Better support for multi-compiler environments.

.github/workflows/lint.yaml

@@ -16,13 +16,10 @@ jobs:
     - name: Install Tools
       run: |
         python -m pip install --upgrade pip
-        python -m pip install black flake8 isort mypy
+        python -m pip install black flake8 mypy
     - name: Black
       run: |
         black --check --verbose stackinator unittests
-    - name: isort
-      run: |
-        isort --check --skip stackinator/repo --diff .
     - name: flake8
       run: |
         flake8 --count --show-source --statistics .

docs/recipes.md

@@ -101,7 +101,7 @@ The `compiler` field describes a list compilers to use to build the software sta
 Each compiler toolchain is specified using toolchain and spec
 
 ```yaml title="compile all packages with [email protected]"
-  compiler
+  compiler:
   - toolchain: gcc
     spec: [email protected]
 ```
@@ -110,20 +110,37 @@ Sometimes two compiler toolchains are required, for example when using the `nvhp
 The example below uses the `nvhpc` compilers with [email protected].
 
 ```yaml title="compile all packages with [email protected]"
-  compiler
-  - toolchain: llvm
-    spec: [email protected]
+  compiler:
   - toolchain: gcc
     spec: [email protected]
+  - toolchain: llvm
+    spec: [email protected]
 ```
 
 !!! note
     If more than one version of gcc has been installed, use the same version that was used to install `nvhpc`.
 
 !!! warning
-    As a rule, use a single compiler wherever possible - keep it simple!
+    Stackinator does not test or support using two versions of gcc in the same toolchain.
 
-    We don't test or support using two versions of gcc in the same toolchain.
+The order of the compilers is significant. The first compiler is the default, and the other compilers will only be used to build packages when explicitly added to a spec.
+For example, in the recipe below, only `netcdf-fortran` will be built with the `nvhpc` toolchain, while the root specs `cmake` and `netcdf-c` and all dependencies will be built using the `gcc` toolchain.
+
+
+```yaml title="compile all packages with [email protected]"
+  compiler:
+  - toolchain: gcc
+    spec: gcc
+  - toolchain: llvm
+    spec: nvhpc
+  specs
+  - cmake
+  - netcdf-c
+  - netcdf-fortran%nvhpc
+```
+
+!!! note
+    This approach is typically used to build Fortran applications and packages with one toolchain (e.g. `nvhpc`), and all of the C/C++ dependencies with a different toolchain (e.g. `gcc`).
 
 ### MPI
 

stackinator/recipe.py

@@ -293,6 +293,20 @@ def generate_environment_specs(self, raw):
                         # TODO: Create a custom exception type
                         raise Exception(f"Unsupported mpi: {mpi_impl}")
 
+        # set constraints that ensure the the main compiler is always used to build packages
+        # that do not explicitly request a compiler.
+        for name, config in environments.items():
+            compilers = config["compiler"]
+            if len(compilers) == 1:
+                config["toolchain_constraints"] = []
+                continue
+            requires = [f"%{compilers[0]['spec']}"]
+            for spec in config["specs"]:
+                if "%" in spec:
+                    requires.append(spec)
+
+            config["toolchain_constraints"] = requires
+
         # An awkward hack to work around spack not supporting creating activation
         # scripts for each file system view in an environment: it only generates them
         # for the "default" view.

stackinator/templates/environments.spack.yaml

@@ -1,4 +1,3 @@
-{% set separator = joiner(', ') %}
 spack:
   include:
 {% if config.packages %}
@@ -13,20 +12,22 @@ spack:
     reuse: false
   specs:
 {% for spec in config.specs %}
-  - {{ spec }}
+  - '{{ spec }}'
 {% endfor %}
   packages:
     all:
-      compiler: [{% for c in config.compiler %}{{ separator() }}{{ c.spec }}{% endfor %}]
+{% set separator = joiner(', ') %}
+      compiler: [{% for c in config.compiler %}{{ separator() }}'{{ c.spec }}'{% endfor %}]
+      require:
+{% set separator = joiner(', ') %}
+      - one_of: [{% for c in config.toolchain_constraints %}{{ separator() }}'{{ c }}'{% endfor %}]
     {% if config.variants %}
-      variants:
-      {% for variant in config.variants %}
-      - {{ variant }}
-      {% endfor %}
+{% set separator = joiner(', ') %}
+      variants: [{% for v in config.variants %}{{ separator() }}'{{ v }}'{% endfor %}]
     {% endif %}
     {% if config.mpi.spec %}
     mpi:
-      require: {{ config.mpi.spec }}
+      require: '{{ config.mpi.spec }}'
     {% endif %}
 {% if config.view %}
   view: