Add the provider contract, error taxonomy, naming, and config (Step 2)

The Provider interface (Create/Get/Delete/ListByTag), Instance, and
CreateRequest that every cloud backend implements — kept independent of
api/v1alpha1 so this package has no CRD-type coupling.

Error taxonomy (ErrNotFound/ErrQuotaExceeded/ErrTransient/ErrPermanent)
wrapped via a multi-error Unwrap() []error, so errors.Is and errors.As
both work off the same value: the reconciler branches on classification,
logs keep the underlying SDK error. Unclassified errors default to
ErrTransient — retrying is always safer than latching Failed.

Deterministic instance naming (SHA-256 -> base32 -> 16 chars, 22 total
with the "proxy-" prefix) satisfying GCP's RFC1035 name rules with
headroom, and idempotency-tested across 10k UIDs with zero collisions.

--providers-config YAML parsing (config.go) with fail-fast validation:
unknown type, duplicate name, missing gcp.project, mismatched
type/config-block, and strict-mode rejection of unknown keys.

internal/provider/registry/registry.go takes its type->constructor map
as a parameter rather than hardcoding it, so the package has zero import
on internal/provider/mock or internal/provider/gcp (neither exists yet —
mock is Step 3, gcp is Step 8) and compiles today. Explicit wiring moves
to the composition root in cmd/main.go (Step 10).

Deferred internal/provider/metrics.go (the WithMetrics decorator) to
Step 9, where the Prometheus vectors it needs actually get built —
nothing in this step depends on it.

internal/provider at 96.2% coverage, internal/provider/registry at 100%.
make test green.

