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registry.go
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registry.go
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package vpcinfo
import (
"context"
"log"
"net"
"sync"
"sync/atomic"
"time"
)
// Registry exposes VPC information to the program.
//
// Registry values are intended to be long-lived, they cache the results they
// observe from the resolver.
//
// Registry values are safe to use concurrently from multiple goroutines.
type Registry struct {
// The resolver used by this registry.
//
// If nil, net.DefaultResolver is used instead.
Resolver Resolver
// Time limit for blocking operations ran by this registry.
//
// Zero means to apply no limit.
Timeout time.Duration
// Limits how long cached entries are retained before attempting to refresh
// them from the resolver.
//
// Defaults to 1 minute.
TTL time.Duration
endpoints cache // endpoints
subnets cache // Subnets
platform cache // Platform
zone cache // Zone
}
// LookupPlatform returns the name of the VPC platform, which will be either
// "aws" or "unknown".
func (r *Registry) LookupPlatform(ctx context.Context) (Platform, error) {
v, err := r.platform.load("platform", r.ttl(), func() (interface{}, error) {
subnets, err := r.LookupSubnets(ctx)
if err != nil {
return nil, err
}
return whereAmI(subnets)
})
p, _ := v.(Platform)
return p, err
}
// LookupSubnets returns the list of subnets in the VPC.
//
// Multiple calls to this method may return the same Subnets value, programs
// should treat it as a read-only value and avoid modifying it to prevent race
// conditions.
func (r *Registry) LookupSubnets(ctx context.Context) (Subnets, error) {
v, err := r.subnets.load("subnets", r.ttl(), func() (interface{}, error) {
ctx, cancel := r.withTimeout(ctx)
defer cancel()
records, err := r.lookupTXT(ctx, "subnets")
if err != nil {
return nil, err
}
subnets := make(Subnets, len(records))
for i, r := range records {
if err := parse(r, &subnets[i]); err != nil {
return nil, err
}
}
return subnets, nil
})
s, _ := v.(Subnets)
return s, err
}
// LookupZone returns the name of the VPC zone that the program is running in.
func (r *Registry) LookupZone(ctx context.Context) (Zone, error) {
v, err := (r.zone.load("zone", r.ttl(), func() (interface{}, error) {
ctx, cancel := r.withTimeout(ctx)
defer cancel()
p, err := r.LookupPlatform(ctx)
if err != nil {
return nil, err
}
return p.LookupZone(ctx)
}))
z, _ := v.(Zone)
return z, err
}
func (r *Registry) lookupTXT(ctx context.Context, name string) ([]string, error) {
v, err := r.endpoints.load("endpoints", r.ttl(), func() (interface{}, error) {
records, err := r.resolver().LookupTXT(ctx, "")
if err != nil {
return nil, err
}
endpoints := make(endpoints)
for _, r := range records {
k, v := splitNameAndValue(r)
endpoints[k] = v
}
return endpoints, nil
})
if err != nil {
return nil, err
}
return r.resolver().LookupTXT(ctx, v.(endpoints)[name])
}
func (r *Registry) resolver() Resolver {
if r.Resolver != nil {
return r.Resolver
}
return net.DefaultResolver
}
func (r *Registry) ttl() time.Duration {
if r.TTL > 0 {
return r.TTL
}
return time.Minute
}
func (r *Registry) withTimeout(ctx context.Context) (context.Context, context.CancelFunc) {
if r.Timeout > 0 {
return context.WithTimeout(ctx, r.Timeout)
}
return context.WithCancel(ctx)
}
type cache struct {
value atomic.Value // *value
reload uint64
mutex sync.Mutex
}
type value struct {
value interface{}
err error
update time.Time
expire time.Time
}
func (c *cache) load(what string, ttl time.Duration, lookup func() (interface{}, error)) (interface{}, error) {
now := time.Now()
v, _ := c.value.Load().(*value)
if v != nil {
if now.Before(v.update) || !atomic.CompareAndSwapUint64(&c.reload, 0, 1) {
return v.value, v.err
}
defer atomic.StoreUint64(&c.reload, 0)
} else {
// Block on the mutex to ensure that at most one goroutine sends a
// request to lookup the value. Only when the cache had no value yet
// would this code path be blocking, otherwise only one goroutine would
// enter it due to the negotiation happening on incrementing the version.
c.mutex.Lock()
defer c.mutex.Unlock()
if v, _ = c.value.Load().(*value); v != nil {
return v.value, v.err
}
}
update := now.Add(ttl / 2)
expire := now.Add(ttl)
val, err := lookup()
// On error, retain the previous value if we are still within the TTL.
if err != nil && v != nil && now.Before(v.expire) {
log.Printf("WARN vpcinfo - retaining previously cached %s entry after failure to refresh: %v", what, err)
val = v.value
err = v.err
// Keep the same expiration time so the cached value is eventually
// removed if we keep failing to update it.
expire = v.expire
} else if err != nil {
log.Printf("ERROR vpcinfo - error loading %s: %v", what, err)
} else {
log.Printf("NOTICE vpcinfo - updated %s to %s", what, val)
}
v = &value{
value: val,
err: err,
update: update,
expire: expire,
}
c.value.Store(v)
return v.value, v.err
}