all: sync build script cgo behavior with Mutagen

Since we've now enabled cgo for Mutagen Compose builds, we also need to
add in the minimum macOS version targeting required for portability.
Moreover, we need to disable cgo if trying to cross-compile from a
non-Darwin platform and warn if this is an issue.

Note that this change hides the FSEventStreamScheduleWithRunLoop
deprecation warning since that function isn't deprecated on the macOS
versions that we're targeting.

Signed-off-by: Jacob Howard <jacob@mutagen.io>

scripts/build.go

@@ -16,6 +16,8 @@ import (
 
 	"github.com/spf13/pflag"
 
+	"github.com/mutagen-io/mutagen/cmd"
+
 	"github.com/mutagen-io/mutagen-compose/pkg/version"
 )
 
@@ -38,6 +40,10 @@ const (
 	// path or extension.
 	composeBaseName = "mutagen-compose"
 
+	// minimumMacOSVersion is the minimum version of macOS that we'll support
+	// (currently pinned to the oldest version of macOS that Go supports).
+	minimumMacOSVersion = "10.13"
+
 	// minimumARMSupport is the value to pass to the GOARM environment variable
 	// when building binaries. We currently specify support for ARMv5. This will
 	// enable software-based floating point. For our use case, this is totally
@@ -98,9 +104,33 @@ func (t Target) appendGoEnv(environment []string) []string {
 	environment = append(environment, fmt.Sprintf("GOOS=%s", t.GOOS))
 	environment = append(environment, fmt.Sprintf("GOARCH=%s", t.GOARCH))
 
-	// Keep cgo enabled only for darwin, as it's required for building github.com/fsnotify/fsevents.
-	if t.GOOS == "darwin" {
+	// If we're building a macOS binary on macOS, then we enable cgo because
+	// we'll need it to access the FSEvents API (currently only required by the
+	// the underlying Docker Compose implementation and not by Mutagen Compose).
+	// We have to enable it explicitly because Go won't enable it when cross
+	// compiling between different Darwin architectures. We also need to tell
+	// the C compiler and external linker to support older versions of macOS.
+	// These flags will tell the C compiler to generate code compatible with the
+	// target version of macOS and tell the external linker what value to embed
+	// for the LC_VERSION_MIN_MACOSX flag in the resulting Mach-O binaries. Go's
+	// internal linker automatically defaults to a relatively liberal (old)
+	// value for this flag, but since we're using an external linker, it
+	// defaults to the current SDK version.
+	//
+	// For all other platforms, we disable cgo. This is essential for our Linux
+	// CI setup, because we build agent executables during testing that we then
+	// run inside Docker containers for our integration tests. These containers
+	// typically run Alpine Linux, and if the agent binary is linked to C
+	// libraries that only exist on the build system, then they won't work
+	// inside the container. We can't disable cgo on a global basis though,
+	// because it's needed for race condition testing. Another reason that it's
+	// good to disable cgo when building agent binaries during testing is that
+	// the release agent binaries will also have cgo disabled (except on macOS),
+	// and we'll want to faithfully recreate that.
+	if t.GOOS == "darwin" && runtime.GOOS == "darwin" {
 		environment = append(environment, "CGO_ENABLED=1")
+		environment = append(environment, fmt.Sprintf("CGO_CFLAGS=-mmacosx-version-min=%s", minimumMacOSVersion))
+		environment = append(environment, fmt.Sprintf("CGO_LDFLAGS=-mmacosx-version-min=%s", minimumMacOSVersion))
 	} else {
 		environment = append(environment, "CGO_ENABLED=0")
 	}
@@ -407,6 +437,19 @@ func build() error {
 		return fmt.Errorf("invalid build mode: %s", mode)
 	}
 
+	// The only platform really suited to cross-compiling for every other
+	// platform at the moment is macOS. This is because FSEvents is used (by
+	// Docker Compose) for file monitoring and that is a C-based API, not
+	// accessible purely via system calls. All of the other platforms can
+	// operate with pure Go compilation.
+	if runtime.GOOS != "darwin" {
+		if mode == "release" {
+			return errors.New("macOS is required for release builds")
+		} else if mode == "slim" {
+			cmd.Warning("macOS agents will be built without cgo support")
+		}
+	}
+
 	// If a macOS code signing identity has been specified, then make sure we're
 	// in a mode where that makes sense.
 	if macosCodesignIdentity != "" && runtime.GOOS != "darwin" {