fix(agent): install agent using an absolute remote path

pkg/agent/install.go

@@ -3,7 +3,9 @@ package agent
 import (
 	"fmt"
 	"os"
+	"path"
 	"runtime"
+	"strings"
 
 	"github.com/google/uuid"
 
@@ -76,6 +78,13 @@ func install(logger *logging.Logger, transport Transport, prompter string, cmdEx
 		remoteFileName = "." + remoteFileName
 	}
 	fullRemotePath := remotePathFromHome(cmdExe, remoteFileName)
+	if posix {
+		if home, homeErr := remoteHomeDirectory(transport); homeErr == nil {
+			fullRemotePath = path.Join(home, remoteFileName)
+		} else {
+			logger.Infof("unable to resolve remote home directory, using ~-relative agent path: %v", homeErr)
+		}
+	}
 
 	if err = transport.Copy(agentExecutable, fullRemotePath); err != nil {
 		return fmt.Errorf("unable to copy agent binary: %w", err)
@@ -109,3 +118,15 @@ func install(logger *logging.Logger, transport Transport, prompter string, cmdEx
 	// Success.
 	return nil
 }
+
+func remoteHomeDirectory(transport Transport) (string, error) {
+	out, err := output(transport, `echo "$HOME"`)
+	if err != nil {
+		return "", fmt.Errorf("unable to query remote home directory: %w", err)
+	}
+	home := strings.TrimSpace(string(out))
+	if !strings.HasPrefix(home, "/") {
+		return "", fmt.Errorf("invalid remote home directory: %q", home)
+	}
+	return home, nil
+}

pkg/agent/install_test.go

@@ -1,4 +1,65 @@
 package agent
 
+import (
+	"os"
+	"os/exec"
+	"testing"
+)
+
 // NOTE: Unfortunately the Install() method can't be tested directly, but it is
 // tested indirectly by integration tests.
+
+type echoTransport struct {
+	stdout      string
+	failCommand bool
+}
+
+func (t *echoTransport) Copy(_, _ string) error { return nil }
+
+func (t *echoTransport) Command(_ string) (*exec.Cmd, error) {
+	if t.failCommand {
+		return exec.Command("false"), nil
+	}
+	return exec.Command("printf", "%s", t.stdout), nil
+}
+
+func (t *echoTransport) ClassifyError(_ *os.ProcessState, _ string) (bool, bool, error) {
+	return false, false, nil
+}
+
+func TestRemoteHomeDirectory(t *testing.T) {
+	testCases := []struct {
+		name        string
+		stdout      string
+		failCommand bool
+		expected    string
+		expectError bool
+	}{
+		{name: "simple", stdout: "/home/ubuntu", expected: "/home/ubuntu"},
+		{name: "trailing newline", stdout: "/home/ubuntu\n", expected: "/home/ubuntu"},
+		{name: "surrounding whitespace", stdout: "  /home/ubuntu \n", expected: "/home/ubuntu"},
+		{name: "empty", stdout: "", expectError: true},
+		{name: "unexpanded variable", stdout: "$HOME", expectError: true},
+		{name: "relative", stdout: "home/ubuntu", expectError: true},
+		{name: "command error", failCommand: true, expectError: true},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			transport := &echoTransport{stdout: testCase.stdout, failCommand: testCase.failCommand}
+			home, err := remoteHomeDirectory(transport)
+			if testCase.expectError {
+				if err == nil {
+					t.Fatalf("expected an error but got home %q", home)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if home != testCase.expected {
+				t.Fatalf("expected home %q but got %q", testCase.expected, home)
+			}
+		})
+	}
+}