Still provide an owner even when /etc/passwd doesn't have the UID (#16)

Especially for using in Docker where USER is otherwise unaccompanied by a `mkuser` or `useradd` or similar RUN call.

pkg/tracing/provider.go

@@ -26,7 +26,21 @@ func NewProviderFromEnv(ctx context.Context, resourceOptions ...resource.Option)
 			resourceOptions,
 			// Allow environment variables to override constant attributes provided by the caller.
 			resource.WithFromEnv(),
-			resource.WithProcess(),
+			// We need to replace the default implementation of WithProcessOwner, as it can fail
+			// if there is no passwd file (for example in Docker containers without /etc/passwd
+			// but with CGO enabled). However, there is no straight-forward way to produce a new
+			// With... grouping, due to package visibility rules. (And there is no way, also due
+			// to visibility, to replace the default process owner provider.) So we have to enumerate
+			// _all_ the calls WithProcess is equivalent to, so we can change the process owner
+			// implementation.
+			resource.WithProcessPID(),
+			resource.WithProcessExecutableName(),
+			resource.WithProcessExecutablePath(),
+			resource.WithProcessCommandArgs(),
+			WithSafeProcessOwner(),
+			resource.WithProcessRuntimeName(),
+			resource.WithProcessRuntimeVersion(),
+			resource.WithProcessRuntimeDescription(),
 		)...,
 	)
 	if err != nil {

pkg/tracing/resource.go

@@ -0,0 +1,46 @@
+package tracing
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/user"
+
+	"go.opentelemetry.io/otel/sdk/resource"
+	semconv "go.opentelemetry.io/otel/semconv/v1.12.0"
+)
+
+type safeProcessOwnerProvider struct{}
+
+var _ resource.Detector = safeProcessOwnerProvider{}
+
+func (safeProcessOwnerProvider) Detect(context.Context) (*resource.Resource, error) {
+	// Re-implement the username logic of
+	// https://cs.opensource.google/go/go/+/refs/tags/go1.19.5:src/os/user/lookup_stubs.go
+	// so we can use the same rules in CGO mode as well.
+	username := os.Getenv("USER")
+	if u, err := user.Current(); err == nil {
+		username = u.Username
+	}
+
+	// Instead of returning an empty user, just convert the ID number to a string
+	// and use that -- so we always provide some sort of user. (Like the 'id' program
+	// would do.) This will allow us tostill provide some form of owner attribute
+	// regardless of any mismatch amongst the Docker USER parameter, the
+	// /etc/passwd file, the Kubernetes runAsUser setting...
+	if username == "" {
+		// ...but on Windows id will be -1: ignore all negatives.
+		if id := os.Getuid(); id >= 0 {
+			username = fmt.Sprintf("%d", id)
+		} else {
+			// Just give up.
+			return resource.Empty(), nil
+		}
+	}
+
+	return resource.NewWithAttributes(semconv.SchemaURL, semconv.ProcessOwnerKey.String(username)), nil
+}
+
+func WithSafeProcessOwner() resource.Option {
+	return resource.WithDetectors(safeProcessOwnerProvider{})
+}