Co-Authored-By: Claude <noreply@anthropic.com>

internal/provider/config.go

@@ -0,0 +1,133 @@
+package provider
+
+import (
+	"fmt"
+	"os"
+
+	"sigs.k8s.io/yaml"
+)
+
+// Fail-with classes accepted by MockConfig.FailWith, shared with the mock
+// provider's fault injection so the two sides never drift on the string
+// values.
+const (
+	FailWithNotFound  = "notfound"
+	FailWithQuota     = "quota"
+	FailWithTransient = "transient"
+	FailWithPermanent = "permanent"
+)
+
+var validFailClasses = map[string]bool{
+	FailWithNotFound:  true,
+	FailWithQuota:     true,
+	FailWithTransient: true,
+	FailWithPermanent: true,
+}
+
+// Config is the top-level shape of the --providers-config file.
+type Config struct {
+	Providers []ProviderConfig `json:"providers"`
+}
+
+// ProviderConfig is one named, typed provider instance — e.g. "gcp-eu" and
+// "gcp-us" can be two ProviderConfigs of Type "gcp" with different GCP
+// blocks. Exactly one of the type-specific blocks below should be set,
+// matching Type.
+type ProviderConfig struct {
+	Name string      `json:"name"`
+	Type string      `json:"type"`
+	Mock *MockConfig `json:"mock,omitempty"`
+	GCP  *GCPConfig  `json:"gcp,omitempty"`
+}
+
+// MockConfig configures the in-memory mock provider.
+type MockConfig struct {
+	// ProvisionDelaySeconds is how long a created instance reports
+	// Provisioning before Running. Default 5.
+	ProvisionDelaySeconds int32 `json:"provisionDelaySeconds,omitempty"`
+	// DeleteDelaySeconds is how long a deleted instance reports Terminated
+	// before Get starts returning ErrNotFound. Default 1.
+	DeleteDelaySeconds int32 `json:"deleteDelaySeconds,omitempty"`
+	// FailNextCreates makes the next N Create calls fail with FailWith,
+	// for exercising the reconciler's error handling in demos.
+	FailNextCreates int `json:"failNextCreates,omitempty"`
+	// FailWith selects the error class injected failures return: one of
+	// FailWithNotFound/FailWithQuota/FailWithTransient/FailWithPermanent.
+	// Default FailWithTransient.
+	FailWith string `json:"failWith,omitempty"`
+}
+
+// GCPConfig configures a named GCP provider instance.
+type GCPConfig struct {
+	// Project is the GCP project ID. Required.
+	Project string `json:"project"`
+	// Network is the VPC network name. Default "default".
+	Network string `json:"network,omitempty"`
+	// NetworkTag is the firewall/network tag applied to created instances.
+	// Default "proxy-operator".
+	NetworkTag string `json:"networkTag,omitempty"`
+	// DiskSizeGB is the boot disk size in GB. Default 10.
+	DiskSizeGB int64 `json:"diskSizeGb,omitempty"`
+}
+
+// LoadConfigFile reads and parses a providers-config file from disk.
+func LoadConfigFile(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("reading providers config %s: %w", path, err)
+	}
+	return LoadConfig(data)
+}
+
+// LoadConfig parses and validates providers-config YAML. It fails fast:
+// unknown provider types, duplicate names, and missing type-specific
+// required fields are all load-time errors here, not runtime surprises
+// discovered only when a provider is actually used.
+func LoadConfig(data []byte) (*Config, error) {
+	var cfg Config
+	if err := yaml.UnmarshalStrict(data, &cfg); err != nil {
+		return nil, fmt.Errorf("parsing providers config: %w", err)
+	}
+	if err := cfg.validate(); err != nil {
+		return nil, fmt.Errorf("validating providers config: %w", err)
+	}
+	return &cfg, nil
+}
+
+func (c *Config) validate() error {
+	if len(c.Providers) == 0 {
+		return fmt.Errorf("providers: at least one provider must be configured")
+	}
+	seen := make(map[string]bool, len(c.Providers))
+	for i, p := range c.Providers {
+		if p.Name == "" {
+			return fmt.Errorf("providers[%d]: name is required", i)
+		}
+		if seen[p.Name] {
+			return fmt.Errorf("providers[%d]: duplicate provider name %q", i, p.Name)
+		}
+		seen[p.Name] = true
+
+		switch p.Type {
+		case "mock":
+			if p.GCP != nil {
+				return fmt.Errorf("providers[%d] %q: type is mock but a gcp block is set", i, p.Name)
+			}
+			if p.Mock != nil && p.Mock.FailWith != "" && !validFailClasses[p.Mock.FailWith] {
+				return fmt.Errorf("providers[%d] %q: unknown mock.failWith %q", i, p.Name, p.Mock.FailWith)
+			}
+		case "gcp":
+			if p.Mock != nil {
+				return fmt.Errorf("providers[%d] %q: type is gcp but a mock block is set", i, p.Name)
+			}
+			if p.GCP == nil || p.GCP.Project == "" {
+				return fmt.Errorf("providers[%d] %q: gcp.project is required", i, p.Name)
+			}
+		case "":
+			return fmt.Errorf("providers[%d] %q: type is required", i, p.Name)
+		default:
+			return fmt.Errorf("providers[%d] %q: unknown provider type %q", i, p.Name, p.Type)
+		}
+	}
+	return nil
+}

