Avoid using stderr explicitly (#3089)

Motivation

It's tempting for us to use the FILE * for stderr whenever we want to
print to it. However, the Linux libc modules aren't appropriately
annotated to call that field constant, and indeed it isn't. This causes
Swift to throw a fit if we even touch it.

Modifications

Introduce an awkward function that issues direct writes to stderr
instead of using the FILE *.

Result

Swift is happier.

Author: Lukasa

Sources/NIOPosix/SelectableEventLoop.swift

@@ -19,6 +19,22 @@ import NIOConcurrencyHelpers
 import NIOCore
 import _NIODataStructures
 
+private func printError(_ string: StaticString) {
+    string.withUTF8Buffer { buf in
+        var buf = buf
+        while buf.count > 0 {
+            // 2 is stderr
+            let rc = write(2, buf.baseAddress, buf.count)
+            if rc < 0 {
+                let err = errno
+                if err == EINTR { continue }
+                fatalError("Unexpected error writing: \(err)")
+            }
+            buf = .init(rebasing: buf.dropFirst(Int(rc)))
+        }
+    }
+}
+
 /// Execute the given closure and ensure we release all auto pools if needed.
 @inlinable
 internal func withAutoReleasePool<T>(_ execute: () throws -> T) rethrows -> T {
@@ -438,12 +454,11 @@ internal final class SelectableEventLoop: EventLoop, @unchecked Sendable {
                     if Self.strictModeEnabled {
                         fatalError("Cannot schedule tasks on an EventLoop that has already shut down.")
                     }
-                    fputs(
+                    printError(
                         """
                         ERROR: Cannot schedule tasks on an EventLoop that has already shut down. \
                         This will be upgraded to a forced crash in future SwiftNIO versions.\n
-                        """,
-                        stderr
+                        """
                     )
                     return false
                 }