Rework erlang version constraints

When the erlang_major and erlang_minor constraint settings were folded
into erlang_version, selects.config_setting_group instances were used
to define platforms for an erlang major with any available minor.

Unfortunately in some cases this causes a cycle in the analysis graph
to fail the analysis phase of some builds (probably depending on a
host erlang's degree of overlap with internal
erlangs). selects.config_setting_group expands to a chain of `alias`
instances, where a `select` statement chooses the actual
value. How exactly this causes a (potential) cycle isn't fully clear,
but this commit works around the issue by declaring extra toolchains
in a manner that allows the same match any minor without the use of
selects.

Author: HoloRin

MODULE.bazel

@@ -37,4 +37,5 @@ use_repo(
 
 register_toolchains(
     "@erlang_config//external:toolchain",
+    "@erlang_config//external:toolchain2",
 )

repositories/BUILD_external.tpl

@@ -27,7 +27,20 @@ toolchain(
         "//:erlang_external",
     ],
     target_compatible_with = [
-        "%{TARGET_COMPATIBLE_WITH}",
+        "//:erlang_%{ERLANG_MAJOR}",
+    ],
+    toolchain = ":erlang",
+    toolchain_type = "%{RULES_ERLANG_WORKSPACE}//tools:toolchain_type",
+    visibility = ["//visibility:public"],
+)
+
+toolchain(
+    name = "toolchain2",
+    exec_compatible_with = [
+        "//:erlang_external",
+    ],
+    target_compatible_with = [
+        "//:erlang_%{ERLANG_MAJOR}_%{ERLANG_MINOR}",
     ],
     toolchain = ":erlang",
     toolchain_type = "%{RULES_ERLANG_WORKSPACE}//tools:toolchain_type",

repositories/BUILD_internal.tpl

@@ -29,9 +29,22 @@ toolchain(
         "//:erlang_internal",
     ],
     target_compatible_with = [
-        "%{TARGET_COMPATIBLE_WITH}",
+        "//:erlang_%{ERLANG_MAJOR}",
     ],
     toolchain = ":erlang",
     toolchain_type = "%{RULES_ERLANG_WORKSPACE}//tools:toolchain_type",
     visibility = ["//visibility:public"],
 )
+
+toolchain(
+    name = "toolchain2",
+    exec_compatible_with = [
+        "//:erlang_internal",
+    ],
+    target_compatible_with = [
+        "//:erlang_%{ERLANG_MAJOR}_%{ERLANG_MINOR}",
+    ],
+    toolchain = ":erlang",
+    toolchain_type = "%{RULES_ERLANG_WORKSPACE}//tools:toolchain_type",
+    visibility = ["//visibility:public"],
+)

repositories/erlang_config.bzl

@@ -17,7 +17,7 @@ def _parse_maybe_semver(version_string):
     if len(parts) > 1:
         return (parts[0], parts[1])
     else:
-        return (parts[0], None)
+        return (parts[0], _ERLANG_VERSION_UNKNOWN.lower())
 
 def _impl(repository_ctx):
     rules_erlang_workspace = repository_ctx.attr.rules_erlang_workspace
@@ -42,19 +42,15 @@ def _impl(repository_ctx):
         )
 
     for (name, props) in erlang_installations.items():
-        if props.minor != None:
-            target_compatible_with = "//:erlang_{}_{}".format(props.major, props.minor)
-        else:
-            target_compatible_with = "//:erlang_{}".format(props.major)
-
         if props.type == INSTALLATION_TYPE_EXTERNAL:
             repository_ctx.template(
                 "{}/BUILD.bazel".format(name),
                 Label("//repositories:BUILD_external.tpl"),
                 {
                     "%{ERLANG_HOME}": props.erlang_home,
                     "%{ERLANG_VERSION}": props.version,
-                    "%{TARGET_COMPATIBLE_WITH}": target_compatible_with,
+                    "%{ERLANG_MAJOR}": props.major,
+                    "%{ERLANG_MINOR}": props.minor,
                     "%{RULES_ERLANG_WORKSPACE}": rules_erlang_workspace,
                 },
                 False,
@@ -68,7 +64,8 @@ def _impl(repository_ctx):
                     "%{URL}": props.url,
                     "%{STRIP_PREFIX}": props.strip_prefix or "",
                     "%{SHA_256}": props.sha256 or "",
-                    "%{TARGET_COMPATIBLE_WITH}": target_compatible_with,
+                    "%{ERLANG_MAJOR}": props.major,
+                    "%{ERLANG_MINOR}": props.minor,
                     "%{RULES_ERLANG_WORKSPACE}": rules_erlang_workspace,
                 },
                 False,
@@ -83,10 +80,12 @@ def _impl(repository_ctx):
         False,
     )
 