internal/provider/config_test.go

@@ -0,0 +1,173 @@
+package provider
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestLoadConfig_valid(t *testing.T) {
+	t.Parallel()
+	data := []byte(`
+providers:
+  - name: mock
+    type: mock
+    mock:
+      provisionDelaySeconds: 2
+  - name: gcp-eu
+    type: gcp
+    gcp:
+      project: my-project
+      network: custom-net
+`)
+	cfg, err := LoadConfig(data)
+	if err != nil {
+		t.Fatalf("LoadConfig() error = %v, want nil", err)
+	}
+	if len(cfg.Providers) != 2 {
+		t.Fatalf("len(cfg.Providers) = %d, want 2", len(cfg.Providers))
+	}
+	if cfg.Providers[0].Mock == nil || cfg.Providers[0].Mock.ProvisionDelaySeconds != 2 {
+		t.Errorf("providers[0].mock = %+v, want ProvisionDelaySeconds=2", cfg.Providers[0].Mock)
+	}
+	if cfg.Providers[1].GCP == nil || cfg.Providers[1].GCP.Project != "my-project" {
+		t.Errorf("providers[1].gcp = %+v, want Project=my-project", cfg.Providers[1].GCP)
+	}
+}
+
+func TestLoadConfig_invalid(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name       string
+		yaml       string
+		wantErrSub string
+	}{
+		{
+			name:       "empty providers list",
+			yaml:       `providers: []`,
+			wantErrSub: "at least one provider",
+		},
+		{
+			name: "missing name",
+			yaml: `
+providers:
+  - type: mock`,
+			wantErrSub: "name is required",
+		},
+		{
+			name: "duplicate name",
+			yaml: `
+providers:
+  - name: mock
+    type: mock
+  - name: mock
+    type: mock`,
+			wantErrSub: "duplicate provider name",
+		},
+		{
+			name: "unknown type",
+			yaml: `
+providers:
+  - name: p1
+    type: azure`,
+			wantErrSub: `unknown provider type "azure"`,
+		},
+		{
+			name: "missing type",
+			yaml: `
+providers:
+  - name: p1`,
+			wantErrSub: "type is required",
+		},
+		{
+			name: "gcp missing project",
+			yaml: `
+providers:
+  - name: gcp-eu
+    type: gcp`,
+			wantErrSub: "gcp.project is required",
+		},
+		{
+			name: "gcp with empty project",
+			yaml: `
+providers:
+  - name: gcp-eu
+    type: gcp
+    gcp:
+      project: ""`,
+			wantErrSub: "gcp.project is required",
+		},
+		{
+			name: "mock type with gcp block",
+			yaml: `
+providers:
+  - name: p1
+    type: mock
+    gcp:
+      project: my-project`,
+			wantErrSub: "type is mock but a gcp block is set",
+		},
+		{
+			name: "gcp type with mock block",
+			yaml: `
+providers:
+  - name: p1
+    type: gcp
+    gcp:
+      project: my-project
+    mock:
+      failNextCreates: 1`,
+			wantErrSub: "type is gcp but a mock block is set",
+		},
+		{
+			name: "unknown mock.failWith",
+			yaml: `
+providers:
+  - name: p1
+    type: mock
+    mock:
+      failWith: oops`,
+			wantErrSub: `unknown mock.failWith "oops"`,
+		},
+		{
+			name: "strict mode rejects unknown top-level key",
+			yaml: `
+providers:
+  - name: p1
+    type: mock
+extraneous: true`,
+			wantErrSub: "parsing providers config",
+		},
+		{
+			name: "strict mode rejects unknown provider key",
+			yaml: `
+providers:
+  - name: p1
+    type: mock
+    bogus: true`,
+			wantErrSub: "parsing providers config",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			_, err := LoadConfig([]byte(tc.yaml))
+			if err == nil {
+				t.Fatalf("LoadConfig() error = nil, want error containing %q", tc.wantErrSub)
+			}
+			if !strings.Contains(err.Error(), tc.wantErrSub) {
+				t.Errorf("LoadConfig() error = %q, want substring %q", err.Error(), tc.wantErrSub)
+			}
+		})
+	}
+}
+
+func TestLoadConfigFile_missingFile(t *testing.T) {
+	t.Parallel()
+	_, err := LoadConfigFile("/nonexistent/providers.yaml")
+	if err == nil {
+		t.Fatal("LoadConfigFile() error = nil, want error")
+	}
+	if !strings.Contains(err.Error(), "reading providers config") {
+		t.Errorf("LoadConfigFile() error = %q, want substring %q", err.Error(), "reading providers config")
+	}
+}

