cmd/go: add a 'buildmode' condition for script tests

In CL 208233 I am fixing a panic that occurs only with a specific
build mode. I want that test to run on all platforms that support that
build mode, but the logic for determining support is somewhat
involved.

For now, I am duplicating that logic into the cmd/internal/sys
package, which already reports platform support for other build flags.

We can refactor cmd/go/internal/work to use the extracted function in
a followup CL.

Updates #35759

Change-Id: Ibbaedde4d1e8f683c650beedd10849bc27e7a6e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/208457
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/script_test.go

@@ -30,6 +30,7 @@ import (
 	"cmd/go/internal/robustio"
 	"cmd/go/internal/txtar"
 	"cmd/go/internal/work"
+	"cmd/internal/sys"
 )
 
 // TestScript runs the tests in testdata/script/*.txt.
@@ -303,6 +304,11 @@ Script:
 					}
 					break
 				}
+				if strings.HasPrefix(cond.tag, "buildmode:") {
+					value := strings.TrimPrefix(cond.tag, "buildmode:")
+					ok = sys.BuildModeSupported(runtime.Compiler, value, runtime.GOOS, runtime.GOARCH)
+					break
+				}
 				if !imports.KnownArch[cond.tag] && !imports.KnownOS[cond.tag] && cond.tag != "gc" && cond.tag != "gccgo" {
 					ts.fatalf("unknown condition %q", cond.tag)
 				}

src/cmd/go/testdata/script/README

@@ -79,6 +79,7 @@ should only run when the condition is satisfied. The available conditions are:
    - [symlink] for testenv.HasSymlink()
    - [exec:prog] for whether prog is available for execution (found by exec.LookPath)
    - [GODEBUG:value] for whether value is one of the comma-separated entries in the GODEBUG variable
+   - [buildmode:value] for whether -buildmode=value is supported
 
 A condition can be negated: [!short] means to run the rest of the line
 when testing.Short() is false. Multiple conditions may be given for a single

src/cmd/internal/sys/supported.go

@@ -43,3 +43,76 @@ func MustLinkExternal(goos, goarch string) bool {
 	}
 	return false
 }
+
+// BuildModeSupported reports whether goos/goarch supports the given build mode
+// using the given compiler.
+func BuildModeSupported(compiler, buildmode, goos, goarch string) bool {
+	// This function mirrors the logic in cmd/go/internal/work.buildModeInit.
+	//
+	// TODO(bcmills): Refactor buildModeInit to use this function so that the two
+	// don't get out of sync.
+
+	if compiler == "gccgo" {
+		return true
+	}
+
+	platform := goos + "/" + goarch
+
+	switch buildmode {
+	case "archive":
+		return true
+
+	case "c-archive":
+		// TODO(bcmills): This seems dubious.
+		// Do we really support c-archive mode on js/wasm‽
+		return platform != "linux/ppc64"
+
+	case "c-shared":
+		switch platform {
+		case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/ppc64le", "linux/s390x",
+			"android/amd64", "android/arm", "android/arm64", "android/386",
+			"freebsd/amd64",
+			"darwin/amd64", "darwin/386",
+			"windows/amd64", "windows/386":
+			return true
+		}
+		return false
+
+	case "default":
+		return true
+
+	case "exe":
+		return true
+
+	case "pie":
+		switch platform {
+		case "linux/386", "linux/amd64", "linux/arm", "linux/arm64", "linux/ppc64le", "linux/s390x",
+			"android/amd64", "android/arm", "android/arm64", "android/386",
+			"freebsd/amd64",
+			"darwin/amd64",
+			"aix/ppc64":
+			return true
+		}
+		return false
+
+	case "shared":
+		switch platform {
+		case "linux/386", "linux/amd64", "linux/arm", "linux/arm64", "linux/ppc64le", "linux/s390x":
+			return true
+		}
+		return false
+
+	case "plugin":
+		switch platform {
+		case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/s390x", "linux/ppc64le",
+			"android/amd64", "android/arm", "android/arm64", "android/386",
+			"darwin/amd64",
+			"freebsd/amd64":
+			return true
+		}
+		return false
+
+	default:
+		return false
+	}
+}