Don't set a default PATH for Windows

Commit ecf070a updated DefaultPathEnv to return
platform specific values, but also defined a default PATH for Windows.

However, Windows does not have a default PATH, and the PATH to use must be set
by the container image. Also see moby/moby@3c177dc,
which removed the default PATH on Windows;

> This is deliberately empty on Windows as the default path will be set by
> the container. Docker has no context of what the default path should be.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

exporter/containerimage/writer.go

@@ -588,7 +588,9 @@ func defaultImageConfig() ([]byte, error) {
 	}
 	img.RootFS.Type = "layers"
 	img.Config.WorkingDir = "/"
-	img.Config.Env = []string{"PATH=" + system.DefaultPathEnv(pl.OS)}
+	if env := system.DefaultPathEnv(pl.OS); env != "" {
+		img.Config.Env = []string{"PATH=" + env}
+	}
 	dt, err := json.Marshal(img)
 	return dt, errors.Wrap(err, "failed to create empty image config")
 }

frontend/dockerfile/dockerfile2llb/convert.go

@@ -505,11 +505,13 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 
 		// make sure that PATH is always set
 		if _, ok := shell.BuildEnvs(d.image.Config.Env)["PATH"]; !ok {
-			var os string
+			var pOS string
 			if d.platform != nil {
-				os = d.platform.OS
+				pOS = d.platform.OS
+			}
+			if env := system.DefaultPathEnv(pOS); env != "" {
+				d.image.Config.Env = append(d.image.Config.Env, "PATH="+env)
 			}
-			d.image.Config.Env = append(d.image.Config.Env, "PATH="+system.DefaultPathEnv(os))
 		}
 
 		// initialize base metadata from image conf

frontend/dockerfile/dockerfile2llb/image.go

@@ -29,6 +29,8 @@ func emptyImage(platform ocispecs.Platform) Image {
 	}
 	img.RootFS.Type = "layers"
 	img.Config.WorkingDir = "/"
-	img.Config.Env = []string{"PATH=" + system.DefaultPathEnv(platform.OS)}
+	if env := system.DefaultPathEnv(platform.OS); env != "" {
+		img.Config.Env = []string{"PATH=" + env}
+	}
 	return img
 }

frontend/gateway/container.go

@@ -316,7 +316,9 @@ func (gwCtr *gatewayContainer) Start(ctx context.Context, req client.StartReques
 	if procInfo.Meta.Cwd == "" {
 		procInfo.Meta.Cwd = "/"
 	}
-	procInfo.Meta.Env = addDefaultEnvvar(procInfo.Meta.Env, "PATH", utilsystem.DefaultPathEnv(gwCtr.platform.OS))
+	if env := utilsystem.DefaultPathEnv(gwCtr.platform.OS); env != "" {
+		procInfo.Meta.Env = addDefaultEnvvar(procInfo.Meta.Env, "PATH", env)
+	}
 	if req.Tty {
 		procInfo.Meta.Env = addDefaultEnvvar(procInfo.Meta.Env, "TERM", "xterm")
 	}

solver/llbsolver/ops/exec.go

@@ -333,7 +333,9 @@ func (e *execOp) Exec(ctx context.Context, g session.Group, inputs []solver.Resu
 	if e.platform != nil {
 		currentOS = e.platform.OS
 	}
-	meta.Env = addDefaultEnvvar(meta.Env, "PATH", utilsystem.DefaultPathEnv(currentOS))
+	if env := utilsystem.DefaultPathEnv(currentOS); env != "" {
+		meta.Env = addDefaultEnvvar(meta.Env, "PATH", env)
+	}
 
 	secretEnv, err := e.loadSecretEnv(ctx, g)
 	if err != nil {

util/system/path.go

@@ -5,10 +5,10 @@ package system
 // ':' character .
 const DefaultPathEnvUnix = "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
 
-// DefaultPathEnvWindows is windows style list of directories to search for
-// executables. Each directory is separated from the next by a colon
-// ';' character .
-const DefaultPathEnvWindows = "c:\\Windows\\System32;c:\\Windows"
+// DefaultPathEnvWindows is deliberately empty on Windows and the default path
+// must be set by the image. BuildKit has no context of what the default
+// path should be.
+const DefaultPathEnvWindows = ""
 
 func DefaultPathEnv(os string) string {
 	if os == "windows" {