internal/provider/errors.go

@@ -0,0 +1,91 @@
+package provider
+
+import (
+	"errors"
+	"fmt"
+)
+
+// The provider error taxonomy. Every error a Provider method returns should
+// be classifiable as exactly one of these four, via Wrap or Class. The
+// reconciler branches on this classification to decide how to react —
+// never on a provider-specific error type.
+var (
+	// ErrNotFound means the instance doesn't exist. Get returning this is
+	// normal, not exceptional.
+	ErrNotFound = errors.New("provider: instance not found")
+	// ErrQuotaExceeded means the request failed because of a cloud quota or
+	// rate limit. The reconciler backs off slowly (minutes, not seconds)
+	// rather than hammering an exhausted quota.
+	ErrQuotaExceeded = errors.New("provider: quota exceeded")
+	// ErrTransient means the request failed for a reason likely to clear on
+	// retry (network blip, 5xx, timeout). The reconciler retries with the
+	// workqueue's normal exponential backoff.
+	ErrTransient = errors.New("provider: transient error")
+	// ErrPermanent means the request failed for a reason that will not
+	// clear on retry (bad config, permission denied, invalid argument). The
+	// reconciler stops retrying and surfaces the failure in status.
+	ErrPermanent = errors.New("provider: permanent error")
+)
+
+// Error wraps a provider-specific error with enough context for logs, while
+// remaining classifiable via errors.Is against one of the four sentinels
+// above and unwrappable via errors.As to the underlying SDK error.
+type Error struct {
+	Class    error  // one of ErrNotFound/ErrQuotaExceeded/ErrTransient/ErrPermanent
+	Op       string // "create", "get", "delete", "list"
+	Provider string // the configured provider name, e.g. "gcp-eu"
+	ID       string // providerID, if known
+	Err      error  // underlying error; may be nil
+}
+
+func (e *Error) Error() string {
+	msg := fmt.Sprintf("provider %s: %s", e.Provider, e.Op)
+	if e.ID != "" {
+		msg += fmt.Sprintf(" %s", e.ID)
+	}
+	msg += ": " + e.Class.Error()
+	if e.Err != nil {
+		msg += fmt.Sprintf(" (%v)", e.Err)
+	}
+	return msg
+}
+
+// Unwrap exposes both the taxonomy sentinel and the underlying error, via
+// Go's multi-error unwrap (errors.Unwrap() []error). This is what lets
+// errors.Is(err, ErrQuotaExceeded) and errors.As(err, &googleErr) both
+// succeed against the same *Error value: errors.Is/As walk every branch.
+func (e *Error) Unwrap() []error {
+	if e.Err != nil {
+		return []error{e.Class, e.Err}
+	}
+	return []error{e.Class}
+}
+
+// Wrap builds an *Error classified under class. err is the underlying
+// SDK/network error and may be nil when there's nothing further to wrap
+// (e.g. classifying a bare HTTP status code).
+func Wrap(class error, op, providerName, id string, err error) error {
+	return &Error{Class: class, Op: op, Provider: providerName, ID: id, Err: err}
+}
+
+// Class returns the taxonomy sentinel matching err — ErrNotFound,
+// ErrQuotaExceeded, ErrPermanent, or ErrTransient, checked via errors.Is so
+// it works against any error built with Wrap regardless of how deeply it's
+// wrapped elsewhere. An unclassified error (nil, or not built with Wrap)
+// defaults to ErrTransient: retrying is always safer than latching Failed
+// on an error nobody has taught this package to recognize.
+func Class(err error) error {
+	if err == nil {
+		return nil
+	}
+	switch {
+	case errors.Is(err, ErrNotFound):
+		return ErrNotFound
+	case errors.Is(err, ErrQuotaExceeded):
+		return ErrQuotaExceeded
+	case errors.Is(err, ErrPermanent):
+		return ErrPermanent
+	default:
+		return ErrTransient
+	}
+}

