Make the existence cache more type-safe.

This makes the uncertain nature of a Bloom filter hit explicit at every
call site.

src/caddy.go

@@ -30,7 +30,7 @@ func ServeCaddy(w http.ResponseWriter, r *http.Request) {
 	domain = strings.ToLower(domain)
 
 	// Run a cheap check as to whether we might be serving the domain.
-	var found = existenceCache.CheckDomain(r.Context(), domain)
+	found := existenceCache.CheckDomain(r.Context(), domain).IsPossible()
 
 	if found {
 		// Run an expensive check as to whether we are actually serving the domain.

src/existence.go

@@ -16,12 +16,38 @@ import (
 	"github.com/bits-and-blooms/bloom/v3"
 )
 
+type Existence int
+
+const (
+	ExistenceImpossible Existence = iota
+	ExistencePossible
+)
+
+func (value Existence) IsImpossible() bool {
+	return value == ExistenceImpossible
+}
+
+func (value Existence) IsPossible() bool {
+	return value == ExistencePossible
+}
+
+func (value Existence) String() string {
+	switch value {
+	case ExistenceImpossible:
+		return "impossible"
+	case ExistencePossible:
+		return "possible"
+	default:
+		return "(invalid)"
+	}
+}
+
 type ExistenceCache interface {
 	// Check if we might be serving the site.
-	CheckSite(ctx context.Context, site string) (found bool)
+	CheckSite(ctx context.Context, site string) (result Existence)
 
 	// Check if we might be serving the domain.
-	CheckDomain(ctx context.Context, domain string) (found bool)
+	CheckDomain(ctx context.Context, domain string) (result Existence)
 
 	// Add the site to the cache.
 	AddSite(ctx context.Context, site string)
@@ -118,38 +144,38 @@ func (c *bloomExistenceCache) refresh(ctx context.Context) error {
 	return nil
 }
 
-func (c *bloomExistenceCache) CheckSite(ctx context.Context, site string) (found bool) {
+func (c *bloomExistenceCache) CheckSite(ctx context.Context, site string) (result Existence) {
 	select {
 	case c.accessCh <- struct{}{}:
 	default:
 	}
 
 	c.filterMu.Lock()
-	found = c.sites.TestString(site)
+	if c.sites.TestString(site) {
+		result = ExistencePossible
+	} else {
+		result = ExistenceImpossible
+	}
 	c.filterMu.Unlock()
 
-	result := "miss"
-	if found {
-		result = "hit"
-	}
 	logc.Printf(ctx, "existence: site %s: %s", site, result)
 	return
 }
 
-func (c *bloomExistenceCache) CheckDomain(ctx context.Context, domain string) (found bool) {
+func (c *bloomExistenceCache) CheckDomain(ctx context.Context, domain string) (result Existence) {
 	select {
 	case c.accessCh <- struct{}{}:
 	default:
 	}
 
 	c.filterMu.Lock()
-	found = c.domains.TestString(domain)
+	if c.domains.TestString(domain) {
+		result = ExistencePossible
+	} else {
+		result = ExistenceImpossible
+	}
 	c.filterMu.Unlock()
 
-	result := "miss"
-	if found {
-		result = "hit"
-	}
 	logc.Printf(ctx, "existence: domain %s: %s", domain, result)
 	return
 }
@@ -170,8 +196,12 @@ func (c *bloomExistenceCache) AddSite(ctx context.Context, site string) {
 
 type dummyExistenceCache struct{}
 
-func (d dummyExistenceCache) CheckSite(context.Context, string) bool { return true }
+func (d dummyExistenceCache) CheckSite(context.Context, string) Existence {
+	return ExistencePossible
+}
 
-func (d dummyExistenceCache) CheckDomain(context.Context, string) bool { return true }
+func (d dummyExistenceCache) CheckDomain(context.Context, string) Existence {
+	return ExistencePossible
+}
 
 func (d dummyExistenceCache) AddSite(context.Context, string) {}

src/pages.go

@@ -139,7 +139,7 @@ func getPage(w http.ResponseWriter, r *http.Request) error {
 	indexManifestCh := make(chan indexManifestResult, 1)
 	go func() {
 		webRoot := makeWebRoot(host, ".index")
-		if !existenceCache.CheckSite(r.Context(), webRoot) {
+		if existenceCache.CheckSite(r.Context(), webRoot).IsImpossible() {
 			close(indexManifestCh)
 			return
 		}
@@ -154,7 +154,8 @@ func getPage(w http.ResponseWriter, r *http.Request) error {
 	sitePath = strings.TrimPrefix(r.URL.Path, "/")
 	if projectName, projectPath, hasProjectSlash := strings.Cut(sitePath, "/"); projectName != "" {
 		webRoot := makeWebRoot(host, projectName)
-		if ValidateProjectName(projectName) == nil && existenceCache.CheckSite(r.Context(), webRoot) {
+		if ValidateProjectName(projectName) == nil &&
+			existenceCache.CheckSite(r.Context(), webRoot).IsPossible() {
 			var projectManifest *Manifest
 			var projectMetadata ManifestMetadata
 			projectManifest, projectMetadata, err = backend.GetManifest(