1 files changed, 129 insertions, 0 deletions
diff --git a/cache.go b/cache.go
new file mode 100644
index 0000000..d43fa77
--- /dev/null
+++ b/cache.go
@@ -0,0 +1,129 @@
+package main
+
+import (
+	"context"
+	"log"
+	"path"
+	"sync"
+	"time"
+)
+
+// A PackageCache holds information about Go packages and their upstream
+// repositories.
+//
+// Given a package name, it looks for a similarly named repository in one of
+// the upstream hosts and holds on to the result for quicker subsequent access.
+type PackageCache struct {
+	// ExpireAfter is how long to consider a found repository valid for. As
+	// repositories do not frequently move around, this could be rather long,
+	// though it also determines how long a user might have to wait for a new
+	// repository to become available if they previously got a 404.
+	ExpireAfter time.Duration
+
+	// UpstreamTimeout is how long we should allow HTTP requests to upstream
+	// hosts to carry on before they are canceled.
+	UpstreamTimeout time.Duration
+
+	// Hosts are the RepoHosts which should be checked for a repository matching
+	// the package name. Hosts are checked in order; if two hosts both have a
+	// repo with the desired name, the repo host which is listed first will
+	// "win".
+	Hosts []RepoHost
+
+	// Logger is a logger for diagnostics
+	Logger *log.Logger
+
+	// CanonicalPrefix is the package prefix to use when naming packages. E.g.,
+	// when looking for a package named "conf", we'll treat the package name as
+	// CanonicalPrefix + "/conf", e.g. burwell.io/conf.
+	CanonicalPrefix string
+
+	entries sync.Map // concurrency-safe map[string]entry
+}
+
+type entry struct {
+	t   time.Time
+	pkg *Package
+}
+
+// A Package represents a Go package's canonical name and its source git
+// repository.
+type Package struct {
+	Name string
+	Repo string
+}
+
+// Get loads package name from the cache, consulting upstream repositories to
+// find a matching repo if not present. As this method is meant to be called in
+// the critical path of an incoming HTTP request, it can also be cancelled with
+// the supplied context.
+func (c *PackageCache) Get(ctx context.Context, name string) (Package, bool) {
+	val, ok := c.entries.Load(name)
+	if !ok {
+		return c.load(ctx, name)
+	}
+	ent := val.(entry)
+
+	if time.Now().After(ent.t.Add(c.ExpireAfter)) {
+		return c.load(ctx, name)
+	}
+
+	if ent.pkg == nil {
+		return Package{}, false
+	}
+
+	return *ent.pkg, true
+}
+
+// load looks for a package in the upstream repos, and if found, stores it in
+// the cache. If no matching repo is found, a nil entry is stored in the map so
+// we don't immediately re-check the upstreams for subsequent requests.
+func (c *PackageCache) load(ctx context.Context, name string) (Package, bool) {
+	pkg, ok := c.find(ctx, name)
+	if !ok {
+		c.entries.Store(name, entry{t: time.Now()})
+		return Package{}, false
+	}
+	c.entries.Store(name, entry{t: time.Now(), pkg: &pkg})
+	return pkg, true
+}
+
+// requestContext creates a context for making HTTP requests to upstream hosts.
+func (c *PackageCache) requestContext(ctx context.Context) (context.Context, context.CancelFunc) {
+	if c.UpstreamTimeout == 0 {
+		return ctx, func() {}
+	}
+	return context.WithTimeout(ctx, c.UpstreamTimeout)
+}
+
+// logf logs a message using the configured logger, if any.
+func (c *PackageCache) logf(msg string, args ...interface{}) {
+	if c.Logger == nil {
+		return
+	}
+	c.Logger.Printf(msg, args...)
+}
+
+// find consults the configured upstream hosts to try to find a matching repo
+// for package name.
+func (c *PackageCache) find(ctx context.Context, name string) (Package, bool) {
+	c.logf("loading %s", name)
+	for _, h := range c.Hosts {
+		reqCtx, cancel := c.requestContext(ctx)
+		defer cancel()
+		repo, ok, err := h.HostsRepo(reqCtx, name)
+		if err != nil {
+			c.logf("error checking upstream for repo %s: %v", name, err)
+			return Package{}, false
+		}
+		if ok {
+			c.logf("found repo %s: %s", name, repo)
+			return Package{
+				Name: path.Join(c.CanonicalPrefix, name),
+				Repo: repo,
+			}, true
+		}
+	}
+	c.logf("could not find repo %s on any host", name)
+	return Package{}, false
+}