internal/provider/errors_test.go

@@ -0,0 +1,70 @@
+package provider
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+// sdkError simulates an underlying provider SDK error type, so tests can
+// assert errors.As reaches through the wrapper to it.
+type sdkError struct{ code int }
+
+func (e *sdkError) Error() string { return fmt.Sprintf("sdk error %d", e.code) }
+
+func TestError_IsAndAs(t *testing.T) {
+	t.Parallel()
+	underlying := &sdkError{code: 429}
+	err := Wrap(ErrQuotaExceeded, "create", "gcp-eu", "zones/z/instances/x", underlying)
+
+	if !errors.Is(err, ErrQuotaExceeded) {
+		t.Error("errors.Is(err, ErrQuotaExceeded) = false, want true")
+	}
+	if errors.Is(err, ErrNotFound) {
+		t.Error("errors.Is(err, ErrNotFound) = true, want false")
+	}
+
+	var sdk *sdkError
+	if !errors.As(err, &sdk) {
+		t.Fatal("errors.As(err, &sdk) = false, want true")
+	}
+	if sdk.code != 429 {
+		t.Errorf("recovered sdkError.code = %d, want 429", sdk.code)
+	}
+}
+
+func TestError_WrapWithNilUnderlying(t *testing.T) {
+	t.Parallel()
+	err := Wrap(ErrPermanent, "get", "mock", "id", nil)
+	if !errors.Is(err, ErrPermanent) {
+		t.Error("errors.Is(err, ErrPermanent) = false, want true")
+	}
+	if err.Error() == "" {
+		t.Error("Error() returned an empty string")
+	}
+}
+
+func TestClass(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		err  error
+		want error
+	}{
+		{"nil returns nil", nil, nil},
+		{"wrapped not found", Wrap(ErrNotFound, "get", "mock", "id", nil), ErrNotFound},
+		{"wrapped quota", Wrap(ErrQuotaExceeded, "create", "gcp", "id", nil), ErrQuotaExceeded},
+		{"wrapped permanent", Wrap(ErrPermanent, "create", "gcp", "id", nil), ErrPermanent},
+		{"wrapped transient", Wrap(ErrTransient, "create", "gcp", "id", nil), ErrTransient},
+		{"unclassified defaults to transient", errors.New("boom"), ErrTransient},
+		{"further-wrapped preserves classification", fmt.Errorf("outer: %w", Wrap(ErrQuotaExceeded, "create", "gcp", "id", nil)), ErrQuotaExceeded},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			if got := Class(tc.err); got != tc.want {
+				t.Errorf("Class(%v) = %v, want %v", tc.err, got, tc.want)
+			}
+		})
+	}
+}

internal/provider/name.go

@@ -0,0 +1,33 @@
+package provider
+
+import (
+	"crypto/sha256"
+	"encoding/base32"
+	"strings"
+
+	"k8s.io/apimachinery/pkg/types"
+)
+
+const namePrefix = "proxy-"
+
+// NameFromUID derives a deterministic instance name from a Proxy CR's UID.
+// Provider.Create implementations key idempotency on this name: a repeat
+// call after a crash must find the existing instance by this name rather
+// than create a duplicate, and the reconciler relies on that to recover
+// providerID after a crash even if status was wiped.
+//
+// SHA-256 of the UID, truncated to the first 10 bytes, RFC 4648 base32
+// (no padding), lowercased: exactly 16 characters, for 22 total with the
+// "proxy-" prefix. That gives 80 bits of collision resistance — a birthday
+// collision only becomes likely around 2^40 objects, against a fleet of
+// tens — while satisfying GCP's instance name rules
+// (^[a-z]([-a-z0-9]{0,61}[a-z0-9])?$, <=63 chars) with headroom, since GCP
+// auto-names some resources (e.g. the boot disk) after the instance name.
+// base32's alphabet (A-Z2-7) is entirely legal once lowercased; base64 is
+// not (has '+', '/', and uppercase), and hex would need 20 characters for
+// the same 80 bits.
+func NameFromUID(uid types.UID) string {
+	sum := sha256.Sum256([]byte(uid))
+	enc := base32.StdEncoding.WithPadding(base32.NoPadding)
+	return namePrefix + strings.ToLower(enc.EncodeToString(sum[:10]))
+}

