Add new setting //go/toolchain:sdk_name to allow the selection of a specific Go toolchain (#4242)

**What type of PR is this?**

Feature

**What does this PR do? Why is it needed?**

This commit adds a new setting
--@io_bazel_rules_go//go/toolchain:sdk_name which can be used to select
a registered Go toolchain via the name provided in go_download_sdk,
go_wrap_sdk, etc.

This fixes the problem of selecting a toolchain where the same Go
version is used, but different experiements are enabled or patches
applied.

**Which issues(s) does this PR fix?**

Fixes #4240

**Other notes for review**

Works also as a workaround for #3582

Author: bluec0re

go/private/go_toolchain.bzl

@@ -123,6 +123,7 @@ def declare_bazel_toolchains(
         minor,
         patch,
         prerelease,
+        sdk_name,
         sdk_type,
         prefix = ""):
     """Declares toolchain targets for each platform."""
@@ -200,6 +201,36 @@ def declare_bazel_toolchains(
         visibility = ["//visibility:private"],
     )
 
+    # use Label constructor to resolve the label relative to the package this bzl file
+    # is located, instead of the context of the caller of declare_bazel_toolchains.
+    # See https://bazel.build/rules/lib/builtins/Label#Label for details.
+    sdk_name_label = Label("//go/toolchain:sdk_name")
+
+    native.config_setting(
+        name = prefix + "match_sdk_name",
+        flag_values = {
+            sdk_name_label: sdk_name,
+        },
+        visibility = ["//visibility:private"],
+    )
+
+    native.config_setting(
+        name = prefix + "match_all_sdks",
+        flag_values = {
+            sdk_name_label: "",
+        },
+        visibility = ["//visibility:private"],
+    )
+
+    selects.config_setting_group(
+        name = prefix + "sdk_name_setting",
+        match_any = [
+            ":" + prefix + "match_all_sdks",
+            ":" + prefix + "match_sdk_name",
+        ],
+        visibility = ["//visibility:private"],
+    )
+
     for p in PLATFORMS:
         if p.cgo:
             # Don't declare separate toolchains for cgo_on / cgo_off.
@@ -222,6 +253,9 @@ def declare_bazel_toolchains(
                 "@io_bazel_rules_go//go/toolchain:" + host_goarch,
             ],
             target_compatible_with = constraints,
-            target_settings = [":" + prefix + "sdk_version_setting"],
+            target_settings = [
+                ":" + prefix + "sdk_name_setting",
+                ":" + prefix + "sdk_version_setting",
+            ],
             toolchain = go_toolchain_repo + "//:go_" + p.name + "-impl",
         )

go/private/sdk.bzl

@@ -227,6 +227,7 @@ def go_toolchains_single_definition(ctx, *, prefix, goos, goarch, sdk_repo, sdk_
     minor = {identifier_prefix}MINOR_VERSION,
     patch = {identifier_prefix}PATCH_VERSION,
     prerelease = {identifier_prefix}PRERELEASE_SUFFIX,
+    sdk_name = "{sdk_repo}",
     sdk_type = "{sdk_type}",
 )
 """.format(

go/toolchain/BUILD.bazel

@@ -14,6 +14,11 @@ string_flag(
     build_setting_default = "",
 )
 
+string_flag(
+    name = "sdk_name",
+    build_setting_default = "",
+)
+
 filegroup(
     name = "all_rules",
     srcs = glob(["*.bzl"]),

go/toolchains.rst

@@ -79,6 +79,40 @@ The SDK version can omit the patch, or include a prerelease part, eg. ``"1"``,
 When ``go_host_sdk`` is used, ``"version"`` can be set to ``host`` to refer to the host Go SDK.
 It can also be set ``remote`` to match any non-host version.
 