-    toolchains = [
-        "@{}//{}:toolchain".format(repository_ctx.name, name)
-        for name in erlang_installations.keys()
-    ]
+    toolchains = []
+    for name in erlang_installations.keys():
+        toolchains.extend([
+            "@{}//{}:toolchain".format(repository_ctx.name, name),
+            "@{}//{}:toolchain2".format(repository_ctx.name, name),
+        ])
 
     repository_ctx.template(
         "defaults.bzl",
@@ -186,25 +185,13 @@ def _default_erlang_dict(repository_ctx):
                 type = INSTALLATION_TYPE_EXTERNAL,
                 version = _ERLANG_VERSION_UNKNOWN,
                 major = _ERLANG_VERSION_UNKNOWN.lower(),
-                minor = None,
+                minor = _ERLANG_VERSION_UNKNOWN.lower(),
                 erlang_home = erlang_home,
             ),
         }
 
 def _build_file_content(erlang_installations):
-    default_installation = erlang_installations[_DEFAULT_EXTERNAL_ERLANG_PACKAGE_NAME]
-
-    if default_installation.minor != None:
-        default_identifier = "{}_{}".format(
-            default_installation.major,
-            default_installation.minor,
-        )
-    else:
-        default_identifier = default_installation.major
-
     build_file_content = """\
-load("@bazel_skylib//lib:selects.bzl", "selects")
-
 package(
     default_visibility = ["//visibility:public"],
 )
@@ -214,44 +201,37 @@ constraint_setting(
     default_constraint_value = ":erlang_external",
 )
 
-constraint_setting(
-    name = "erlang_version",
-    default_constraint_value = ":erlang_{default_identifier}",
-)
-
 constraint_value(
     name = "erlang_external",
     constraint_setting = ":erlang_internal_external",
 )
 
-""".format(
-        default_identifier = default_identifier,
-    )
-
-    external_installations = {
-        name: props
-        for (name, props) in erlang_installations.items()
-        if props.type == INSTALLATION_TYPE_EXTERNAL
-    }
-    if len(erlang_installations) > len(external_installations):
-        build_file_content += """\
 constraint_value(
     name = "erlang_internal",
     constraint_setting = ":erlang_internal_external",
 )
 
 """
 
+    default_installation = erlang_installations[_DEFAULT_EXTERNAL_ERLANG_PACKAGE_NAME]
+
+    build_file_content += """\
+constraint_setting(
+    name = "erlang_version",
+    default_constraint_value = ":erlang_{}_{}",
+)
+
+""".format(default_installation.major, default_installation.minor)
+
     major_by_minors = {}
     for (name, props) in erlang_installations.items():
         minors = major_by_minors.get(props.major, [])
-        if props.minor != None and props.minor not in minors:
+        if props.minor not in minors:
             minors.append(props.minor)
         major_by_minors[props.major] = minors
 
     for (major, minors) in major_by_minors.items():
-        if len(minors) == 0:
-            build_file_content += """\
+        build_file_content += """\
 constraint_value(
     name = "erlang_{major}",
     constraint_setting = ":erlang_version",
@@ -264,16 +244,10 @@ platform(
     ],
 )
 
-""".format(
-                major = major,
-            )
-        else:
-            constraints = []
-            for minor in minors:
-                constraints.append(
-                    ":erlang_{}_{}".format(major, minor),
-                )
-                build_file_content += """\
+""".format(major = major)
+
+        for minor in minors:
+            build_file_content += """\
 constraint_value(
     name = "erlang_{major}_{minor}",
     constraint_setting = ":erlang_version",
@@ -288,22 +262,4 @@ platform(
 
 """.format(major = major, minor = minor)
 
-            build_file_content += """\
-selects.config_setting_group(
-    name = "erlang_{major}",
-    match_any = {constraints},
-)
-
-platform(
-    name = "erlang_{major}_platform",
-    constraint_values = [
-        ":erlang_{major}",
-    ],
-)
-
-""".format(
-                major = major,
-                constraints = constraints,
-            )
-
     return build_file_content

test/MODULE.bazel

@@ -28,4 +28,5 @@ use_repo(
 
 register_toolchains(
     "@erlang_config//internal:toolchain",
+    "@erlang_config//internal:toolchain2",
 )