internal/provider/name_test.go

@@ -0,0 +1,61 @@
+package provider
+
+import (
+	"fmt"
+	"regexp"
+	"testing"
+
+	"k8s.io/apimachinery/pkg/types"
+)
+
+var nameRegexp = regexp.MustCompile(`^proxy-[a-z2-7]{16}$`)
+
+func TestNameFromUID_idempotent(t *testing.T) {
+	t.Parallel()
+	uid := types.UID("f47ac10b-58cc-4372-a567-0e02b2c3d479")
+	got1 := NameFromUID(uid)
+	got2 := NameFromUID(uid)
+	if got1 != got2 {
+		t.Errorf("NameFromUID(%q) is not idempotent: %q != %q", uid, got1, got2)
+	}
+}
+
+func TestNameFromUID_charsetAndLength(t *testing.T) {
+	t.Parallel()
+	tests := []types.UID{
+		"f47ac10b-58cc-4372-a567-0e02b2c3d479",
+		"",
+		"a",
+		"00000000-0000-0000-0000-000000000000",
+		"ffffffff-ffff-ffff-ffff-ffffffffffff",
+	}
+	for _, uid := range tests {
+		t.Run(string(uid), func(t *testing.T) {
+			t.Parallel()
+			name := NameFromUID(uid)
+			if !nameRegexp.MatchString(name) {
+				t.Errorf("NameFromUID(%q) = %q, does not match %s", uid, name, nameRegexp)
+			}
+			if len(name) > 63 {
+				t.Errorf("NameFromUID(%q) = %q, length %d exceeds GCP's 63-char limit", uid, name, len(name))
+			}
+			if len(name) != len(namePrefix)+16 {
+				t.Errorf("NameFromUID(%q) = %q, length %d, want %d", uid, name, len(name), len(namePrefix)+16)
+			}
+		})
+	}
+}
+
+func TestNameFromUID_distinctAcrossFleet(t *testing.T) {
+	t.Parallel()
+	const n = 10000
+	seen := make(map[string]types.UID, n)
+	for i := range n {
+		uid := types.UID(fmt.Sprintf("00000000-0000-0000-0000-%012d", i))
+		name := NameFromUID(uid)
+		if prior, ok := seen[name]; ok {
+			t.Fatalf("collision: NameFromUID(%q) == NameFromUID(%q) == %q", uid, prior, name)
+		}
+		seen[name] = uid
+	}
+}

