platforms define sys_signame with a size, which is imported
+      // into Swift as a tuple.
+      withUnsafeBytes(of: sys_signame) { sys_signame in
+        sys_signame.withMemoryRebound(to: UnsafePointer<CChar>.self) { sys_signame in
+          if signal > 0 && signal < sys_signame.count {
+            signalName = String(validatingCString: sys_signame[Int(signal)])?.uppercased()
+          }
+        }
+      }
+#elseif os(Linux)
+#if !SWT_NO_DYNAMIC_LINKING
+      signalName = _sigabbrev_np?(signal).flatMap(String.init(validatingCString:))
+#endif
+#elseif os(Windows) || os(WASI)
+      // These platforms do not have API to get the programmatic name of a
+      // signal constant.
+#else
+#warning("Platform-specific implementation missing: signal names unavailable")
+#endif
+
+      if let signalName {
+        return ".signal(SIG\(signalName) → \(signal))"
+      }
+      return ".signal(\(signal))"
     }
   }
 }

Tests/TestingTests/ExitTestTests.swift

@@ -13,6 +13,15 @@ private import _TestingInternals
 
 #if !SWT_NO_EXIT_TESTS
 @Suite("Exit test tests") struct ExitTestTests {
+  @Test("Signal names are reported (where supported)") func signalName() {
+    let exitStatus = ExitStatus.signal(SIGABRT)
+#if SWT_TARGET_OS_APPLE || os(Linux) || os(FreeBSD) || os(OpenBSD) || os(Android)
+    #expect(String(describing: exitStatus) == ".signal(SIGABRT → \(SIGABRT))")
+#else
+    #expect(String(describing: exitStatus) == ".signal(\(SIGABRT))")
+#endif
+  }
+
   @Test("Exit tests (passing)") func passing() async {
     await #expect(processExitsWith: .failure) {
       exit(EXIT_FAILURE)