Test conflict between conda and mamba with same base env in CI

.github/workflows/unix_impl.yml

@@ -192,3 +192,49 @@ jobs:
           cd micromamba/test-server
           ./generate_gpg_keys.sh
           ./testserver_pkg_signing.sh
+
+  mamba_conda_root_prefix_tests:
+    name: mamba-conda-root-prefix tests
+    runs-on: ${{ inputs.os }}
+    if: startsWith(inputs.os, 'ubuntu')
+    steps:
+      - name: Checkout mamba repository
+        uses: actions/checkout@v4
+      - name: Create environment with conda
+        uses: mamba-org/setup-micromamba@v1
+        with:
+          environment-name: conda_env
+          create-args: >-
+            conda
+      - name: Create conda environment containing mamba
+        shell: bash -l {0} -euo pipefail -x
+        run: |
+          # Get a recent version of mamba to have it tested (mamba 2.x)
+          conda create -n mamba_in_conda_env mamba=2.0.0rc5 -c conda-forge/label/mamba_dev -y
+      - name: Run tests using conda and a nested mamba
+        shell: bash -l {0} -euo pipefail -x
+        run: |
+          conda info
+          source "$(conda info --base)/etc/profile.d/conda.sh"  # Initialize Conda
+          conda activate mamba_in_conda_env
+          # Testing with conda and mamba having the same root prefix
+          export MAMBA_ROOT_PREFIX="$(conda info --base)"
+          mamba create -n mamba_env_test xtensor -c conda-forge -y
+          mamba env list | grep mamba_env_test || { echo "mamba_env_test not found in env list"; exit 1; }
+          mamba info | grep "base environment" | grep "$(conda info --base)" || { echo "Expected base environment path not found in mamba info"; exit 1; }
+          mamba list -n mamba_env_test | grep xtensor || { echo "xtensor not found in mamba_env_test"; exit 1; }
+          eval "$(mamba shell hook --shell bash)"
+          mamba activate
+          # Since no target prefix has been specified, we should fall back on the root prefix
+          mamba info | grep "environment"  | grep "base (active)" || { echo "Expected base environment not active"; exit 1; }
+          mamba info | grep "env location" | grep "$(conda info --base)" || { echo "Expected environment not active"; exit 1; }
+          # Activate another env - specified
+          mamba activate -n mamba_env_test
+          mamba info | grep "environment"  | grep "mamba_env_test (active)" || { echo "Expected mamba_env_test environment not active"; exit 1; }
+          # Get back to use conda without changing the environment
+          conda info | grep "shell level"  | grep "2" || { echo "Expected conda shell level = 2"; exit 1; }
+          conda info | grep "active environment"  | grep "mamba_env_test" || { echo "Expected conda environment not active"; exit 1; }
+          # Activate another environment using conda
+          conda activate mamba_in_conda_env
+          conda info | grep "shell level"  | grep "3" || { echo "Expected conda shell level = 3"; exit 1; }
+          conda info | grep "active environment"  | grep "mamba_in_conda_env" || { echo "Expected nested conda environment not active"; exit 1; }