internal/provider/provider.go

@@ -0,0 +1,109 @@
+// Package provider defines the contract every cloud provider backend
+// implements: Create/Get/Delete/ListByTag for a single VM, keyed by an
+// opaque providerID. Concrete implementations live in subpackages (mock,
+// gcp); this package has no dependency on any of them, so a type→
+// constructor registry can be assembled at the composition root (cmd/main.go)
+// without an import cycle.
+package provider
+
+import (
+	"context"
+	"time"
+)
+
+// InstanceState is a provider's lifecycle state for a single VM.
+type InstanceState string
+
+const (
+	StateProvisioning InstanceState = "Provisioning"
+	StateRunning      InstanceState = "Running"
+	StateStopped      InstanceState = "Stopped"
+	StateTerminated   InstanceState = "Terminated"
+)
+
+// GC contract: every cloud resource a provider creates must carry these two
+// labels/tags. Orphan GC (internal/gc) relies on both — Managed to find
+// resources it owns at all, UID to decide whether a resource is still
+// claimed by a live Proxy CR.
+const (
+	LabelManaged    = "proxy-operator-managed"
+	LabelManagedYes = "true"
+	LabelUID        = "proxy-operator-uid"
+)
+
+// Instance is a provider's view of a single VM.
+type Instance struct {
+	// ID is the opaque providerID, stable for the life of the VM.
+	ID string
+	// IP is the VM's current address, empty until it's assigned one.
+	IP string
+	// State is the VM's current lifecycle state.
+	State InstanceState
+	// UID is the LabelUID value read back off the resource — the owning
+	// Proxy CR's UID, or "" if the resource predates this label (shouldn't
+	// happen for anything this operator created, but Get/ListByTag callers
+	// must tolerate it rather than panic).
+	UID string
+	// CreatedAt is when the provider created the resource. Orphan GC uses
+	// this to skip young instances that may still be mid-create, avoiding a
+	// race with an in-flight Create whose status write hasn't landed yet.
+	CreatedAt time.Time
+}
+
+// Placement is the provider-opaque placement/size configuration a Create
+// call needs. It deliberately does not import api/v1alpha1 — this package
+// stays independent of the CRD types; the reconciler maps
+// v1alpha1.PlacementSpec to this struct when calling Create. Providers may
+// ignore fields that don't apply to them.
+type Placement struct {
+	Region      string
+	Zone        string
+	MachineType string
+	Image       string
+}
+
+// CreateRequest carries everything a provider needs to create a VM.
+type CreateRequest struct {
+	// Name is the deterministic instance name, already derived from the
+	// owning CR's UID via NameFromUID. Create must be idempotent keyed on
+	// this name: a repeat call after a crash must find the existing
+	// instance rather than create a duplicate.
+	Name string
+	// UID is the owning Proxy CR's UID. Create must tag/label the created
+	// resource with LabelUID=UID and LabelManaged=LabelManagedYes.
+	UID string
+
+	Namespace string
+	ProxyName string
+
+	Placement Placement
+	// CloudInit is the already-resolved user-data (a Secret reference, if
+	// used, has already been read by the caller).
+	CloudInit string
+	Port      int32
+}
+
+// Provider is the contract every cloud backend implements. Kept
+// deliberately minimal: this is the same interface five future providers
+// must satisfy.
+type Provider interface {
+	// Create starts VM creation and returns as soon as the request is
+	// submitted — it does not block until the VM is running. Must be
+	// idempotent by req.Name, so a repeat call after a crash finds the
+	// existing VM instead of duplicating it.
+	Create(ctx context.Context, req CreateRequest) (providerID string, err error)
+
+	// Get returns the current state of a previously created instance.
+	// Returning (nil, ErrNotFound) is a normal, expected outcome — it
+	// drives the reconciler's replacement and adoption logic, not an
+	// exceptional condition.
+	Get(ctx context.Context, providerID string) (*Instance, error)
+
+	// Delete is idempotent: deleting an instance that no longer exists is
+	// not an error.
+	Delete(ctx context.Context, providerID string) error
+
+	// ListByTag returns every instance this operator has ever tagged with
+	// LabelManaged=LabelManagedYes, for orphan GC.
+	ListByTag(ctx context.Context) ([]Instance, error)
+}

internal/provider/registry/registry.go

@@ -0,0 +1,44 @@
+// Package registry assembles a set of named providers from a
+// provider.Config by dispatching each entry's Type through a
+// caller-supplied map of constructors.
+//
+// This package deliberately never imports internal/provider/mock or
+// internal/provider/gcp. If it did, and internal/provider ever needed to
+// import this package (e.g. to expose a default registry), that would be an
+// import cycle: internal/provider/mock already imports internal/provider
+// for the Provider interface. Keeping the type→constructor map external —
+// supplied by the composition root in cmd/main.go — avoids the cycle
+// entirely and keeps this package trivially testable with fake
+// constructors.
+package registry
+
+import (
+	"context"
+	"fmt"
+
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+)
+
+// Constructor builds a provider.Provider from its config block.
+type Constructor func(ctx context.Context, cfg provider.ProviderConfig) (provider.Provider, error)
+
+// Build constructs every provider listed in cfg, dispatching on
+// ProviderConfig.Type through builtins. Fails fast: an unknown type or a
+// constructor error aborts the whole build rather than returning a
+// partially-usable provider set that would misbehave once a caller reaches
+// for the missing entry.
+func Build(ctx context.Context, cfg *provider.Config, builtins map[string]Constructor) (map[string]provider.Provider, error) {
+	providers := make(map[string]provider.Provider, len(cfg.Providers))
+	for _, pc := range cfg.Providers {
+		ctor, ok := builtins[pc.Type]
+		if !ok {
+			return nil, fmt.Errorf("provider %q: unknown type %q", pc.Name, pc.Type)
+		}
+		p, err := ctor(ctx, pc)
+		if err != nil {
+			return nil, fmt.Errorf("provider %q: %w", pc.Name, err)
+		}
+		providers[pc.Name] = p
+	}
+	return providers, nil
+}

