Fix crashes in GCC 4.x shared-libs builds, due to recent symbol visib…

…ility changes (#675)

**Issue:**
When building with GCC 4.8 and 4.9 and `-DBUILD_SHARED_LIBS=ON`, many tests were crashing. The crashes would say something like "free(): invalid pointer" and the stack trace would point into the c++ string headers.

**Things we tried**
- Crash stopped if we commented out the [recently-added line in CMakeLists.txt](https://github.com/awslabs/aws-crt-cpp/blob/80f4736126e14223a0c3882d3b734607143731f3/CMakeLists.txt#L305) line that sets `CXX_VISIBILITY_PRESET hidden`. So obviously symbol visibility has something to do with it.
- We tried adding adding `AWS_CRT_CPP_API` to the templated `StlAllocator` class (see edfed14), which stopped GCC 4.x from crashing, but then the [Windows compiler didn't like that](https://github.com/awslabs/aws-crt-cpp/actions/runs/11636848409/job/32409003392#step:2:2373).

**Description of Changes:** 
Don't set visibility to hidden for GCC < 5. That's how we had it before. GCC 4.x is very old. This isn't the first time it's disappointed us

.github/workflows/ci.yml

@@ -17,6 +17,11 @@ env:
   AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
   AWS_REGION: us-east-1
 
+# cancel in-progress builds after a new commit
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   linux-compat-use-openssl:
     runs-on: ubuntu-22.04 # latest
@@ -128,12 +133,15 @@ jobs:
 
   linux-shared-libs:
     runs-on: ubuntu-22.04 # latest
+    strategy:
+      matrix:
+        compiler: [gcc-4.8, gcc-11] # oldest, latest
     steps:
         # We can't use the `uses: docker://image` version yet, GitHub lacks authentication for actions -> packages
     - name: Build ${{ env.PACKAGE_NAME }}
       run: |
         aws s3 cp s3://aws-crt-test-stuff/ci/${{ env.BUILDER_VERSION }}/linux-container-ci.sh ./linux-container-ci.sh && chmod a+x ./linux-container-ci.sh
-        ./linux-container-ci.sh ${{ env.BUILDER_VERSION }} aws-crt-${{ env.LINUX_BASE_IMAGE }} build -p ${{ env.PACKAGE_NAME }} --cmake-extra=-DBUILD_SHARED_LIBS=ON
+        ./linux-container-ci.sh ${{ env.BUILDER_VERSION }} aws-crt-${{ env.LINUX_BASE_IMAGE }} build -p ${{ env.PACKAGE_NAME }} --compiler=${{ matrix.compiler }} --cmake-extra=-DBUILD_SHARED_LIBS=ON
 
   linux-openssl-static:
     runs-on: ubuntu-22.04 # latest

CMakeLists.txt

@@ -302,7 +302,13 @@ endif()
 
 set_target_properties(${PROJECT_NAME} PROPERTIES LINKER_LANGUAGE CXX)
 set_target_properties(${PROJECT_NAME} PROPERTIES CXX_STANDARD ${CMAKE_CXX_STANDARD})
-set_target_properties(${PROJECT_NAME} PROPERTIES CXX_VISIBILITY_PRESET hidden VISIBILITY_INLINES_HIDDEN ON)
+
+# Hide symbols by default
+# Except for ancient GCC, because it leads to crashes in shared-lib builds
+# see: https://github.com/awslabs/aws-crt-cpp/pull/675
+if(NOT (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0"))
+    set_target_properties(${PROJECT_NAME} PROPERTIES CXX_VISIBILITY_PRESET hidden VISIBILITY_INLINES_HIDDEN ON)
+endif()
 
 aws_prepare_symbol_visibility_args(${PROJECT_NAME} "AWS_CRT_CPP")