Fix for s2n_prelude.h Changes (#299)

Author: waahm7

.github/workflows/s2n-prelude-changes.yml

@@ -0,0 +1,32 @@
+# Detect changes to s2n_prelude.h to update our `Package.swift` and stay in sync with it.
+# See: https://github.com/awslabs/aws-crt-swift/pull/299 for updating the Package.swift.
+
+name: s2n_prelude.h Change Detector
+
+on: [push]
+
+jobs:
+  check-for-changes:
+
+    runs-on: ubuntu-24.04 # latest
+
+    steps:
+    - name: Checkout Sources
+      uses: actions/checkout@v4
+      with:     
+        submodules: true 
+
+    - name: Check s2n_prelude.h
+      run: |
+        TMPFILE=$(mktemp)
+          echo "116f1525acbc94c91b0ee2ea2af9fdef aws-common-runtime/s2n/utils/s2n_prelude.h" > $TMPFILE
+        md5sum --check $TMPFILE
+
+    # No further steps if successful
+
+    - name: Echo fail
+      if: failure()
+      run: |
+          echo "The aws-crt-swift has a hack to manually define macros which are defined in s2n_prelude.h in Package.Swift.
+                This check will fail whenever s2n_prelude.h is updated by the S2N team. You should make sure that Package.Swift is updated accordingly
+                with the s2n_prelude.h changes and then run `md5sum aws-common-runtime/s2n/utils/s2n_prelude.h` and update the value above."

Package.swift

@@ -100,8 +100,17 @@ packageTargets.append(.target(
     publicHeadersPath: "api",
     cSettings: [
         .headerSearchPath("./"),
-        .define("POSIX_C_SOURCE=200809L"),
-        .define("S2N_NO_PQ")
+        .define("S2N_NO_PQ"),
+        // This is a hack to get around the fact that S2N uses the compiler option `-include`
+        // to include `s2n_prelude.h` in all .c files. Since SwiftPM doesn't support compiler flags,
+        // we manually define the macros from `s2n_prelude.h`. When SwiftPM supports compiler flags
+        // or building packages using CMake, this hack should be removed.
+        // We are not defining `S2N_API` because we don't need to expose any symbols from S2N in crt-swift.
+        .define("_S2N_PRELUDE_INCLUDED"),
+        .define("S2N_BUILD_RELEASE"),
+        .define("_FORTIFY_SOURCE", to: "2"),
+        .define("POSIX_C_SOURCE", to: "200809L"),
+
     ]
 ))
 #endif
@@ -146,9 +155,6 @@ var awsCChecksumsExcludes = [
     "cmake",
     "tests"]
 
-// swift never uses Microsoft Visual C++ compiler
-awsCChecksumsExcludes.append("source/intel/visualc")
-
 // Hardware accelerated checksums are disabled because SwiftPM doesn't like the necessary compiler flags.
 // We can add it once SwiftPM has the necessary support for CPU flags or builds C libraries
 // using CMake.