+If you would like to use a specific Go SDK target, pass the flag ``--@io_bazel_rules_go//go/toolchain:sdk_name="name"``.
+This can be useful if there are several `go_download_sdk`_ / `go_host_sdk`_ / `go_local_sdk`_ / `go_wrap_sdk`_
+with the same Go SDK version, but have different experiments enabled or patches applied:
+
+.. code:: bzl
+
+    # WORKSPACE
+
+    go_download_sdk(
+      name = "go_sdk",
+      version = "1.23.5",
+    )
+
+    # select with bazel build --@io_bazel_rules_go//go/toolchain:sdk_name=go_sdk_with_rangefunc
+    go_download_sdk(
+      name = "go_sdk_with_rangefunc",
+      version = "1.23.5",
+      experiments = ["rangefunc"],
+    )
+
+    # MODULE.bazel
+
+    go_sdk.download(
+      name = "go_sdk",
+      version = "1.23.5",
+    )
+
+    # select with bazel build --@io_bazel_rules_go//go/toolchain:sdk_name=go_sdk_with_rangefunc
+    go_sdk.download(
+      name = "go_sdk_with_rangefunc",
+      version = "1.23.5",
+      experiments = ["rangefunc"],
+    )
+
 The toolchain
 ~~~~~~~~~~~~~
 

tests/core/go_download_sdk/go_download_sdk_test.go

@@ -142,13 +142,44 @@ load("@io_bazel_rules_go//go:deps.bzl", "go_download_sdk")
 
 go_download_sdk(
     name = "go_sdk",
-		goarch = "amd64",
-		goos = "windows",
-		version = "1.20.4",
+	goarch = "amd64",
+	goos = "windows",
+	version = "1.20.4",
 )
 `,
 			fetchOnly: "@go_sdk//:BUILD.bazel",
 		},
+		{
+			desc: "multiple_sdks_by_name",
+			rule: `
+load("@io_bazel_rules_go//go:deps.bzl", "go_download_sdk", "go_host_sdk")
+
+go_download_sdk(
+    name = "go_sdk",
+    version = "1.23.5",
+)
+go_download_sdk(
+    name = "go_sdk_1_17",
+    version = "1.17",
+)
+go_download_sdk(
+    name = "go_sdk_1_17_1",
+    version = "1.17.1",
+)
+go_download_sdk(
+    name = "go_sdk_with_experiments",
+    version = "1.23.5",
+	experiments = ["rangefunc"],
+)
+`,
+			optToWantVersion: map[string]string{
+				"": "go1.23.5 X:nocoverageredesign",
+				"--@io_bazel_rules_go//go/toolchain:sdk_name=go_sdk_1_17_1":           "go1.17.1",
+				"--@io_bazel_rules_go//go/toolchain:sdk_name=go_sdk_1_17":             "go1.17",
+				"--@io_bazel_rules_go//go/toolchain:sdk_name=go_sdk":                  "go1.23.5 X:nocoverageredesign",
+				"--@io_bazel_rules_go//go/toolchain:sdk_name=go_sdk_with_experiments": "go1.23.5 X:nocoverageredesign,rangefunc",
+			},
+		},
 	} {
 		t.Run(test.desc, func(t *testing.T) {
 			origWorkspaceData, err := ioutil.ReadFile("WORKSPACE")

tests/core/starlark/sdk_tests.bzl

@@ -30,6 +30,7 @@ _123_PREFIX_PRERELEASE_SUFFIX = "rc1"
     minor = _123_PREFIX_MINOR_VERSION,
     patch = _123_PREFIX_PATCH_VERSION,
     prerelease = _123_PREFIX_PRERELEASE_SUFFIX,
+    sdk_name = "sdk_repo",
     sdk_type = "download",
 )
 """,
@@ -69,6 +70,7 @@ def _go_toolchains_single_definition_without_version_test(ctx):
     minor = _123_PREFIX_MINOR_VERSION,
     patch = _123_PREFIX_PATCH_VERSION,
     prerelease = _123_PREFIX_PRERELEASE_SUFFIX,
+    sdk_name = "sdk_repo",
     sdk_type = "download",
 )
 """,