internal/pkg: cache extern responses

This prepares substitute computation for shallow extern replacements.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg.go

@@ -770,6 +770,8 @@ type Cache struct {
 
 	// Optional external cache implementation.
 	extern External
+	// Caches responses from extern.
+	externCache map[unique.Handle[ID]]unique.Handle[Checksum]
 	// Synchronises access to extern.
 	externMu sync.RWMutex
 
@@ -930,6 +932,7 @@ func readlinkChecksum(a *check.Absolute, buf *Checksum) error {
 // SetExternal sets e as the [External] implementation of c.
 func (c *Cache) SetExternal(e External) {
 	c.externMu.Lock()
+	c.externCache = make(map[unique.Handle[ID]]unique.Handle[Checksum])
 	c.extern = e
 	c.externMu.Unlock()
 }
@@ -1849,37 +1852,45 @@ func (r *RContext) NewMeasuredReader(
 }
 
 // tryExtern attempts to obtain an [Artifact] outcome from extern.
-func (c *Cache) tryExtern(ctx context.Context, id unique.Handle[ID]) (
+func (c *Cache) tryExtern(
+	ctx context.Context,
+	id unique.Handle[ID],
+	status *io.ReadCloser,
+) (
 	unique.Handle[Checksum],
-	io.ReadCloser,
 	error,
 ) {
 	c.externMu.RLock()
 	defer c.externMu.RUnlock()
 
-	if c.extern == nil {
-		return zeroChecksum, nil, nil
+	checksum, ok := c.externCache[id]
+	if !ok {
+		if c.extern == nil {
+			return zeroChecksum, nil
+		}
+
+		v, err := c.extern.Artifact(ctx, id)
+		if err != nil {
+			return zeroChecksum, err
+		}
+		if v == nil {
+			return zeroChecksum, nil
+		}
+		checksum = unique.Make(*v)
+
+		var got unique.Handle[Checksum]
+		if _, got, err = c.Cure(c.extern.Checksum(checksum)); err != nil {
+			return checksum, err
+		} else if got != checksum {
+			return zeroChecksum, &ChecksumMismatchError{got.Value(), checksum.Value()}
+		}
 	}
 
-	v, err := c.extern.Artifact(ctx, id)
-	if err != nil {
-		return zeroChecksum, nil, err
+	var err error
+	if status != nil {
+		*status, err = c.extern.Status(&RContext{common{ctx, c}}, id)
 	}
-	if v == nil {
-		return zeroChecksum, nil, nil
-	}
-	checksum := unique.Make(*v)
-
-	var got unique.Handle[Checksum]
-	if _, got, err = c.Cure(c.extern.Checksum(checksum)); err != nil {
-		return checksum, nil, err
-	} else if got != checksum {
-		return zeroChecksum, nil, &ChecksumMismatchError{got.Value(), checksum.Value()}
-	}
-
-	var status io.ReadCloser
-	status, err = c.extern.Status(&RContext{common{ctx, c}}, id)
-	return checksum, status, err
+	return checksum, err
 }
 
 // cure implements Cure without acquiring a read lock on abortMu. cure must not
@@ -2225,7 +2236,7 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 			externChecksum unique.Handle[Checksum]
 			externStatus   io.ReadCloser
 		)
-		if externChecksum, externStatus, err = c.tryExtern(ctx, id); err != nil {
+		if externChecksum, err = c.tryExtern(ctx, id, &externStatus); err != nil {
 			if c.msg.IsVerbose() {
 				c.msg.Verbosef("extern %s: %v", reportName(ca, id), err)
 			}