internal/pkg: remove status acquisition from extern helper

Doing this together just makes it awkward to query status early, having to deal with cleaning up.

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

internal/pkg/pkg.go

@@ -1881,11 +1881,7 @@ func (c *Cache) tryLocal(id unique.Handle[ID]) (unique.Handle[Checksum], error)
 }
 
 // tryExtern attempts to obtain an [Artifact] outcome from extern.
-func (c *Cache) tryExtern(
-	ctx context.Context,
-	id unique.Handle[ID],
-	status *io.ReadCloser,
-) (
+func (c *Cache) tryExtern(ctx context.Context, id unique.Handle[ID]) (
 	unique.Handle[Checksum],
 	error,
 ) {
@@ -1914,12 +1910,7 @@ func (c *Cache) tryExtern(
 			return zeroChecksum, &ChecksumMismatchError{got.Value(), checksum.Value()}
 		}
 	}
-
-	var err error
-	if status != nil {
-		*status, err = c.extern.Status(&RContext{common{ctx, c}}, id)
-	}
-	return checksum, err
+	return checksum, nil
 }
 
 // cureMany concurrently collects outcome of multiple [Artifact].
@@ -2254,11 +2245,8 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 
 		defer f.destroy(&err)
 
-		var (
-			externChecksum unique.Handle[Checksum]
-			externStatus   io.ReadCloser
-		)
-		if externChecksum, err = c.tryExtern(ctx, id, &externStatus); err != nil {
+		var externChecksum unique.Handle[Checksum]
+		if externChecksum, err = c.tryExtern(ctx, id); err != nil {
 			if c.msg.IsVerbose() {
 				c.msg.Verbosef("extern %s: %v", reportName(ca, id), err)
 			}
@@ -2266,9 +2254,6 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 		}
 		if externChecksum != zeroChecksum {
 			if checksum != zeroChecksum && externChecksum != checksum {
-				if externStatus != nil {
-					_ = externStatus.Close()
-				}
 				err = &ChecksumMismatchError{externChecksum.Value(), checksum.Value()}
 				if c.msg.IsVerbose() {
 					c.msg.Verbosef("extern %s: %v", reportName(ca, id), err)
@@ -2276,6 +2261,14 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 				return
 			}
 
+			var externStatus io.ReadCloser
+			c.externMu.RLock()
+			externStatus, err = c.extern.Status(&RContext{common{ctx, c}}, id)
+			c.externMu.RUnlock()
+			if err != nil {
+				return
+			}
+
 			checksum = externChecksum
 			checksums = Encode(checksum.Value())
 			checksumPathname = c.base.Append(