internal/provider/registry/registry_test.go

@@ -0,0 +1,92 @@
+package registry
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+)
+
+// stubProvider is a minimal provider.Provider satisfying the interface for
+// registry tests, which care about wiring, not provider behavior.
+type stubProvider struct{ name string }
+
+func (s *stubProvider) Create(ctx context.Context, req provider.CreateRequest) (string, error) {
+	return "", nil
+}
+func (s *stubProvider) Get(ctx context.Context, providerID string) (*provider.Instance, error) {
+	return nil, provider.ErrNotFound
+}
+func (s *stubProvider) Delete(ctx context.Context, providerID string) error { return nil }
+func (s *stubProvider) ListByTag(ctx context.Context) ([]provider.Instance, error) {
+	return nil, nil
+}
+
+func stubConstructor(name string) Constructor {
+	return func(ctx context.Context, cfg provider.ProviderConfig) (provider.Provider, error) {
+		return &stubProvider{name: name}, nil
+	}
+}
+
+func TestBuild_dispatchesByType(t *testing.T) {
+	t.Parallel()
+	cfg := &provider.Config{Providers: []provider.ProviderConfig{
+		{Name: "mock", Type: "mock"},
+		{Name: "gcp-eu", Type: "gcp"},
+	}}
+	builtins := map[string]Constructor{
+		"mock": stubConstructor("mock"),
+		"gcp":  stubConstructor("gcp"),
+	}
+
+	providers, err := Build(context.Background(), cfg, builtins)
+	if err != nil {
+		t.Fatalf("Build() error = %v, want nil", err)
+	}
+	if len(providers) != 2 {
+		t.Fatalf("len(providers) = %d, want 2", len(providers))
+	}
+	if _, ok := providers["mock"]; !ok {
+		t.Error(`providers["mock"] missing`)
+	}
+	if _, ok := providers["gcp-eu"]; !ok {
+		t.Error(`providers["gcp-eu"] missing, keyed by config name not type`)
+	}
+}
+
+func TestBuild_unknownType(t *testing.T) {
+	t.Parallel()
+	cfg := &provider.Config{Providers: []provider.ProviderConfig{
+		{Name: "p1", Type: "azure"},
+	}}
+	_, err := Build(context.Background(), cfg, map[string]Constructor{"mock": stubConstructor("mock")})
+	if err == nil {
+		t.Fatal("Build() error = nil, want error for unknown type")
+	}
+}
+
+func TestBuild_constructorErrorAbortsWholeBuild(t *testing.T) {
+	t.Parallel()
+	failing := errors.New("boom")
+	cfg := &provider.Config{Providers: []provider.ProviderConfig{
+		{Name: "mock", Type: "mock"},
+		{Name: "bad", Type: "bad"},
+	}}
+	builtins := map[string]Constructor{
+		"mock": stubConstructor("mock"),
+		"bad": func(ctx context.Context, cfg provider.ProviderConfig) (provider.Provider, error) {
+			return nil, failing
+		},
+	}
+	providers, err := Build(context.Background(), cfg, builtins)
+	if err == nil {
+		t.Fatal("Build() error = nil, want error from failing constructor")
+	}
+	if !errors.Is(err, failing) {
+		t.Errorf("Build() error = %v, want it to wrap %v", err, failing)
+	}
+	if providers != nil {
+		t.Errorf("Build() providers = %v, want nil on failure (no partial result)", providers)
+	}
+}